The small differences in the specific gravity values given in the above table are probably due to the presence of various impurities. Since coloured diamonds always contain a small amount of impurity, the specific gravity will vary with the colour, as is shown in the table below.
Colourless diamond 3.521
Other values sometimes given are : colourless diamoud 3.519, light yellow and green 3.521, for colourless Cape diamonds 3.520, and for yellow diamonds from the same locality 3.524.
Determinations which give results much above or much below the mean value of the specific gravity of diamond, namely, and specially those which approximate to the extreme values, 3.3 and 3.7, must be regarded either as inaccurate or as having been made on impure material.
Black carbonado has a much lower specific gravity than pure crystals of diamond, values ranging from 3.141 to 3.416 having been determined. This is due to the porous nature of the material, the numerous air spaces enclosed in its substance causing it to be lighter, bulk for bulk, than are crystals of diamond.
HARDNESS OF DIAMOND.
In respect to its hardness, diamond stands alone among all other substances, whether natural or artificial. The hardness possessed by some artificial substances, such, for example, as crystallised boron and carborundum, does, however, approach that of diamond. This gem stands at the head of Mohs' scale, and receives the number 10 as a measure of its hardness. Between this and the next hardest natural substance, namely corundum (ruby and sapphire), there is a wide gap, the difference in hardness between diamond and corundum being far greater than that which exists between corundum and talc, the softest of all minerals. The unique degree of hardness thus possessed by diamond renders it easily recognisable, since it scratches all other substances without exception, and is itself scratched by none.
It is a remarkable fact that degrees of hardness exist in diamonds among themselves, this being shown by the fact that diamonds from one locality are capable of scratching those from other localities. Thus the Australian stones are harder than those from South Africa, which are said to be the softest of all diamonds; and the beautiful black diamonds of Borneo are harder than those of other colours. It is also remarkable that many South African diamonds gradually assume their characteristic hardness only after a more or less prolonged exposure to air.
Diamond forms no exception to the general rule that the hardness of a crystal is not everywhere the same. It has been observed that the powder obtained by rubbing the surface of diamond crystals in the operation of bruting, which will be described in the section on diamond cutting, is more efficacious in the process of grinding than is the powder obtained by pounding up large fragments of diamond. It may naturally be inferred from this, that diamond crystals must be harder on the exterior than in the interior. On the surface itself, however, differences in hardness are distinctly perceptible, some faces of the crystal being more easily scratched than others, while on each face there are certain directions along which scratching may be more readily effected than in others. This being so, it follows that the process of grinding will also be more difficult in certain directions and on certain parts of the stone than in others. This subject will, however, be treated more fully in the section devoted to diamond cutting.
The great degree of hardness possessed by diamond renders it exceptionally suitable for use in personal ornament, since the sharpness of the edges and corners of the cut stone and the lustre of the polished facets are retained in spite of long continued wear.
Several important technical applications of diamond depend on the enormous hardness it possesses; these will be fully discussed in a special section, and we need only mention here the use of diamond powder in the cutting of the harder precious stones and of diamond itself. The hard Australian diamonds, however, are unattacked by the powder of other softer diamonds, they can only be worked by the help of their own powder.
In spite of their enormous hardness, the diamond crystals found in river-sands and gravels often show signs of wear and tear, their edges and corners being rounded and their surfaces dull and roughened. This has been effected by long ages of grinding against the pebbles and quartz-grains, and occasionally the precious stones of river-sands and gravels; thus even diamond itself cannot escape the action of time.
The hardness of diamond has often been confused with its frangibility or brittleness. It has been supposed, especially in ancient and medieval times, that hammer and anvil may be shattered but not the diamond which lies between. This statement was made by Pliny, the great naturalist of ancient days, who was killed in 79 A.D. at the first historic eruption of Vesuvius. He proceeded to say further that the fragmentation of a diamond may he effected by subjecting it to a preliminary immersion in the warm blood of a goat, but that even under these ceircuinstances the hammer and anvil will also be broken! According to Albertus Magnus (1205 - 1280), the blood is more efficacious if the goat has previously drunk wine or eaten parsley!
Such being the views then held respecting the unbreakable and indestructible character of the diamond, it is easy to understand why the Greek word adamas, signifying unconquerable, should have been applied to this stone, although its application to the diamond is singularly inappropriate and inatetnirate when its extreme frangibility is considered. Many a doubtful stone has been subinitted to the test of the hammer, with the belief that the blow would he resisted only if the stone were a genuine diamond. Probably litany beautiful stones have been sacrificed to this old belief. As a matter of fact, diamond is easily fractured, a vety moderate blow from a hammer sufficing for the purpose; its perfect cleavage places it among the most brittle of minerals.
OPTICAL CHARACTERS OF DIAMOND
In its pure condition diamond is most beautifully clear and transparent; the presence of enclosures of foreign matter, however, often diminishes the natural transparency of the stone, in some cases causing almost complete opacity. Dark colored diamonds, especially brown and black specimens, are frequently transparent only at their edges, and black diamonds are often completely opaque. The transparency of a crystal depends also upon the condition of its surface; if this should be roughened, as will he the case after a prolonged rolling about on the bed of a river, the stone will appear dull and cloudy, although its interior may be perfectly transparent, as is evident when the rough surfaces have been removed by cutting.
The luster on the smooth face of a diamond, besides being extraordinarily strong and brilliant, is very peculiar; in character and is intermediate between the luster of glass and that of metal. Being characteristic of diamond, it is known as adamantine luster, and is shown by very few minerals and by still fewer precious stones. It is therefore possible after a little practice to readily distinguish diamond from other transparent substances, such as glass, quartz, etc., by the character of the luster alone. As we have already seen, however, there is an artificial glass known as strass, which possesses an adamantine luster and which is therefore much used in the manufacture of imitation diamonds.
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting Diamond trade
Rafal Swiecki, geological engineer email contact
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