Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form, but diamond almost never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material,  properties that are utilized in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.
Because the arrangement of atoms in diamond is extremely rigid, few types of impurity can contaminate it (two exceptions being boron and nitrogen). Small numbers of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (defects), green (radiation exposure), purple, pink, orange or red. Diamond also has relatively high optical dispersion (ability to disperse light of different colors).
Most natural diamonds have ages between 1billion and 3.5billion years. Most were formed at depths between 150 and 250 kilometers (93 and 155mi) in the Earth's mantle, although a few have come from as deep as 800 kilometers (500mi). Under high pressure and temperature, carbon-containing fluids dissolved minerals and replaced them with diamonds. Much more recently (tens to hundreds of million years ago), they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites.
Synthetic diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gas by chemical vapor deposition (CVD). Imitation diamonds can also be made out of materials such as cubic zirconia and silicon carbide. Natural, synthetic and imitation diamonds are most commonly distinguished using optical techniques or thermal conductivity measurements.

Material properties

Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond. The two most common allotropes of pure carbon are diamond and graphite. In graphite the bonds are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. In diamond they are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors.[3][4] Tetrahedra are rigid, the bonds are strong, and of all known substances diamond has the greatest number of atoms per unit volume, which is why it is both the hardest and the least compressible.[5][6] It also has a high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520 kg/m³ in pure diamond.[1] In graphite, the bonds between nearest neighbors are even stronger but the bonds between planes are weak, so the planes can easily slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable.[7]
Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness and thermal conductivity (900–{{val|2320|u=watts per centimeter per kelvin ),[8] as well as wide bandgap and high optical dispersion.[9]  Diamond's ignition point is 720–800°C in oxygen and 850–1000°C in air.
Thermodynamics

The equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally. The pressure changes linearly between 1.7GPa at 0K and 12GPa at 5000K (the diamond/graphite/liquid triple point).[10][11]
However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20°C (293K) and 1 standard atmosphere (0.10MPa), the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible.[6] However, at temperatures above about 4500K, diamond rapidly converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000K, a pressure of 35GPa is needed.[10]
Above the triple point, the melting point of diamond increases slowly with increasing pressure; but at pressures of hundreds of GPa, it decreases.[12] At high pressures, silicon and germanium have a BC8 body-centered cubic crystal structure, and a similar structure is predicted for carbon at high pressures. At 0K, the transition is predicted to occur at 1100GPa.[13]

Crystal structure

The most common crystal structure of diamond is called diamond cubic. It is formed of unit cells (see the figure) stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell.[14] Each side of the unit cell is 3.57 angstroms in length.[15]
A diamond cubic lattice can be thought of as two interpenetrating face-centered cubic lattices with one displaced by 1/4 of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point.[15] Looked at from a <1 1 1> crystallographic direction, it is formed of layers stacked in a repeating ABCABC ... pattern. Diamonds can also form an ABAB ... structure, which is known as hexagonal diamond or lonsdaleite, but this is far less common and is formed under different conditions from cubic carbon.[16]
Crystal habit

Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles. As diamond's crystal structure has a cubic arrangement of the atoms, they have many facets that belong to a cube, octahedron, rhombicosidodecahedron, tetrakis hexahedron or disdyakis dodecahedron. The crystals can have rounded off and unexpressive edges and can be elongated. Diamonds (especially those with rounded crystal faces) are commonly found coated in nyf, an opaque gum-like skin.[17]
Some diamonds have opaque fibers. They are referred to as opaque if the fibers grow from a clear substrate or fibrous if they occupy the entire crystal. Their colors range from yellow to green or gray, sometimes with cloud-like white to gray impurities. Their most common shape is cuboidal, but they can also form octahedra, dodecahedra, macles or combined shapes. The structure is the result of numerous impurities with sizes between 1 and 5 microns. These diamonds probably formed in kimberlite magma and sampled the volatiles.[18]
Diamonds can also form polycrystalline aggregates. There have been attempts to classify them into groups with names such as boart, ballas, stewartite and framesite, but there is no widely accepted set of criteria.[18] Carbonado, a type in which the diamond grains were sintered (fused without melting by the application of heat and pressure), is black in color and tougher than single crystal diamond.[19] It has never been observed in a volcanic rock. There are many theories for its origin, including formation in a star, but no consensus.[18][20][21]
Mechanical properties
Hardness

Diamond is the hardest known natural material on both the Vickers scale and the Mohs scale. Diamond's great hardness relative to other materials has been known since antiquity, and is the source of its name.
Diamond hardness depends on its purity, crystalline perfection and orientation: hardness is higher for flawless, pure crystals oriented to the <111> direction (along the longest diagonal of the cubic diamond lattice).[22] Therefore, whereas it might be possible to scratch some diamonds with other materials, such as boron nitride, the hardest diamonds can only be scratched by other diamonds and nanocrystalline diamond aggregates.
The hardness of diamond contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching—perhaps contributing to its popularity as the preferred gem in engagement or wedding rings, which are often worn every day.
The hardest natural diamonds mostly originate from the Copeton and Bingara fields located in the New England area in New South Wales, Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is associated with the crystal growth form, which is single-stage crystal growth. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice, all of which affect their hardness. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce diamonds that are harder than the diamonds used in hardness gauges.[23]
Toughness
Somewhat related to hardness is another mechanical property toughness, which is a material's ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 7.5–10 MPa·m1/2.[24][25] This value is good compared to other ceramic materials, but poor compared to most engineering materials such as engineering alloys, which typically exhibit toughnesses over 100 MPa·m1/2. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond has a cleavage plane and is therefore more fragile in some orientations than others. Diamond cutters use this attribute to cleave some stones, prior to faceting.[26] "Impact toughness" is one of the main indexes to measure the quality of synthetic industrial diamonds.
Yield strength
Diamond has compressive yield strength of 130–140GPa.[27] This exceptionally high value, along with the hardness and transparency of diamond, are the reasons that diamond anvil cells are the main tool for high pressure experiments.[28] These anvils have reached pressures of 600GPa.[29] Much higher pressures may be possible with nanocrystalline diamonds.[28][30]
Electrical conductivity
Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most diamonds, which are excellent electrical insulators.[31] The conductivity and blue color originate from boron impurity. Boron substitutes for carbon atoms in the diamond lattice, donating a hole into the valence band.[31]
Substantial conductivity is commonly observed in nominally undoped diamond grown by chemical vapor deposition. This conductivity is associated with hydrogen-related species adsorbed at the surface, and it can be removed by annealing or other surface treatments.[32][33]
Surface property
Diamonds are naturally lipophilic and hydrophobic, which means the diamonds' surface cannot be wet by water, but can be easily wet and stuck by oil. This property can be utilized to extract diamonds using oil when making synthetic diamonds. However, when diamond surfaces are chemically modified with certain ions, they are expected to become so hydrophilic that they can stabilize multiple layers of water ice at human body temperature.[34]
The surface of diamonds is partially oxidized. The oxidized surface can be reduced by heat treatment under hydrogen flow. That is to say, this heat treatment partially removes oxygen-containing functional groups. But diamonds (sp3C) are unstable against high temperature (above about 400°C (752°F)) under atmospheric pressure. The structure gradually changes into sp2C above this temperature. Thus, diamonds should be reduced under this temperature.[35]
Chemical stability
Diamonds are not very reactive. Under room temperature diamonds do not react with any chemical reagents including strong acids and bases. A diamond's surface can only be oxidized at temperatures above about 850°C (1,560°F) in air. Diamond also reacts with fluorine gas above about 700°C (1,292°F).
Color

