This article is part of a series onHealthcare reform in the
United States of America
History
Debate
Legislation
Preceding
Social Security Amendments of 1965
EMTALA (1986)
HIPAA (1996)
Medicare Modernization Act (2003)
PSQIA (2005)
Superseded
Affordable Health Care for America (H.R. 3962)
America's Affordable Health Choices (H.R. 3200)
Baucus Health Bill (S. 1796)
Proposed
American Health Care Act (2017)
United States National Health Care Act (2009, H.R. 676)
Healthy Americans Act (2007, 2009)
Health Security Act (H.R. 3600)
Latest enacted
Affordable Care Act (H.R. 3590)
Health Care and Education Reconciliation Act
 (H.R. 4872)
Reforms
Obama administration proposals
Public opinion
Reform advocacy groups
Rationing
Insurance coverage
Systems
Free market
Health insurance exchange
Nationalized insurance
Publicly-funded
Single-payer
Canadian vs. American
Two-tier
Universal
Third-party payment models
All-payer rate setting
Capitation
Fee-for-service
Global payment
 United Statesportal
 Health careportalvt
Health insurance coverage in the United States is provided by several public and private sources. During 2016, the U.S. population overall was approximately 325 million, with 53 million persons 65 years of age and over covered by the federal Medicare program. The 272 million non-institutional persons under age 65 either obtained their coverage from employer-based (155 million) or non-employer based (90 million) sources, or were uninsured (27 million).[1] Approximately 15 million military personnel received coverage through the Veteran's Administration.[2] During the year 2016, 91.2% of Americans had health insurance coverage.[3] Despite being among the top world economic powers, the US remains the sole industrialized nation in the world without universal health care coverage.[4][5]
Prohibitively high cost is the primary reason Americans give for problems accessing health care.[5] At over 27 million, higher than the entire population of Australia, the number of people without health insurance coverage in the United States is one of the primary concerns raised by advocates of health care reform. Lack of health insurance is associated with increased mortality, in the range 30-90 thousand deaths per year, depending on the study.[6][7]
Multiple surveys indicate the number of uninsured fell between 2013-2016 due to expanded Medicaid eligibility and health insurance exchanges established due to the Patient Protection and Affordable Care Act, also known as the "ACA" or "Obamacare".  According to the United States Census Bureau, in 2012 there were 45.6 million people in the US (14.8% of the under-65 population) who were without health insurance. Following the implementation of major ACA provisions in 2013, this figure fell by 18.3 million or 40%, to 27.3 million by 2016 or 8.6% of the under-65 population.[8][1]
The Census Bureau reported that the number of uninsured persons rose from 27.3 million in 2016 to 28.0 million in 2017. The uninsured rate rose from 8.6% in 2016 to 8.7% in 2017.[8] This was the first increase in the number and rate of uninsured since 2010. Further, the Commonwealth Fund estimated in May 2018 that the number of uninsured increased by 4 million from early 2016 to early 2018. The rate of those uninsured increased from 12.7% in 2016 to 15.5% under their methodology. The impact was greater among lower-income adults, who had a higher uninsured rate than higher-income adults. Regionally, the South and West had higher uninsured rates than the North and East. Further, those 18 states that have not expanded Medicaid had a higher uninsured rate than those that did.[9]
The causes of this rate of uninsurance remain a matter of political debate.  Nearly half those without insurance cite its cost as the primary factor. Rising insurance costs have contributed to a trend in which fewer employers are offering health insurance, and many employers are managing costs by requiring higher employee contributions. Many of the uninsured are the working poor or are unemployed.[10]
Overview
Health insurance coverage is provided by several public and private sources in the United States. Analyzing these statistics is more challenging due to persons with multiple sources of insurance, such as those with coverage under both an employer plan and Medicaid. During 2016, the U.S. population overall was approximately 325 million, with 53 million persons 65 years of age and over covered by the federal Medicare program. For the remaining 272 million non-institutional persons under age 65:
There were 155 million with employer-based coverage, 90 million with other coverage, and 27 million uninsured.