Diamond has a wide bandgap of 5.5eV corresponding to the deep ultraviolet wavelength of 225 nanometers. This means that pure diamond should transmit visible light and appear as a clear colorless crystal. Colors in diamond originate from lattice defects and impurities. The diamond crystal lattice is exceptionally strong, and only atoms of nitrogen, boron and hydrogen can be introduced into diamond during the growth at significant concentrations (up to atomic percents). Transition metals nickel and cobalt, which are commonly used for growth of synthetic diamond by high-pressure high-temperature techniques, have been detected in diamond as individual atoms; the maximum concentration is 0.01% for nickel[36] and even less for cobalt. Virtually any element can be introduced to diamond by ion implantation.[37]
Nitrogen is by far the most common impurity found in gem diamonds and is responsible for the yellow and brown color in diamonds. Boron is responsible for the blue color.[9] Color in diamond has two additional sources: irradiation (usually by alpha particles), that causes the color in green diamonds, and plastic deformation of the diamond crystal lattice. Plastic deformation is the cause of color in some brown[38] and perhaps pink and red diamonds.[39] In order of increasing rarity, yellow diamond is followed by brown, colorless, then by blue, green, black, pink, orange, purple, and red.[26] "Black", or Carbonado, diamonds are not truly black, but rather contain numerous dark inclusions that give the gems their dark appearance. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless. Most diamond impurities replace a carbon atom in the crystal lattice, known as a carbon flaw. The most common impurity, nitrogen, causes a slight to intense yellow coloration depending upon the type and concentration of nitrogen present.[26] The Gemological Institute of America (GIA) classifies low saturation yellow and brown diamonds as diamonds in the normal color range, and applies a grading scale from "D" (colorless) to "Z" (light yellow). Diamonds of a different color, such as blue, are called fancy colored diamonds and fall under a different grading scale.[26]
In 2008, the Wittelsbach Diamond, a 35.56-carat (7.112g) blue diamond once belonging to the King of Spain, fetched over US$24million at a Christie's auction.[40] In May 2009, a 7.03-carat (1.406g) blue diamond fetched the highest price per carat ever paid for a diamond when it was sold at auction for 10.5million Swiss francs (6.97million euros, or US$9.5million at the time).[41] That record was, however, beaten the same year: a 5-carat (1.0g) vivid pink diamond was sold for $10.8million in Hong Kong on December 1, 2009.[42]
Identification
Diamonds can be identified by their high thermal conductivity. Their high refractive index is also indicative, but other materials have similar refractivity. Diamonds cut glass, but this does not positively identify a diamond because other materials, such as quartz, also lie above glass on the Mohs scale and can also cut it. Diamonds can scratch other diamonds, but this can result in damage to one or both stones. Hardness tests are infrequently used in practical gemology because of their potentially destructive nature.[43] The extreme hardness and high value of diamond means that gems are typically polished slowly, using painstaking traditional techniques and greater attention to detail than is the case with most other gemstones;[44] these tend to result in extremely flat, highly polished facets with exceptionally sharp facet edges. Diamonds also possess an extremely high refractive index and fairly high dispersion. Taken together, these factors affect the overall appearance of a polished diamond and most diamantaires still rely upon skilled use of a loupe (magnifying glass) to identify diamonds "by eye".[45]
Geology
Diamonds are extremely rare, with concentrations of at most parts per billion in source rock.[18] Before the 20th century, most diamonds were found in alluvial deposits. Loose diamonds are also found along existing and ancient shorelines, where they tend to accumulate because of their size and density.[46]:149 Rarely, they have been found in glacial till (notably in Wisconsin and Indiana), but these deposits are not of commercial quality.[46]:19 These types of deposit were derived from localized igneous intrusions through weathering and transport by wind or water.[47]
Most diamonds come from the Earth's mantle, and most of this section discusses those diamonds. However, there are other sources. Some blocks of the crust, or terranes, have been buried deep enough as the crust thickened so they experienced ultra-high-pressure metamorphism. These have evenly distributed microdiamonds that show no sign of transport by magma. In addition, when meteorites strike the ground, the shock wave can produce high enough temperatures and pressures for microdiamonds and nanodiamonds to form.[47] Impact-type microdiamonds can be used as an indicator of ancient impact craters.[48] Popigai crater in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact.[49]
A common misconception is that diamonds are formed from highly compressed coal. Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first land plants. It is possible that diamonds can form from coal in subduction zones, but diamonds formed in this way are rare, and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal.[50][51]
Surface distribution

Diamonds are far from evenly distributed over the Earth. A rule of thumb known as Clifford's rule states that they are almost always found in kimberlites on the oldest part of cratons, the stable cores of continents with typical ages of 2.5 billion years or more.[47][52]:314 However, there are exceptions. The Argyle diamond mine in Australia, the largest producer of diamonds by weight in the world, is located in a mobile belt, also known as an orogenic belt,[53] a weaker zone surrounding the central craton that has undergone compressional tectonics. Instead of kimberlite, the host rock is lamproite. Lamproites with diamonds that are not economically viable are also found in the United States, India and Australia.[47] In addition,  diamonds in the Wawa belt of the Superior province in Canada and microdiamonds in the island arc of Japan are found in a type of rock called lamprophyre.[47]
Kimberlites can be found in narrow (1 to 4 metres) dikes and sills, and in pipes with diameters that range from about 75 m to 1.5 km. Fresh rock is dark bluish green to greenish gray, but after exposure rapidly turns brown and crumbles.[54] It is hybrid rock with a chaotic mixture of small minerals and rock fragments (clasts) up to the size of watermelons. They are a mixture of xenocrysts and xenoliths (minerals and rocks carried up from the lower crust and mantle), pieces of surface rock, altered minerals such as serpentine, and new minerals that crystallized during the eruption. The texture varies with depth. The composition forms a continuum with carbonatites, but the latter have too much oxygen for carbon to exist in a pure form. Instead, it is locked up in the mineral calcite (CaCO3).[47]
All three of the diamond-bearing rocks (kimberlite, lamproite and lamprophyre) lack certain minerals (melilite and kalsilite) that are incompatible with diamond formation. In kimberlite, olivine is large and conspicuous, while lamproite has Ti-phlogopite and lamprophyre has biotite and amphibole. They are all derived from magma types that erupt rapidly from small amounts of melt, are rich in volatiles and magnesium oxide, and are less oxidizing than more common mantle melts such as basalt. These characteristics allow the melts to carry diamonds to the surface before they dissolve.[47]
Exploration
See List of diamond mines

Kimberlite pipes can be difficult to find. They weather quickly (within a few years after exposure) and tend to have lower topographic relief than surrounding rock. If they are visible in outcrops, the diamonds are never visible because they are so rare. In any case, kimberlites are often covered with vegetation, sediments, soils or lakes. In modern searches, geophysical methods such as aeromagnetic surveys, electrical resistivity and gravimetry, help identify promising regions to explore. This is aided by isotopic dating and modeling of the geological history. Then surveyors must go to the area and collect samples, looking for kimberlite fragments or indicator minerals. The latter have compositions that reflect the conditions where diamonds form, such as extreme melt depletion or high pressures in eclogites. However, indicator minerals can be misleading; a better approach is geothermobarometry, where the compositions of minerals are analyzed as if they were in equilibrium with mantle minerals.[47]
Finding kimberlites requires persistence, and only a small fraction contain diamonds that are commercially viable. The only major discoveries since about 1980 have been in Canada. Since existing mines have lifetimes of as little as 25 years, there could be a shortage of new diamonds in the future.[47]
Ages
Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes. Depending on the elemental abundances, one can look at the decay of rubidium to strontium, samarium to neodymium, uranium to lead, argon-40 to argon-39, or rhenium to osmium. Those found in kimberlites have ages ranging from 1 to 3.5 billion years, and there can be multiple ages in the same kimberlite, indicating multiple episodes of diamond formation. The kimberlites themselves are much younger. Most of them have ages between tens of millions and 300 million years old, although there are some older exceptions (Argyle, Premier and Wawa). Thus, the kimberlites formed independently of the diamonds and served only to transport them to the surface.[18][47] Kimberlites are also much younger than the cratons they have erupted through. The reason for the lack of older kimberlites is unknown, but it suggests there was some change in mantle chemistry or tectonics. No kimberlite has erupted in human history.[47]
Origin in mantle

Most gem-quality diamonds come from depths of 150–250 km in the lithosphere. Such depths occur below cratons in mantle keels, the thickest part of the lithosphere. These regions have high enough pressure and temperature to allow diamonds to form and they are not convecting, so diamonds can be stored for billions of years until a kimberlite eruption samples them.[47]
Host rocks in a mantle keel include harzburgite and lherzolite, two type of peridotite. The most dominant rock type in the upper mantle, peridotite is an igneous rock consisting mostly of the minerals olivine and pyroxene; it is low in silica and high in magnesium. However, diamonds in peridotite rarely survive the trip to the surface.[47] Another common source that does keep diamonds intact is eclogite, a metamorphic rock that typically forms from basalt as an oceanic plate plunges into the mantle at a subduction zone.[18]
A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660 km, a region that includes the transition zone. They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of majorite (a form of garnet with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800 km.[18]
Diamond is thermodynamically stable at high pressures and temperatures, with the phase transition from graphite occurring at greater temperatures as the pressure increases. Thus, underneath continents it becomes stable at temperatures of 950 degrees Celsius and pressures of 4.5 gigapascals, corresponding to depths of 150 kilometers or greater. In subduction zones, which are colder, it becomes stable at temperatures of 800 degrees C and pressures of 3.5 gigapascals. At depths greater than 240 km, iron-nickel metal phases are present and carbon is likely to be either dissolved in them or in the form of carbides. Thus, the deeper origin of some diamonds may reflect unusual growth environments.[18][47]
In 2018 the first known natural samples of a phase of ice called Ice VII were found as inclusions in diamond samples. The inclusions formed at depths between 400 and 800 km, straddling the upper and lower mantle, and provide evidence for water-rich fluid at these depths.[55][56]
Carbon sources
The amount of carbon in the mantle is not well constrained, but its concentration is estimated at 0.5 to 1 parts per thousand.[47] It has two stable isotopes, 12C and 13C, in a ratio of approximately 99:1 by mass. This ratio has a wide range in meteorites, which implies that it was probably also broad in the early Earth. It can also be altered by surface processes like photosynthesis. The fraction is generally compared to a standard sample using a ratio δ13C expressed in parts per thousand. Common rocks from the mantle such as basalts, carbonatites and kimberlites have ratios between −8 and −2. On the surface, organic sediments have an average of −25 while carbonates have an average of 0.[18]
Populations of diamonds from different sources have distributions of δ13C that vary markedly. Peridotitic diamonds are mostly within the typical mantle range; eclogitic diamonds have values from −40 to +3, although the peak of the distribution is in the mantle range. This variability implies that they are not formed from carbon that is primordial (having resided in the mantle since the Earth formed). Instead, they are the result of tectonic processes, although (given the ages of diamonds) not necessarily the same tectonic processes that act in the present.[47]
Formation and growth
Diamonds in the mantle form through a metasomatic process where a C-O-H-N-S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C-O-H-N-S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (e.g., CO2 or CO3) or oxidation of a reduced phase such as methane.[18]
Using probes such as polarized light, photoluminescence and cathodoluminescence, a series of growth zones can be identified in diamonds. The characteristic pattern in diamonds from the lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher temperatures and pressures as well as the transport of the diamonds by convection.[47]
Transport to the surface

Geological evidence supports a model in which kimberlite magma rose at 4–20 meters per second, creating an upward path by hydraulic fracturing of the rock. As the pressure decreases, a vapor phase exsolves from the magma, and this helps to keep the magma fluid. At the surface, the initial eruption explodes out through fissures at high speeds (over 200m/s (450mph)). Then, at lower pressures, the rock is eroded, forming a pipe and producing fragmented rock (breccia). As the eruption wanes, there is pyroclastic phase and then metamorphism and hydration produces serpentinites.[47]
In space

Although diamonds on Earth are rare, they are very common in space. In meteorites, about three percent of the carbon is in the form of nanodiamonds, having diameters of a few nanometers. Sufficiently small diamonds can form in the cold of space because their lower surface energy makes them more stable than graphite. The isotopic signatures of some nanodiamonds indicate they were formed outside the Solar System in stars.[57]
High pressure experiments predict that large quantities of diamonds condense from methane into a "diamond rain" on the ice giant planets Uranus and Neptune.[58][59][60] Some extrasolar planets may be almost entirely composed of diamond.[61]
Diamonds may exist in carbon-rich stars, particularly white dwarfs. One theory for the origin of carbonado, the toughest form of diamond, is that it originated in a white dwarf or supernova.[62][63] Diamonds formed in stars may have been the first minerals.[64]
Industry

The most familiar uses of diamonds today are as gemstones used for adornment, and as industrial abrasives for cutting hard materials. The markets for gem-grade and industrial-grade diamonds value diamonds differently.

Gem-grade diamonds

The dispersion of white light into spectral colors is the primary gemological characteristic of gem diamonds. In the 20th century, experts in gemology developed methods of grading diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds: these are its mass in carats (a carat begin equal to 0.2 grams), cut (quality of the cut is graded according to proportions, symmetry and polish), color (how close to white or colorless; for fancy diamonds how intense is its hue), and clarity (how free is it from inclusions). A large, flawless diamond is known as a paragon.[65]
A large trade in gem-grade diamonds exists. Although most gem-grade diamonds are sold newly polished, there is a well-established market for resale of polished diamonds (e.g. pawnbroking, auctions, second-hand jewelry stores, diamantaires, bourses, etc.). One hallmark of the trade in gem-quality diamonds is its remarkable concentration: wholesale trade and diamond cutting is limited to just a few locations; in 2003, 92% of the world's diamonds were cut and polished in Surat, India.[66] Other important centers of diamond cutting and trading are the Antwerp diamond district in Belgium, where the International Gemological Institute is based, London, the Diamond District in New York City, the Diamond Exchange District in Tel Aviv, and Amsterdam. One contributory factor is the geological nature of diamond deposits: several large primary kimberlite-pipe mines each account for significant portions of market share (such as the Jwaneng mine in Botswana, which is a single large-pit mine that can produce between 12,500,000 and 15,000,000 carats (2,500 and 3,000kg) of diamonds per year[67]). Secondary alluvial diamond deposits, on the other hand, tend to be fragmented amongst many different operators because they can be dispersed over many hundreds of square kilometers (e.g., alluvial deposits in Brazil).
The production and distribution of diamonds is largely consolidated in the hands of a few key players, and concentrated in traditional diamond trading centers, the most important being Antwerp, where 80% of all rough diamonds, 50% of all cut diamonds and more than 50% of all rough, cut and industrial diamonds combined are handled.[68] This makes Antwerp a de facto "world diamond capital".[69] The city of Antwerp also hosts the Antwerpsche Diamantkring, created in 1929 to become the first and biggest diamond bourse dedicated to rough diamonds.[70] Another important diamond center is New York City, where almost 80% of the world's diamonds are sold, including auction sales.[68]
The De Beers company, as the world's largest diamond mining company, holds a dominant position in the industry, and has done so since soon after its founding in 1888 by the British imperialist Cecil Rhodes. De Beers is currently the world's largest operator of diamond production facilities (mines) and distribution channels for gem-quality diamonds. The Diamond Trading Company (DTC) is a subsidiary of De Beers and markets rough diamonds from De Beers-operated mines. De Beers and its subsidiaries own mines that produce some 40% of annual world diamond production. For most of the 20th century over 80% of the world's rough diamonds passed through De Beers,[71] but by 2001–2009 the figure had decreased to around 45%,[72] and by 2013 the company's market share had further decreased to around 38% in value terms and even less by volume.[73] De Beers sold off the vast majority of its diamond stockpile in the late 1990s – early 2000s[74] and the remainder largely represents working stock (diamonds that are being sorted before sale).[75] This was well documented in the press[76] but remains little known to the general public.
As a part of reducing its influence, De Beers withdrew from purchasing diamonds on the open market in 1999 and ceased, at the end of 2008, purchasing Russian diamonds mined by the largest Russian diamond company Alrosa.[77]  As of January 2011, De Beers states that it only sells diamonds from the following four countries: Botswana, Namibia, South Africa and Canada.[78] Alrosa had to suspend their sales in October 2008 due to the global energy crisis,[79] but the company reported that it had resumed selling rough diamonds on the open market by October 2009.[80]  Apart from Alrosa, other important diamond mining companies include BHP Billiton, which is the world's largest mining company;[81] Rio Tinto Group, the owner of the Argyle (100%), Diavik (60%), and Murowa (78%) diamond mines;[82] and Petra Diamonds, the owner of several major diamond mines in Africa.

Further down the supply chain, members of The World Federation of Diamond Bourses (WFDB) act as a medium for wholesale diamond exchange, trading both polished and rough diamonds. The WFDB consists of independent diamond bourses in major cutting centers such as Tel Aviv, Antwerp, Johannesburg and other cities across the USA, Europe and Asia.[26] In 2000, the WFDB and The International Diamond Manufacturers Association established the World Diamond Council to prevent the trading of diamonds used to fund war and inhumane acts. WFDB's additional activities include sponsoring the World Diamond Congress every two years, as well as the establishment of the International Diamond Council (IDC) to oversee diamond grading.
Once purchased by Sightholders (which is a trademark term referring to the companies that have a three-year supply contract with DTC), diamonds are cut and polished in preparation for sale as gemstones ('industrial' stones are regarded as a by-product of the gemstone market; they are used for abrasives).[83] The cutting and polishing of rough diamonds is a specialized skill that is concentrated in a limited number of locations worldwide.[83] Traditional diamond cutting centers are Antwerp, Amsterdam, Johannesburg, New York City, and Tel Aviv. Recently, diamond cutting centers have been established in China, India, Thailand, Namibia and Botswana.[83] Cutting centers with lower cost of labor, notably Surat in Gujarat, India, handle a larger number of smaller carat diamonds, while smaller quantities of larger or more valuable diamonds are more likely to be handled in Europe or North America. The recent expansion of this industry in India, employing low cost labor, has allowed smaller diamonds to be prepared as gems in greater quantities than was previously economically feasible.[68]
Diamonds prepared as gemstones are sold on diamond exchanges called bourses. There are 28 registered diamond bourses in the world.[84] Bourses are the final tightly controlled step in the diamond supply chain; wholesalers and even retailers are able to buy relatively small lots of diamonds at the bourses, after which they are prepared for final sale to the consumer. Diamonds can be sold already set in jewelry, or sold unset ("loose"). According to the Rio Tinto Group, in 2002 the diamonds produced and released to the market were valued at US$9billion as rough diamonds, US$14billion after being cut and polished, US$28billion in wholesale diamond jewelry, and US$57billion in retail sales.[85]
Cutting

Mined rough diamonds are converted into gems through a multi-step process called "cutting".
Diamonds are extremely hard, but also brittle and can be split up by a single blow. Therefore, diamond cutting is traditionally considered as a delicate procedure requiring skills, scientific knowledge, tools and experience. Its final goal is to produce a faceted jewel where the specific angles between the facets would optimize the diamond luster, that is dispersion of white light, whereas the number and area of facets would determine the weight of the final product. The weight reduction upon cutting is significant and can be of the order of 50%.[86] Several possible shapes are considered, but the final decision is often determined not only by scientific, but also practical considerations. For example, the diamond might be intended for display or for wear, in a ring or a necklace, singled or surrounded by other gems of certain color and shape.[87] Some of them may be considered as classical, such as round, pear, marquise, oval, hearts and arrows diamonds, etc. Some of them are special, produced by certain companies, for example, Phoenix, Cushion, Sole Mio diamonds, etc.[88]
The most time-consuming part of the cutting is the preliminary analysis of the rough stone. It needs to address a large number of issues, bears much responsibility, and therefore can last years in case of unique diamonds. The following issues are considered:
The hardness of diamond and its ability to cleave strongly depend on the crystal orientation. Therefore, the crystallographic structure of the diamond to be cut is analyzed using X-ray diffraction to choose the optimal cutting directions.
Most diamonds contain visible non-diamond inclusions and crystal flaws. The cutter has to decide which flaws are to be removed by the cutting and which could be kept.
The diamond can be split by a single, well calculated blow of a hammer to a pointed tool, which is quick, but risky. Alternatively, it can be cut with a diamond saw, which is a more reliable but tedious procedure.[87][89]
After initial cutting, the diamond is shaped in numerous stages of polishing. Unlike cutting, which is a responsible but quick operation, polishing removes material by gradual erosion and is extremely time consuming. The associated technique is well developed; it is considered as a routine and can be performed by technicians.[90] After polishing, the diamond is reexamined for possible flaws, either remaining or induced by the process. Those flaws are concealed through various diamond enhancement techniques, such as repolishing, crack filling, or clever arrangement of the stone in the jewelry. Remaining non-diamond inclusions are removed through laser drilling and filling of the voids produced.[43]
Marketing