Of the 90 million with other coverage, 57 million were covered by Medicaid and Children's Health Insurance Program (CHIP), 12 million were covered by the ACA/Obamacare exchanges, 11 million were covered by the ACA Medicaid expansion, and 10 million had other coverage, such as private insurance purchased outside the ACA exchanges.
Of the 12 million on the ACA exchanges, 10 million received subsidies and 2 million did not.[1]
Of the 27 million uninsured, 21 million (79%) were U.S. citizens while 6 million (21%) were non-citizens, including both documented and undocumented immigrants. In 2015, 45% of the uninsured were white, 32% were Hispanic, and 15% were black.
Approximately 15 million institutional (military) personnel were covered by the Veteran's Administration in 2015.[11]
The uninsured rate fell from a peak of 18.2% in 2010 to 10.5% by 2015, due primarily to ACA/Obamacare along with improvements in the economy.
States that expanded Medicaid under Obamacare had lower uninsured rates than states that did not.
Inability to afford insurance was the primary reason cited by persons without coverage (46%).[10]
Lack of health insurance is associated with increased mortality, in the range 30-90 thousand deaths per year, depending on the study. This figure is calculated based on 1 additional death per 300-800 persons without health insurance, on a base of 27 million uninsured persons.[6]
Estimates of the number of uninsured

The Congressional Budget Office (CBO) estimated the number of uninsured at 27 million in 2016, about 10% of the under-65 population of 272 million.[1] Gallup estimated in July 2014 that the uninsured rate for adults (persons 18 years of age and over) was 13.4% as of Q2 2014, down from 18.0% in Q3 2013 when the health insurance exchanges created under the Patient Protection and Affordable Care Act (PPACA or "Obamacare") first opened. The uninsured rate fell across nearly all demographic groups.[13] The Commonwealth Fund reported that the uninsured rate among adults 19-64 declined from 20% in Q3 2013 to 15% in Q2 2014, meaning approximately 9.5 million more adults had health insurance.[14]
The United States Census Bureau annually reports statistics on the uninsured. The 2016 Census Bureau Health Insurance highlights summary report states that:
"The uninsured rate decreased between 2015 and 2016 by 0.3 percentage points as measured by the CPS ASEC. In 2016, the percentage of people without health insurance coverage for the entire calendar year was 8.8 percent, or 28.1 million, lower than the rate and number of uninsured in 2015 (9.1 percent or 29.0 million).
The percentage of people with health insurance coverage for all or part of 2016 was 91.2 percent, higher than the rate in 2015 (90.9 percent).
In 2016, private health insurance coverage continued to be more prevalent than government coverage, at 67.5 percent and 37.3 percent, respectively. Of the subtypes of health insurance coverage, employer-based insurance covered 55.7 percent of the population for some or all of the calendar year, followed by Medicaid (19.4 percent), Medicare (16.7 percent), direct-purchase (16.2 percent), and military coverage (4.6 percent).
Between 2015 and 2016, the rate of Medicare coverage increased by 0.4 percentage points to cover 16.7 percent of people for part or all of 2016 (up from 16.3 percent in 2015). There was no statistically significant difference between 2015 and 2016 for any other subtype of health insurance.
Between 2015 and 2016, the percentage of people without health insurance coverage dropped for most ages under 65, with generally larger decreases for working-age adults (aged 19 to 64).
The percentage of uninsured children under age 19, 5.4 percent, did not significantly change between 2015 and 2016.
In 2016, the uninsured rate for children under age 19 in poverty, 7.0 percent, was higher than the uninsured rate for children not in poverty, 5.0 percent.
In 2016, non-Hispanic Whites had the lowest uninsured rate among race and Hispanic origin groups, at 6.3 percent. The uninsured rates for Blacks and Asians were higher than for non-Hispanic Whites, at 10.5 percent and 7.6 percent, respectively. Hispanics had the highest uninsured rate, at 16.0 percent.
Between 2015 and 2016, the percentage of people without health insurance at any time during the year fell 0.4 percentage points for non-Hispanic Whites, down to 6.3 percent. There was no statistical change in the uninsured rate for Blacks, Asians, or Hispanics during this period.