Marketing has significantly affected the image of diamond as a valuable commodity.
N. W. Ayer & Son, the advertising firm retained by De Beers in the mid-20th century, succeeded in reviving the American diamond market. And the firm created new markets in countries where no diamond tradition had existed before. N. W. Ayer's marketing included product placement, advertising focused on the diamond product itself rather than the De Beers brand, and associations with celebrities and royalty. Without advertising the De Beers brand, De Beers was advertising its competitors' diamond products as well,[91] but this was not a concern as De Beers dominated the diamond market throughout the 20th century. De Beers' market share dipped temporarily to 2nd place in the global market below Alrosa in the aftermath of the global economic crisis of 2008, down to less than 29% in terms of carats mined, rather than sold.[92]  The campaign lasted for decades but was effectively discontinued by early 2011. De Beers still advertises diamonds, but the advertising now mostly promotes its own brands, or licensed product lines, rather than completely "generic" diamond products.[92]  The campaign was perhaps best captured by the slogan "a diamond is forever".[93]  This slogan is now being used by De Beers Diamond Jewelers,[94] a jewelry firm which is a 50%/50% joint venture between the De Beers mining company and LVMH, the luxury goods conglomerate.
Brown-colored diamonds constituted a significant part of the diamond production, and were predominantly used for industrial purposes. They were seen as worthless for jewelry (not even being assessed on the diamond color scale). After the development of Argyle diamond mine in Australia in 1986, and marketing, brown diamonds have become acceptable gems.[95][96] The change was mostly due to the numbers: the Argyle mine, with its 35,000,000 carats (7,000kg) of diamonds per year, makes about one-third of global production of natural diamonds;[97] 80% of Argyle diamonds are brown.[98]
Industrial-grade diamonds

Industrial diamonds are valued mostly for their hardness and thermal conductivity, making many of the gemological characteristics of diamonds, such as the 4 Cs, irrelevant for most applications. 80% of mined diamonds (equal to about 135,000,000 carats (27,000kg) annually) are unsuitable for use as gemstones and are used industrially.[99] In addition to mined diamonds, synthetic diamonds found industrial applications almost immediately after their invention in the 1950s; another 570,000,000 carats (114,000kg) of synthetic diamond is produced annually for industrial use (in 2004; in 2014 it is 4,500,000,000 carats (900,000kg), 90% of which is produced in China). Approximately 90% of diamond grinding grit is currently of synthetic origin.[100]
The boundary between gem-quality diamonds and industrial diamonds is poorly defined and partly depends on market conditions (for example, if demand for polished diamonds is high, some lower-grade stones will be polished into low-quality or small gemstones rather than being sold for industrial use). Within the category of industrial diamonds, there is a sub-category comprising the lowest-quality, mostly opaque stones, which are known as bort.[101]
Industrial use of diamonds has historically been associated with their hardness, which makes diamond the ideal material for cutting and grinding tools. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial applications of this property include diamond-tipped drill bits and saws, and the use of diamond powder as an abrasive. Less expensive industrial-grade diamonds, known as bort, with more flaws and poorer color than gems, are used for such purposes.[102] Diamond is not suitable for machining ferrous alloys at high speeds, as carbon is soluble in iron at the high temperatures created by high-speed machining, leading to greatly increased wear on diamond tools compared to alternatives.[103]
Specialized applications include use in laboratories as containment for high-pressure experiments (see diamond anvil cell), high-performance bearings, and limited use in specialized windows.[101] With the continuing advances being made in the production of synthetic diamonds, future applications are becoming feasible. The high thermal conductivity of diamond makes it suitable as a heat sink for integrated circuits in electronics.[104]
Mining

Approximately 130,000,000 carats (26,000kg) of diamonds are mined annually, with a total value of nearly US$9billion, and about 100,000kg (220,000lb) are synthesized annually.[105]
Roughly 49% of diamonds originate from Central and Southern Africa, although significant sources of the mineral have been discovered in Canada, India, Russia, Brazil, and Australia.[100] They are mined from kimberlite and lamproite volcanic pipes, which can bring diamond crystals, originating from deep within the Earth where high pressures and temperatures enable them to form, to the surface. The mining and distribution of natural diamonds are subjects of frequent controversy such as concerns over the sale of blood diamonds or conflict diamonds by African paramilitary groups.[106] The diamond supply chain is controlled by a limited number of powerful businesses, and is also highly concentrated in a small number of locations around the world.
Only a very small fraction of the diamond ore consists of actual diamonds. The ore is crushed, during which care is required not to destroy larger diamonds, and then sorted by density. Today, diamonds are located in the diamond-rich density fraction with the help of X-ray fluorescence, after which the final sorting steps are done by hand. Before the use of X-rays became commonplace,[86] the separation was done with grease belts; diamonds have a stronger tendency to stick to grease than the other minerals in the ore.[26]

Historically, diamonds were found only in alluvial deposits in Guntur and Krishna district of the Krishna River delta in Southern India.[107] India led the world in diamond production from the time of their discovery in approximately the 9th century BC[108][109] to the mid-18th century AD, but the commercial potential of these sources had been exhausted by the late 18th century and at that time India was eclipsed by Brazil where the first non-Indian diamonds were found in 1725.[108] Currently, one of the most prominent Indian mines is located at Panna.[110]
Diamond extraction from primary deposits (kimberlites and lamproites) started in the 1870s after the discovery of the Diamond Fields in South Africa.[111]
Production has increased over time and now an accumulated total of 4,500,000,000 carats (900,000kg) have been mined since that date.[112] Twenty percent of that amount has been mined in the last five years, and during the last 10 years, nine new mines have started production; four more are waiting to be opened soon. Most of these mines are located in Canada, Zimbabwe, Angola, and one in Russia.[112]
In the U.S., diamonds have been found in Arkansas, Colorado, New Mexico, Wyoming, and Montana.[113][114] In 2004, the discovery of a microscopic diamond in the U.S. led to the January 2008 bulk-sampling of kimberlite pipes in a remote part of Montana. The Crater of Diamonds State Park in Arkansas is open to the public, and is the only mine in the world where members of the public can dig for diamonds.[114]
Today, most commercially viable diamond deposits are in Russia (mostly in Sakha Republic, for example Mir pipe and Udachnaya pipe), Botswana, Australia (Northern and Western Australia) and the Democratic Republic of the Congo.[115]
In 2005, Russia produced almost one-fifth of the global diamond output, according to the British Geological Survey. Australia boasts the richest diamantiferous pipe, with production from the Argyle diamond mine reaching peak levels of 42 metric tons per year in the 1990s.[113][116]
There are also commercial deposits being actively mined in the Northwest Territories of Canada and Brazil.[100]
Diamond prospectors continue to search the globe for diamond-bearing kimberlite and lamproite pipes.
Political issues

In some of the more politically unstable central African and west African countries, revolutionary groups have taken control of diamond mines, using proceeds from diamond sales to finance their operations. Diamonds sold through this process are known as conflict diamonds or blood diamonds.[106]
In response to public concerns that their diamond purchases were contributing to war and human rights abuses in central and western Africa, the United Nations, the diamond industry and diamond-trading nations introduced the Kimberley Process in 2002.[117] The Kimberley Process aims to ensure that conflict diamonds do not become intermixed with the diamonds not controlled by such rebel groups. This is done by requiring diamond-producing countries to provide proof that the money they make from selling the diamonds is not used to fund criminal or revolutionary activities. Although the Kimberley Process has been moderately successful in limiting the number of conflict diamonds entering the market, some still find their way in. According to the International Diamond Manufacturers Association, conflict diamonds constitute 2–3% of all diamonds traded.[118] Two major flaws still hinder the effectiveness of the Kimberley Process: (1) the relative ease of smuggling diamonds across African borders, and (2) the violent nature of diamond mining in nations that are not in a technical state of war and whose diamonds are therefore considered "clean".[117]
The Canadian Government has set up a body known as the Canadian Diamond Code of Conduct[119] to help authenticate Canadian diamonds. This is a stringent tracking system of diamonds and helps protect the "conflict free" label of Canadian diamonds.[120]
Synthetics, simulants, and enhancements
Synthetics

Synthetic diamonds are diamonds manufactured in a laboratory, as opposed to diamonds mined from the Earth. The gemological and industrial uses of diamond have created a large demand for rough stones. This demand has been satisfied in large part by synthetic diamonds, which have been manufactured by various processes for more than half a century. However, in recent years it has become possible to produce gem-quality synthetic diamonds of significant size.[46]  It is possible to make colorless synthetic gemstones that, on a molecular level, are identical to natural stones and so visually similar that only a gemologist with special equipment can tell the difference.[121]
The majority of commercially available synthetic diamonds are yellow and are produced by so-called high-pressure high-temperature (HPHT) processes.[122] The yellow color is caused by nitrogen impurities. Other colors may also be reproduced such as blue, green or pink, which are a result of the addition of boron or from irradiation after synthesis.[123]

Another popular method of growing synthetic diamond is chemical vapor deposition (CVD). The growth occurs under low pressure (below atmospheric pressure). It involves feeding a mixture of gases (typically 1 to 99 methane to hydrogen) into a chamber and splitting them to chemically active radicals in a plasma ignited by microwaves, hot filament, arc discharge, welding torch or laser.[124] This method is mostly used for coatings, but can also produce single crystals several millimeters in size (see picture).[105]
As of 2010, nearly all 5,000million carats (1,000 tonnes) of synthetic diamonds produced per year are for industrial use. Around 50% of the 133million carats of natural diamonds mined per year end up in industrial use.[121][125]  Mining companies' expenses average 40 to 60US dollars per carat for natural colorless diamonds, while synthetic manufacturers' expenses average $2,500 per carat for synthetic, gem-quality colorless diamonds.[121]:79  However, a purchaser is more likely to encounter a synthetic when looking for a fancy-colored diamond because nearly all synthetic diamonds are fancy-colored, while only 0.01% of natural diamonds are.[126]
Simulants