Between 2015 and 2016, the percentage of people without health insurance coverage at the time of interview decreased in 39 states. Eleven states and the District of Columbia did not have a statistically significant change in their uninsured rate."[15]
Underinsured
Those who are insured may be underinsured such that they cannot afford adequate medical care. A 2003 estimated that 16million U.S. adults were underinsured in 2003, disproportionately affecting those with lower incomes – 73% of the underinsured in the study population had annual incomes below 200% of the federal poverty level.[16]
Coverage gaps also occur among the insured population. Johns Hopkins University professor Vicente Navarro stated in 2003, "the problem does not end here, with the uninsured. An even larger problem is the underinsured" and "The most credible estimate of the number of people in the United States who have died because of lack of medical care was provided by a study carried out by Harvard Medical School Professors Himmelstein and Woolhandler.[17] They concluded that almost 100,000 people died in the U.S. yearly because of lack of needed care."[18] Another study focusing on the effect of being uninsured found that individuals with private insurance were less likely to be diagnosed with late-stage cancer than either the uninsured or Medicaid beneficiaries.[19] A study examining the effects of health insurance cost-sharing more generally found that chronically ill patients with higher co-payments sought less care for both minor and serious symptoms while no effect on self-reported health status was observed. The authors concluded that the effect of cost sharing should be carefully monitored.[20]
Coverage gaps and affordability also surfaced in a 2007 international comparison by the Commonwealth Fund. Among adults surveyed in the U.S., 37% reported that they had foregone needed medical care in the previous year because of cost; either skipping medications, avoiding seeing a doctor when sick, or avoiding other recommended care. The rate was higher – 42% –, among those with chronic conditions. The study reported that these rates were well above those found in the other six countries surveyed: Australia, Canada, Germany, the Netherlands, New Zealand, and the UK.[21] The study also found that 19% of U.S. adults surveyed reported serious problems paying medical bills, more than double the rate in the next highest country.
Coverage trends under President Trump
Gains in healthcare coverage under President Obama began to reverse under President Trump. The Commonwealth Fund estimated in May 2018 that the number of uninsured increased by 4 million from early 2016 to early 2018. The rate of those uninsured increased from 12.7% in 2016 to 15.5%. This was due to two factors: 1) Not addressing specific weaknesses in the ACA; and 2) Actions by the Trump administration that exacerbated those weaknesses. The impact was greater among lower-income adults, who had a higher uninsured rate than higher-income adults. Regionally, the South and West had higher uninsured rates than the North and East. Further, those 18 states that have not expanded Medicaid had a higher uninsured rate than those that did.[9]
Uninsured demographic

The Kaiser Family Foundation reported in October 2016 that there were 27.2 million uninsured under age 65, roughly 10% of the 272 million persons in that group. Kaiser reported that:
2.6 million were in the "coverage gap" due to the 19 states that chose not to expand the Medicaid program under the ACA/Obamacare, meaning their income was above the Medicaid eligibility limit but below the threshold for subsidies on the ACA exchanges (~44% to 100% of the federal poverty level or FPL);
5.4 million were undocumented immigrants;
4.5 million had an employer's insurance offer (making them ineligible for ACA/Obamacare coverage) but declined it;
3.0 million were ineligible for financial assistance under ACA/Obamacare due to sufficiently high income;
6.4 million were eligible for Medicaid or other public healthcare program but did not pursue it; and
5.3 million were eligible for ACA/Obamacare tax credits but did not enroll in the program.
An estimated 46% cited costs as a barrier to getting insurance coverage.
Nearly 12 million (43%) of persons were eligible for financial assistance (Medicaid or ACA subsidies) but did not enroll to obtain it.[23]
The Census Bureau reports that in 2007 nearly 37 million of the uninsured were employment-age adults (ages 18 to 64) and more than 27 million worked at least part-time. Approximately 61% of the roughly 45 million uninsured live in households with incomes under $50,000 (13.5 million below $25,000 and 14.5 million at $25,000 to $49,000).[24] And 38% live in households with incomes of $50,000 or more (8.5 million at $50,000 to $74,999 and 9.1 million at $75,000 or more). As stated by the Census Bureau, people of Hispanic origin were the most affected by being uninsured; nearly a third of Hispanics lack health insurance. In 2004, about 33% of Latinos were uninsured as opposed to 10% of white, non-Latinos[25]
However, this rate decreased slightly from 2006 to 2007, from 15.3 to 14.8 million, a decrease of 2 percentage points (34.1% to 32.1%).