A diamond simulant is a non-diamond material that is used to simulate the appearance of a diamond, and may be referred to as diamante. Cubic zirconia is the most common. The gemstone moissanite (silicon carbide) can be treated as a diamond simulant, though more costly to produce than cubic zirconia. Both are produced synthetically.[127]
Enhancements

Diamond enhancements are specific treatments performed on natural or synthetic diamonds (usually those already cut and polished into a gem), which are designed to better the gemological characteristics of the stone in one or more ways. These include laser drilling to remove inclusions, application of sealants to fill cracks, treatments to improve a white diamond's color grade, and treatments to give fancy color to a white diamond.[128]
Coatings are increasingly used to give a diamond simulant such as cubic zirconia a more "diamond-like" appearance. One such substance is diamond-like carbon—an amorphous carbonaceous material that has some physical properties similar to those of the diamond. Advertising suggests that such a coating would transfer some of these diamond-like properties to the coated stone, hence enhancing the diamond simulant. Techniques such as Raman spectroscopy should easily identify such a treatment.[129]
Identification
Early diamond identification tests included a scratch test relying on the superior hardness of diamond. This test is destructive, as a diamond can scratch another diamond, and is rarely used nowadays. Instead, diamond identification relies on its superior thermal conductivity. Electronic thermal probes are widely used in the gemological centers to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. This test takes about two to three seconds.[130]
Whereas the thermal probe can separate diamonds from most of their simulants, distinguishing between various types of diamond, for example synthetic or natural, irradiated or non-irradiated, etc., requires more advanced, optical techniques. Those techniques are also used for some diamonds simulants, such as silicon carbide, which pass the thermal conductivity test. Optical techniques can distinguish between natural diamonds and synthetic diamonds. They can also identify the vast majority of treated natural diamonds.[131] "Perfect" crystals (at the atomic lattice level) have never been found, so both natural and synthetic diamonds always possess characteristic imperfections, arising from the circumstances of their crystal growth, that allow them to be distinguished from each other.[132]
Laboratories use techniques such as spectroscopy, microscopy and luminescence under shortwave ultraviolet light to determine a diamond's origin.[131] They also use specially made instruments to aid them in the identification process. Two screening instruments are the DiamondSure and the DiamondView, both produced by the DTC and marketed by the GIA.[133]
Several methods for identifying synthetic diamonds can be performed, depending on the method of production and the color of the diamond. CVD diamonds can usually be identified by an orange fluorescence. D-J colored diamonds can be screened through the Swiss Gemmological Institute's[134] Diamond Spotter. Stones in the D-Z color range can be examined through the DiamondSure UV/visible spectrometer, a tool developed by De Beers.[132] Similarly, natural diamonds usually have minor imperfections and flaws, such as inclusions of foreign material, that are not seen in synthetic diamonds.
Screening devices based on diamond type detection can be used to make a distinction between diamonds that are certainly natural and diamonds that are potentially synthetic. Those potentially synthetic diamonds require more investigation in a specialized lab. Examples of commercial screening devices are D-Screen (WTOCD / HRD Antwerp) and Alpha Diamond Analyzer (Bruker / HRD Antwerp).
Stolen diamonds
Occasionally, large thefts of diamonds take place. In February 2013 armed robbers carried out a raid at Brussels Airport and escaped with gems estimated to be worth US$50M (£32M; €37M). The gang broke through a perimeter fence and raided the cargo hold of a Swiss-bound plane. The gang have since been arrested and large amounts of cash and diamonds recovered.[135]
The identification of stolen diamonds presents a set of difficult problems. Rough diamonds will have a distinctive shape depending on whether their source is a mine or from an alluvial environment such as a beach or river—alluvial diamonds have smoother surfaces than those that have been mined. Determining the provenance of cut and polished stones is much more complex.
The Kimberley Process was developed to monitor the trade in rough diamonds and prevent their being used to fund violence. Before exporting, rough diamonds are certificated by the government of the country of origin. Some countries, such as Venezuela, are not party to the agreement. The Kimberley Process does not apply to local sales of rough diamonds within a country.
Diamonds may be etched by laser with marks invisible to the naked eye. Lazare Kaplan, a US-based company, developed this method. However, whatever is marked on a diamond can readily be removed.[136][137]
History

The name diamond is derived from the ancient Greek αδάμας (adámas), "proper", "unalterable", "unbreakable", "untamed", from ἀ- (a-), "un-" + δαμάω (damáō), "I overpower", "I tame".[138] Diamonds are thought to have been first recognized and mined in India, where significant alluvial deposits of the stone could be found many centuries ago along the rivers Penner, Krishna and Godavari. Diamonds have been known in India for at least 3,000 years but most likely 6,000 years.[108]
Diamonds have been treasured as gemstones since their use as religious icons in ancient India. Their usage in engraving tools also dates to early human history.[139][140] The popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns.[93]
In 1772, the French scientist Antoine Lavoisier used a lens to concentrate the rays of the sun on a diamond in an atmosphere of oxygen, and showed that the only product of the combustion was carbon dioxide, proving that diamond is composed of carbon.[141] Later in 1797, the English chemist Smithson Tennant repeated and expanded that experiment.[142] By demonstrating that burning diamond and graphite releases the same amount of gas, he established the chemical equivalence of these substances.[44]
See also

Diamondoid
List of diamonds
List of largest rough diamonds
List of minerals
Lonsdaleite
Superhard material

Books

. (2007) Gemological Institute of America (GIA)
Tyson, Peter (November 2000). . Retrieved March 10, 2005.

How long does it take diamond to form from carbon?


Medieval stained glass is the coloured and painted glass of medieval Europe from the 10th century to the 16th century. For much of this period stained glass windows were the major pictorial art form, particularly in northern France, Germany and England, where windows tended to be larger than in southern Europe (in Italy, for example, frescos were more common).

Stained glass windows were used predominantly in churches, but were also found in wealthy domestic settings and public buildings such as town halls, though surviving examples of secular glass are very rare indeed.  The purpose of stained glass windows in a church was both to enhance the beauty of their setting and to inform the viewer through narrative or symbolism. The subject matter was generally religious in churches, though "portraits" and heraldry are often included, and many narrative scenes give valuable insights into the medieval world.
History

Early stained glass
Window glass was in use from at least the 1st century AD and colored and painted window glass for use in religious buildings was also manufactured at an early date. The earliest extant example of ecclesiastical stained glass is possibly that from San Vitale in Ravenna, Italy. A clear glass roundel with a depiction of Christ in Majesty, thought to be the 6th century, was discovered here.[1][2]
Some of the earliest known examples of colored window glass, datable to c. 800–820, were recovered in excavations at the Abbey of San Vicenzo in Volturno, Italy.[3] Glass of the same colour ranges and similar date is also found in England, at the monastic sites of Jarrow and Monkwearmouth, and at other sites in the north of England.[4][5] These examples are not painted. However, it was not until the advent of the monumental cathedral and church building campaigns in the 11th and 12th centuries that the demand for colored glass began to increase significantly, reaching its highest level in the 14th and 15th centuries. The oldest surviving stained glass windows still in situ are thought to be the Prophet Windows in Augsburg Cathedral, of c. 1065.
A useful 12th-century source on medieval glass manufacture is the De Divers Artibus of Theophilus Presbyter. Theophilus was a Benedictine Monk, believed by some scholars to be Roger of Helmarshausen, a metal glass and pigment worker who practiced in the late 11th and early 12th centuries.[6][7] De Divers Artibus describes a number of craft processes including glassmaking and glassworking.
In the 9th century, CE or earlier (books 1 and 2 of a 3-vol Mss. completed by another in the 12th, Eraclius, in De coloribus et artibus Romanorum, also gives an account of methods for producing colored glass, although he may have copied much of his text from the Naturalis Historia of Pliny the Elder of c. 77AD.[8] Now considered 'Inaccurate' date on works of Eraclius[9] Later in the medieval period  the son of Anthony of Pisa, artist and instructor Cennino Cennini, and, in 1556, Georgius Agricola[10] also contributed texts with aspects of glassmaking and glassworking.
Composition, manufacture and distribution
Prior to c. 1000, most coloured glass was of a soda-lime-silica composition. In Northern Europe soda glass was eventually almost totally superseded by potash-lime-silica glass (Forest glass). Forest glass continued to be used in stained glass for the duration of the medieval period[11] until soda glass again began to be used in the 16th century.
The potash (K2O) found in Forest Glass was derived from wood ash. In De Divers Artibus, Theophilus describes the use of beech wood as the preferred source of ash.[12] Other plant matter, such as bracken, was also used.[13] As well as containing potash, beech ash comprises an assortment of compounds including iron and manganese oxides, which are particularly important for generating colour in glass.
Medieval stained glass panels could be created either by the cylinder blown sheet or crown glass (window) method.
Forest glass was manufactured in Burgundy and Lorraine near the Rhein; in Flanders; and in Normandy, in the Seine and Loire Valleys. It was distributed throughout mainland north-west Europe and Britain in the form of ready-made sheets.[14] The application of painted decoration to and final shaping of the sheets was carried out at glass working centres close by the final destination of the glass.[15]
Colour

The colour of glass could be affected by many factors. Sources of silica were often impure, with iron oxide being one of the most common impurities. The greenish hue of uncoloured glass is usually owing to the presence of a mixture of ferrous (Fe2+) and ferric (Fe3+) ions in the glass matrix. It is also possible that ‘impurities’ could be introduced at the fritting stage within the glass furnace, leading to the addition of further alumina, silica and iron oxides.[16]
Inherent colour
Inherent colour refers to the colours that may be formed in the molten glass by manipulating the furnace environment. Theophilus describes molten glass changing to a ‘saffron yellow colour’ which will eventually transform to a reddish yellow on further heating, he also refers to a ‘tawny colour, like flesh’ which, upon further heating will become ‘a light purple’ and later ‘a reddish purple, and exquisite’.[17]
These colour changes are the result of the behaviour, under redox conditions, of the iron and manganese oxides which are naturally present in beech wood ash.
In the glass melt the iron and manganese behave as follows:
In an oxidising environment metal (and some non-metal) ions will lose electrons. In iron oxides, Fe2+ (ferrous) ions will become Fe3+ (ferric) ions. In molten glass this will result in a change in glass colour from pale blue to yellow/brown. In a reducing environment the iron will gain electrons and colour will change from yellow/brown to pale blue. Similarly manganese will change in colour depending on its oxidation state. The lower oxidation state of manganese (Mn2+) is yellow in common glass while the higher oxidation states (Mn3+ or higher) is purple. A combination of the two states will give a pink glass.
As the manganese and iron may each be in different states of oxidation, the combination of the two can result in a broad range of attractive colours. Manganese in its fully oxidised state, if not present in too great a mass, will also act as a decolourant of glass if the iron is in its yellow, ferric form. The two colours in effect cancel each other out to produce a clear glass.
Experimental manufacture of potash glass emulating Theophilus’ recipes has resulted in colours ranging from colourless to yellow, amber, brown, green, blue, pink and purple.[18][19][20] Variation in colour hue and depth would also probably be affected by the source of the beech wood ash, depending on the soil chemistry where the beech tree grew, the age of the tree and the climate conditions.[21][22]
Some of the stronger reds, blues and greens that are a feature of medieval stained glass rely on the addition of copper oxides.
Deliberately added colourants

Eraclius’ De Coloribus et Artibus includes instructions for creating green and red glass by adding copper (probably in the form of ore or copper filings) to the batch, a method practiced since ancient times.  (The chapters on how to make red, green and blue glass are missing from De Divers Artibus.)
As with the iron/manganese colours, the colours formed by the addition of copper oxide to the glass are dependent on the different oxidation states of the added copper. In an oxidising environment blue cupric (Cu2+) ions are formed, in a strongly reducing environment red colloidal cuprous (Cu1+) oxide is formed and if reoxidised then green cuprous (Cu1+) oxide results.
The production of bright reds and blues in particular was straightforward, as the addition of copper to the mix resulted in the reliable creation of red, blue and green. The dominance of red and blue in Romanesque and Gothic glass is obvious. However, at York Minster, for example, it was demonstrated that 90% of the medieval glass was coloured by means of iron/manganese content.[23]
Medieval blue soda glass
Early medieval glass was soda-based, and although the use of soda glass in Northern Europe was almost wholly superseded by forest glass after c. 1000, there are some examples of a richly coloured blue glass (identified through XRF analysis) that was produced in the medieval period using soda as the alkali.[24] In the UK, a considerable quantity of blue soda glass has been identified in stained glass from York Minster, and in excavations at Old Sarum and Winchester. In France, in Chartres Cathedral and St Denis in Paris, soda glass has also been found,[25] and no doubt there must be many other examples.
Evidence of recycling Roman tesserae to produce window glass in the 9th century has been identified at the Benedictine Monastery of San Vincenzo, Molise, Italy.[26] Theophilus, in the 12th century, was also aware of such practice. He states that mosaic tesserae: ‘little square stones’ from ‘ancient pagan buildings’ along with ‘various small vessels in the same colours’ could be used to produce glass: ‘they even melt the blue in their furnaces, adding a little of the clear white to it, and they make from it blue glass sheets which are costly and very useful in windows.’[27] According to Theophilus, the French were particularly skilled at this process.
Cox suggests that the examples analysed at York could indeed be ‘Roman, or slightly later, glass re-melted in France and imported into England’.[28]
Flashing
Glass comprising multiple layers of clear and (usually) red glass was known to exist in the 12th and 13th centuries. The manufacturing process is not known.[29] Flashing was developed in the 15th century, and refers to the superimposition of a thin layer of coloured glass onto another coloured or uncoloured glass sheet. The procedure may have involved dipping a small sphere of molten glass into a molten uncoloured glass and blowing this into a cylinder form (the cylinder blown sheet process) which was then cut into sections and flattened in an annealing oven.[30] Red, or ruby, copper-based glass, is usually flashed as the colour is too dense to be used alone. Other glass colours may also be flashed. These techniques could be remarkably sophisticated as demonstrated by 15thc. glass from the Carthusian Monastery of Pavia, where layered glasses of blue and violet; green and uncoloured; and red and uncoloured have been identified.[31]
Applied paint and silver stain

Paint
The paint applied to glass was a type of enamel, usually dark brown or black, formed from a mixture of: ground copper or iron oxide; powdered glass; wine, urine or vinegar; and gum arabic.[32] other recipes could include sugar, treacle or vegetable oil.[33] This ‘paint’ was applied in a series of washes, with fine details added last. Both the external and internal faces of the glass could be painted, adding depth to the overall composition. The enamel was fixed by ‘firing’ the glass in an annealing oven.
Silver stain
Producing a strong clear yellow could be difficult in early stained glass as it relied upon the careful control of furnace conditions in order to create the appropriate reducing or oxidising environment.[34] The introduction of silver stain in the early 14th century not only provided a solution to this difficulty, but also allowed greater flexibility in the way in which colour could be used. The first datable example of the use of silver stain is in the parish church of Le Mesnil-Villeman, Manche, France (1313).[35] Silver stain was a combination of silver nitrate or silver sulphide blended with pipe clay and applied to (usually) clear glass.[36]  This technique enabled a more flexible approach to glass painting, allowing, for example, the hair of a figure to be painted on the same piece of glass as the head. It was also used to highlight details of canopywork or grisaille, and later it was added to the surface of coloured glass, to create a wider variety of glass hues.[37]
See also
Stained glass
Stained glass conservation

Bibliography
Azzoni, C. B., Di Martino, D., Marchesi, V., Messiga. B., Riccardi, M. P., (2005). Colour Attributes of Medieval Window Panes: Electron Paramagnetic Resonance and Probe Microanalyses on Stained Glass Windows from Pavia Carthusian Monastery. Archaeometry 47/2, 381–388.
Cox, G. A., Gillies, K. J. S., 1986. The X-Ray Fluorescence Analysis of Medieval Durable Blue Soda Glass from York Minster. Archaeometry, 28/1, 57–68.
Cramp, R. J., 1975. Window Glass From the Monastic Site of Jarrow: Problems of Interpretation. Journal of Glass Studies, 17, 88–96.
Davison, S., 2003. Conservation and Restoration of Glass. Oxford: Butterworth-Heinemann.
Freestone, I. 1992. Theophilus and the Composition of Medieval Glass, Materials Issues in Art and Archaeology III, Pittsburgh, Pennsylvania: Materials Research Society, 739–745.
Hawthorne, J. G, Stanley Smith, C. (eds), 1979.Theophilus: On Divers Arts. New York: Dover.
Henderson, J. 1992. Early medieval glass technology: the calm before the storm. In S. Jennings and A. Vince (eds) Medieval Europe 1992: Volume 3 Technology and Innovation, 175–179.
Heyworth, M., 1992. Evidence for Early Medieval Glass Working. In S. Jennings and A. Vince (eds) Medieval Europe 1992: Volume 3 Technology and Innovation, 169–174.
Marks, R., 1993. Stained Glass in England During the Middle Ages. London: Routledge.
Newton, R., and Davison, S., 1997, Conservation of Glass, Butterworth–Heinemann, Oxford.
Royce-Roll, D., 1994. The Colors of Romanesque Stained Glass. Journal of Glass Studies,36, 71–80.
Smedley, J., W., Jackson, C. M., Booth, C.A., 1998. Back to the Roots: The Raw Materials, Glass Recipes and Glassmaking Practices of Theophilus. In: McCray, P. (ed). The Prehistory and History of Ancient Glassmaking, 145–165.
Stern, W. B., and Gerber, Y., 2004, Potassium-Calcium Glass: New Data and Experiments. Archaeometry 46/1, 137–156.
Further reading

Why do Catholic churches have stained glass?