The state with the highest percentage of uninsured was Texas (24.1% average for three years, 2004–2006). New Mexico has the second highest percentage of residents without health insurance at 22%.[26]
It has been estimated that nearly one-fifth of the uninsured population is able to afford insurance, almost one quarter is eligible for public coverage, and the remaining 56% need financial assistance (8.9% of all Americans).[27] An estimated 5 million of those without health insurance are considered "uninsurable" because of pre-existing conditions.[28] A recent study concluded that 15% of people shopping online for health insurance are considered "uninsurable" because of a pre-existing condition, or for being overweight.  This label does not necessarily mean they can never get health insurance, but that they will not qualify for standard individual coverage. People with similar health status can be covered via employer-provided health insurance, Medicare, or Medicaid.[29]
Uninsured children and young adults
The current estimate for uninsured children does not greatly differ from past estimates. In 2009 the Census Bureau states that 10.0 percent or 7.5 million children under the age of 18 were medically uninsured. Children living in poverty are 15.1 percent more likely than other children to be uninsured. The lower the income of a household the more likely it is they are uninsured. In 2009, a household with an annual income of 25,000 or less was only 26.6 percent likely not to have medical insurance and those with an annual income of 75,000 or more were only 9.1 percent unlikely to be insured.[30]  According to the Census Bureau, in 2007, there were 8.1 million uninsured children in the US. Nearly 8 million young adults (those aged 18–24), were uninsured, representing 28.1% of their population. Young adults make up the largest age segment of the uninsured, are the most likely to be uninsured, and are one of the fastest growing segments of the uninsured population. They often lose coverage under their parents' health insurance policies or public programs when they reach age 19.  Others lose coverage when they graduate from college. Many young adults do not have the kind of stable employment that would provide ongoing access to health insurance.[31][32]
According to the Congressional Budget Office the plan the way it is now would have to cover unmarried dependents under their parents' insurance up to age 26.  These changes also affect large employers, including self-insured firms, so that the firm bears the financial responsibility of providing coverage.  The only exception to this is policies that were maintained continuously before the enactment of this legislation.  Those policies would be grandfathered in.[33]
Non-citizens

Non-citizens are more likely to be uninsured than citizens, with a 43.8% uninsured rate.  This is attributable to a higher likelihood of working in a low-wage job that does not offer health benefits, and restrictions on eligibility for public programs. The longer a non-citizen immigrant has been in the country, the less likely they are to be uninsured.  In 2006, roughly 27% of immigrants entering the country before 1970 were uninsured, compared to 45% of immigrants entering the country in the 1980s and 49% of those entering between 2000 and 2006.
Most uninsured non-citizens are recent immigrants; almost half entered the country between 2000 and 2006, and 36% entered during the 1990s.  Foreign-born non-citizens accounted for over 40% of the increase in the uninsured between 1990 and 1998, and over 90% of the increase between 1998 and 2003.  One reason for the acceleration after 1998 may be restrictions imposed by the Personal Responsibility and Work Opportunity Reconciliation Act (PRWORA) of 1996.  Almost seven out of ten (68%) of uninsured non-citizens live in California, Texas, Florida, or New York.[34]
Downturn effects

A report by the Kaiser Family Foundation in April 2008 found that US economic downturns place a significant strain on state Medicaid and SCHIP programs.  The authors estimated that a 1% increase in the unemployment rate increase Medicaid and SCHIP enrollment by 1 million, and increase the number uninsured by 1.1 million.  State spending on Medicaid and SCHIP would increase by $1.4 billion (total spending on these programs would increase by $3.4 billion).  This increased spending would occur while state government revenues were declining.  During the last downturn, the Jobs and Growth Tax Relief Reconciliation Act of 2003 (JGTRRA) included federal assistance to states, which helped states avoid tightening their Medicaid and SCHIP eligibility rules.  The authors conclude that Congress should consider similar relief for the current economic downturn.[35]
History
Prior to the Patient Protection and Affordable Care Act, medical underwriting was common, but after the law came into effect in 2014 it became effectively prohibited.