Main Page
Warframe is a free-to-play cooperative third-person shooter video game developed and published by Digital Extremes. Originally released for Microsoft Windows in March 2013, it was later ported to the PlayStation 4, Xbox One, and Nintendo Switch. In Warframe, players control members of the Tenno, a race of ancient warriors who have awoken from centuries of cryosleep to find themselves at war in a planetary system with different factions. The Tenno use their powered Warframes along with powerful weapons and abilities to complete missions. While many of the game's missions use procedurally-generated levels, newer updates have included large open world areas similar to other massively multiplayer online games. The game includes elements of shooting and melee games, parkour, and role-playing to allow players to advance their Tenno character with improved gear.
The concepts for Warframe had been in place at Digital Extremes since 2000 under the name Dark Sector. The company reintroduced Dark Sector in 2004 in preparation for release on the upcoming seventh generation consoles but could not find a publisher due to its theme. Ultimately Digital Extremes released a game called Dark Sector in 2008, far different from their original plan. By 2012, looking at the success of free-to-play games, the developers took their earlier Dark Sector ideas and art assets, to present it as Warframe.
Initially, the growth of Warframe was slow, hindered by moderate critical reviews and low player counts. However, since its release, the game has experienced positive growth through development by Digital Extremes over the years. The game seen around 38 million players in 2018 and it is now one of Digital Extremes' most successful titles, with it receiving a large chunk of developer attention through the introduction of new game content and modes. The game is supported by microtransactions, which lets player purchase in-game items for real money that would otherwise normally be obtained through grinding.
Setting
Set in an extremely far future, players control the members of the Tenno, a race of ancient warriors who have awoken from a century long cryosleep to find themselves at war with the Grineer, a matriarchal race of militarized and deteriorated human clones built upon metal, blood, and war; the Corpus, a mega-corporation with advanced robotics and laser technology built upon profit; the Infested, disfigured victims of the Technocyte virus, a reference to Dark Sector and the Sentients, a race of self-replicating machines returning from a stellar system known as the Tau system after being driven back centuries ago in an ancient war. To fight back, the Tenno use remotely controlled bio-mechanical suits to channel their unique abilities – the eponymous Warframes.[1]
All of the factions encountered in the game, including the Tenno, were created by or are splinter groups of the old Orokin Empire, an ancient civilization and former reigning power in the Solar system that was led by an elite caste of trans-humans known as the Orokin. Although most of them are long dead by the time of the Tenno's awakening, their lingering presence can be still be felt throughout the Solar system.
Gameplay
Warframe is an online action game that includes elements of shooters, RPG and stealth games. The player creates their Tenno character, which includes a basic armor unit called a "Warframe" which provides the player with special abilities, basic weapons (a melee weapon, guns, and sidearms), and a ship. Through the ship's console, they can select any of the available missions to them. A main storyline set of missions requires players to complete certain missions across planets and moons in the solar system, to be able to access relays that they can progress to other planets or locations. Other missions rotate over time as part of the game's living universe; these can include missions with special rewards and community challenges to allow all players to reap benefits if they are successfully met. Aboard the ship, the player can also manage all other functions for their Tenno, including managing their arsenal of equipment, customizing their Warframe and weapons, crafting new equipment, and access the in-game store.
Missions can be played alone or with up to four players in a player versus environment cooperative manner. Each mission is given a ranking that indicates how difficult the mission is. Missions are generally played on randomly generated maps composed of "tiles" of map sections. Missions have various objectives, such as defeating a certain number of enemies, collecting data from terminals without activating alarms, rescuing prisoners, or defending points on the map for set periods of time. Newer updates have added space-bound combat using Archwings, and a large open-field environment where numerous bounties can be completed. Players can use their weapons, special abilities, and a number of parkour style moves to navigate through and overpower forces within these missions. Downed players may be revived a limited number of times on any map. Once complete, players are rewarded with in-game items, as well as in-game currency and items picked up while exploring the map; failure to complete a mission causes these rewards to be lost. In addition to cooperative missions, the game includes player versus player (PvP) content through the multiplayer "Conclave", which also rewards the player for placing high in such matches.
Players and their equipment also gain experience and level up from successful missions; equipment with higher levels can do more damage and support more "mods", special cards that can be slotted into the equipment to change its attributes or provide passive bonuses and abilities. Mods are dropped by enemies during missions and may be part of the rewards, and are generally given out following a rarity distribution, with more powerful mods being more elusive to acquire. Alongside mods, players have other means of improving their equipment, including conditional upgrades called Arcane Enhancements and Riven Mods, weapon-exclusive mods whose weapon, buffs and stats are determined upon unlocking. Another type of reward are equipment blueprints, which can be used to construct new Warframe parts or weapons; blueprints and their resulting equipment may also be purchased directly using in-game money called Platinum. Players need to have specific quantities of construction materials (found from missions and their rewards) to build these items.
Warframe is designed to be free-to-play, and has avoided using any pay to win elements; all Warframes, weapons, and other equipment is possible to acquire in-game over time through grinding, though monetization can simplify the process.[2] New weapons, Warframes, equipment, and blueprints to construct such equipment can be purchased in the market, using either Credits earned in-game, or Platinum, a premium currency that can be purchased via microtransaction or traded for with other players in-game. Platinum is also required to purchase additional enhancements, such as arsenal slots for Warframes and weapons, as well as items that enhance the mod capacity of gear.[3]  Though, there are some cosmetic items that can only be obtained through in-game payments.
Development
As Dark Sector
The origins of Warframe came out of Canadian studio Digital Extremes' original vision for their first original game, Dark Sector. Prior to that point, Digital Extremes was known as a work-for-hire studio, working alongside other studios to help complete development; this included working with Epic Games for Unreal Tournament (1999) and its sequels Unreal Tournament 2003 and Unreal Tournament 2004.[2] Epic had looked to bring Digital Extremes into their studio, but found there would be issues with the Canadian government that interfered with the merger, and the studios agreed to go their separate ways.[2]
Wanting to establish themselves as a lead studio, Digital Extremes came up with the idea of Dark Sector, which they first announced in February 2000, describing the game as combining "the intense action elements of Unreal Tournament with the scope and character evolution of a persistent online universe".[4] In early interviews, Digital Extreme said that the gameplay for Dark Sector would have had players as bounty hunters and assassins in a dark science fiction setting, with each character having a bounty on their head, making them targets for other players.[5]
The studio used their vision of Dark Sector to try to secure a publisher, but this only led to more offers for work-for-hire.[2] The company remained quiet on Dark Sector for about four years, re-announcing in early 2004 a revised Dark Sector, now to be a stylish, science-fiction single player experience with stealth elements inspired by the Metal Gear Solid series, and a story they considered a mix of Metal Gear Solid and The Dark Crystal set in space, within a larger setting like that of Frank Herbert's Dune universe.[2] Much of the game's art style was informed by the French artist Jean Giraud, aka Moebius.[6] The player-character, belonging to a race called the Tenno, and enemies would wear high-tech suits that would give them unique abilities.[5] This re-announcement included a scripted demo to show their vision of the game's gameplay and graphics.[7] The game was announced just as both the first consoles of the seventh generation, the Xbox 360 and PlayStation 3, had been teased, and Digital Extremes started to look for a publisher to release the games on these platforms.[5] The game received a good deal of attention from its video, including coverage by CNN on the upcoming console generation.[8]
Digital Extreme's creative director Steve Sinclair spent about a year on the road following the re-announcement of Dark Sector to find a publisher, but most rejected the idea; Sinclair said most publishers were not impressed with the science fiction setting, and instead encouraged them to change the setting to modern day, within World War II (which was popular at the time due to the Call of Duty series), and even the American Civil War.[5][4] When Sinclair returned to the studio, they tried to rework the setting, even trying a superhero genre, without luck. Matters were complicated as they were also attempting to develop their own engine, the Evolution engine, to support the game and the new consoles, switching away from the familiar Unreal Engine.[5] Ultimately, Digital Extremes dropped most of the science fiction elements, and moved the gameplay towards a more Resident Evil survivor-horror approach. Digital Extremes did keep one element of the original concept for the released game, that being the protagonist named "Tenno".[2] The Dark Sector released in 2008 was far different from their original vision. Dark Sector received average reviews, and was not a major financial windfall for the studio, leading them back to doing work for hire over the next four years, including BioShock, BioShock 2, Homefront and The Darkness 2.[5]
Around 2011, Digital Extremes were finding themselves struggling for work-for-hire contracts.[2] While the studio had been forced to issue some layoffs, they were still at about 250 people at this time.[6] Looking again to develop their own IP and to try to take advantage of the growth in free-to-play games, Digital Extremes looked back to the original Dark Sector concept from 2004 and looked to develop it as a free-to-play game. This decision was made in early 2012, and required the team to create a prototype within one to two months, as Sinclair and Digital Extreme's CEO James Schmalz were going to shop the game around to publishers at that year's Game Developers Conference in March 2012.[4][2] They took several assets from the abandoned 2004 concept, and developed this as Warframe. At GDC, Sinclair and Schmalz found publishers still cold on the idea: Western publishers were not keen on the science fiction setting, while a large unnamed Korean publisher warned him that they would "fail" as Western developers did not know how to properly support free-to-play games with quality content.[5] Another concern raised by these publishers was that Warframe was based on player-versus-environmental gameplay, which differed significantly with other free-to-play titles at the time that were mostly player-versus-player.[9] Disheartened, they returned to the studio and decided that they would publish Warframe on their own. They built out a playable version of the game, at the time known as Lotus in about nine months.[2] Alongside this, the studio developed the necessary server architecture to support the game and the microtransaction system they had envisioned for it.[2]
Release as Warframe
Warframe was publicly announced in June 2012[10] with its closed beta launched in October 2012.[5] Player feedback helped to refine the game's structure. An early change in the beta in early 2013 was their monetization scheme to avoid pay to win scenarios. For example, initially, each warframe had a skill tree that the player could unlock completely through missions and gaining experience. An extended version of the tree was available if the player augmented the warframe with an in-game item, then only purchasable through microtransactions.[6] When players complained about this feature, they stripped the pay to win elements and adopted the mantra of keeping the game as free to play, requiring that players did not have to spend any money to get an item within the game.[6] To support the game, they borrowed the idea of offering for sale "Founder's Packs" that would grant in-game items and currency, an idea that had been successfully used on Kickstarter projects.[6]
Digital Extremes found it difficult to get attention from the press as around 2012-2013, free to play games were typically shunned by game journalists.[6] Unfavorable comparisons had been made to Destiny, a highly-anticipated title due out in 2014, that also tarnished Warframe's presence.[9]  Coupled with low player counts, Digital Extremes were not sure how long they could continue supporting the game. However, Digital Extremes found they had a small but dedicated group of players that latched onto the title, buying into the game through Founder's Packs, telling their friends about the game, and interacting with the developers to provide feedback which was integrated into the game's design.[3] Further, they discovered that when popular streamers like TotalBiscuit covered the beta, they drew more players to the game.[6]
The open beta for Warframe launched in March 2013 for the Windows platform, with the game available from their own server systems.  Warframe was released at the same time that the studio was also completing development for the April 2013 Star Trek game to tie into the release of the film Star Trek Into Darkness. The Star Trek game was critically panned, leading to financial hardships at the studio and forcing them to layoff developers.[4] Warframe itself was not a critical hit with gaming publications, receiving average reviews; as IGN reviewed in 2013, the game was "fun, but a little bland".[3][11] Digital Extremes was planning to release Warframe for the PlayStation 4 as well, but that console was not available until November 2013, so to try to get more players, they decided to offer the game on Steam, which further grew the player base.[6] Some days after the Steam launch, Digital Extremes had been able to start drawing in enough funding to maintain the viability of the studio.[6][4]
Once the game turned profitable, Digital Extremes found themselves in the position of needing to generate content for the game to maintain its audience. Because they retained their 250-person staff throughout this process, they were able to expand upon content quickly, and soon hired in another 250 developers for Warframe.[6] Community input was critical for Digital Extremes for new content and improvements. One major change after release was an update to the game's movement system, titled "Parkour 2.0" that was released in 2015. They had found before this, players discovers ways to rapidly traverse levels by a trick known as "coptering" using specific weapons, warframes, and upgrades. Though Digital Extremes had considered these movements to be game-breaking and considered removing the abilities altogether, they realized players liked to have exotic moves like this available to them, and thus created the Parkour 2.0 system that, while reining in how extensive these moves could be, fully supported the type of ninja-like movements that players wanted.[6] Another example was a brief additional that allowed players to spend a small amount of the premium in-game currency Platinum to get a random color that they could use for customization purposes. Players reacted very negatively to this, as some found it would take a large amount of Platinum to get the color they wanted. Digital Extremes removed this random factor and instead added means to purchase such customization options directly. The company has also avoided the use of loot boxes that other free-to-play games are often criticized for.[3]
The studio had found it important to release new content regularly to keep a stream of income from the game.[6] They were also faced with the problem that to understand all of Warframe's systems required some commitment by the player, and players that felt it was too much would washout after a few hours.  This led to them investing more into the player community to keep them up to speed while helping players understand what the game's systems offered. This included starting a weekly developer video games "Devstream" on YouTube hosted by community manager Rebecca Ford (who also voices the in-game character Lotus),[2] starting a fan convention called TennoCon, and working with Twitch.tv as a partner to promote certain streamers and offer Warframe rewards within the game.[6]
In 2016, Digital Extremes was acquired by the Chinese investment company Leyou.[12] Leyou since provides necessary funding for Digital Extremes to grow, but has little influence on the direction that the developers take Warframe.[9]
"The Second Dream" expansion
On December 2015, Digital Extremes released Warframe's first cinematic story quest, "The Second Dream". This quest features prominent characters from the game and introduces a new faction, the dreaded Sentients. Also, and most importantly, The Second Dream serves as an "Awakening" to the Tenno's true nature, as more than a mere Warframe, more than human, "but once a child, like any other". Completion of this quest grants access to a new game mechanic named Focus and allows the player to enter the battlefield as themselves, temporarily, through Transcendence. During Transcendence, the Warframe is temporarily deactivated (Provided the player isn't using Excalibur Umbra), and a spectral form of the Tenno themselves enters the battlefield, channeling one of five Focus Abilities, depending on which of the five Focus Schools the player chose during the quest's events.[13]
"The War Within" expansion
On November 2016, Warframe's second cinematic quest was released, titled "The War Within". This quest sends the player on the chase for Teshin, the master and overseer of the Conclave, as he is seen suspiciously gathering Kuva from the pods of the newly awakened Tenno. Tracking Teshin across the solar system leads to the discovery of the Kuva Fortress, a massive asteroid under Grineer control where the (so far only known as a legend) Twin Grineer Queens reside. The Queens are shown to have their origins as far back as the Old Empire, and Teshin is revealed to be a Dax Soldier, meaning he was under their command due to them being of Orokin origin thus gaining the ability to wield the Kuva Scepter. The Queens cause an overload on the connection between Tenno and Warframe, forcing the Tenno to seek them out themselves, slowly discovering their Void powers. On the mission's climax, the Tenno unlocks Transference (which replaces Transcendence), an ability which allows them to roam independently of their Warframe at will, weakens the Elder Grineer Queen and has the option to kill her or "Let her rot", since all Grineer bodies decay over time due to excessive cloning. This quest also introduces an Alignment system to the game, with possible options being Sun, Neutral and Moon. This alignment has so far not had any consequence in gameplay, leaving its purpose unknown.[14]
"Plains of Eidolon" expansion
An update to the game in November 2017, titled "Plains of Eidolon", added an open-world area to the game. The Plains are a semi-open world, initially accessible through a "hub" named Cetus, a settlement on Earth where a people named the Ostrons reside, then directly through the player's ship. As the game describes them, the Ostrons are "A tight-knit band of hucksters and merchants." This expansion added Warframe's first open-world experience to the game, the ability for the player to gain reputation with the Ostrons, side-activities of fishing and mining, a Bounty system, consisting of five missions of ascending difficulty, where the player can choose to play any mission they would like regardless of whether the previous ones have been completed, a new quest named Saya's Vigil which rewarded the blueprint for the warframe Gara, more customization options for the Tenno's combat pets, Kubrows (dogs) and Kavats (cats/ocelots), and the ability for the Tenno themselves to wield their own modular weapon, called an Amplifier, or for short, Amp. And another modular blade called a "Zaw". Finally, the "Plains of Eidolon" offer a new series of boss fights to the game, the titular Eidolons. These Sentient-origin titans require extreme gear and in most cases teamwork to take down for unique rewards.[15][16]
"The Sacrifice" expansion
An update to the game in June 2018, titled "The Sacrifice", added the third cinematic story to the game. Following on the events of Warframe's previous cinematic story quests, The Second Dream and The War Within, The Sacrifice sends the Tenno on a hunt across the solar system for a rogue Warframe known as Umbra. This quest provides insight on Umbra's past, the ability to gain Umbra into arsenal after the quest's climactic point, and information on the origins of the Warframes themselves, answering multiple questions, but creating even more. The Sacrifice also features the Alignment system introduced in "The War Within".[17][18]
"Fortuna" expansion
The next expansion, "Fortuna", was released on PC on November 8, 2018. The update focuses on the titular Fortuna Solaris Debt Internment Colony, which serves as a hub for the game's second open-world map, Orb Vallis. The people of Fortuna were enslaved by a Corpus known as Nef Anyo which uses ancient Orokin devices that made gallons of coolant for the Workstation and trade center on Venus. The area expands upon concepts introduced in Plains of Eidolon, along with new activities, and the ability to obtain a hoverboard-styled vehicle known as a K-Drive.[19][20][21][22] This update also adds more modular items such as a plasma pistol called a "Kitgun", and a robotic companion called a "MOA".
Release
Digital Extremes started the Warframe closed beta for Microsoft Windows on October 24, 2012. Since then it has had several versions and hotfix releases,[23][24] and open beta was launched on March 25, 2013.[25] A PlayStation 4 version was also developed, and was released at the console's launch in November 2013,[26] while the Xbox One version of the game launched on September 2, 2014.[27] The PS4 version was ported to Japan on February 22, 2014,[28] followed by the Xbox One version on September 2, 2014.[29] A Nintendo Switch version was announced in July 2018 and was ported by Panic Button, and was released on November 20, 2018.[30][31] The various versions of Warframe do not support cross-platform play, as Digital Extremes said they currently lacked the capability to keep all platforms updated simultaneously.[32] However, with each console release, Digital Extremes provides a temporary window to allow players on Windows to copy and transfer their accounts to the console version; these become separate accounts that progress separately on Windows and on the console.[33]
Reception
Reception
Warframe received "mixed or average reviews" on all platforms according to the review aggregation website Metacritic.[48][49][50] GameZone's Mike Splechta said of the PlayStation 4 version, "If you already enjoy games like Monster Hunter which require you to farm for items in order to craft better ones, Warframe follows that very same formula, except with much more satisfying and faster paced combat."[41] However, as of 2018 PC Gamer said that "Warframe's growth doesn't resemble a well-tended plant—it's more like a mutant science experiment. Game systems are haphazardly stitched onto one other in ways that are sometimes incoherent, but oddly charming all the same."[45]
The game is among one of the most-played games available on Steam.[51] Digital Extremes attributes the success of the title to the frequent updates they are developing for the game and the game's fanbase. Digital Extremes describes the game as a "rogue success", as the game is able to secure and sustain a large number of players without gaining significant attention from other people.[52] More than 26 million players had played the game since launch by April 2016,[4] and by March 2018, five years from its open beta, had reached 38 million players.[53] In July 2016, Digital Extremes launched its first Warframe-dedicated "Tennocon" in London, Ontario, an event that drew 1000 players, where they announced news of upcoming features and updates to the game.[54] and has since run the event annually.
The game was nominated for "Best Ongoing Game" at The Game Awards 2017,[55] and won the People's Voice Award for "Action" at the 2018 Webby Awards.[56] It was also nominated for the "Still Playing Award" at the 2018 Golden Joystick Awards,[57][58] and for "Fan Favorite Shooter Game" and "Fan Favorite Fall Release" with Fortuna at the Gamers' Choice Awards.[59]

 at MobyGames