Medical underwriting made difficult for many consumers to purchase coverage on the individual market. Medical underwriting meant that insurance companies screen applicants for pre-existing conditions and reject those with serious conditions such as arthritis, cancer, and heart disease, but also such common ailments as acne, being 20pounds over or under weight, and old sports injuries.[36] In 2008, an estimated 5million of those without health insurance were considered "uninsurable" because of pre-existing conditions.[37]
Proponents of medical underwriting argue that it ensures that individual health insurance premiums are kept as low as possible.[38] Critics of medical underwriting believe that it unfairly prevents people with relatively minor and treatable pre-existing conditions from obtaining health insurance.[39]
One large industry survey found that 13% of applicants for individual health insurance who went through medical underwriting were denied coverage in 2004. Declination rates increased significantly with age, rising from 5% for those under 18 to just under one-third for those aged 60 to 64.[40] Among those who were offered coverage, the study found that 76% received offers at standard premium rates, and 22% were offered higher rates.[41] The frequency of increased premiums also increased with age, so for applicants over 40, roughly half were affected by medical underwriting, either in the form of denial or increased premiums. In contrast, almost 90% of applicants in their 20s were offered coverage, and three-quarters of those were offered standard rates. Seventy percent of applicants age 60–64 were offered coverage, but almost half the time (40%) it was at an increased premium. The study did not address how many applicants who were offered coverage at increased rates chose to decline the policy. A study conducted by the Commonwealth Fund in 2001 found that, among those aged 19 to 64 who sought individual health insurance during the previous three years, the majority found it unaffordable, and less than a third ended up purchasing insurance. This study did not distinguish between consumers who were quoted increased rates due to medical underwriting and those who qualified for standard or preferred premiums.[42] Some states have outlawed medical underwriting as a prerequisite for individually purchased health coverage.[43] These states tend to have the highest premiums for individual health insurance.[44]
Causes

Americans who are uninsured may be so because their job does not offer insurance; they are unemployed and cannot pay for insurance; or they may be financially able to buy insurance but consider the cost prohibitive.[45]  During 2009 the continued low employment rate has negatively affected those who had previously been enrolled in employment-based insurance policies. Census Bureau states a 55 percent drop. Other uninsured Americans have chosen to join a health care sharing ministry as an alternative to insurance.[46]
Low-income workers are less likely than higher income individuals to be offered coverage by their employer (or by their spouse's employer) and less able to afford buying it on their own. Beginning with wage and price controls during World War II, and cemented by an income tax exemption ruling in 1954, most working Americans have received their health insurance from their employers.[47] However, recent trends have shown an ongoing decline in employer-sponsored health insurance benefits. In 2000, 68% of small companies with 3 to 199 workers offered health benefits. Since that time, that number has continued to drop to 2007, when 59% offered health benefits. For large firms with 200 or more workers, in 2000, 99% of employers offered health benefits; in 2007, that number stayed the same. On average, considering firms of all numbers of employees, in 2000, 69% offered health insurance, and that number has fallen nearly every year since, to 2007, when 60% of employers offered health insurance.[48]
One study published in 2008 found that people of average health are least likely to become uninsured if they have large group health coverage, more likely to become uninsured if they have small group coverage, and most likely to become uninsured if they have individual health insurance. But, "for people in poor or fair health, the chances of losing coverage are much greater for people who had small-group insurance than for those who had individual insurance." The authors attribute these results to the combination in the individual market of high costs and guaranteed renewability of coverage. Individual coverage costs more if it is purchased after a person becomes unhealthy but "provides better protection (compared to group insurance) against high premiums for already individually insured people who become high risk."  Healthy individuals are more likely to drop individual coverage than less-expensive, subsidized employment-based coverage, but group coverage leaves them "more vulnerable to dropping or losing any and all coverage than does individual insurance" if they become seriously ill.[49]
Roughly a quarter of the uninsured are eligible for public coverage but are not enrolled.[50][51] Possible reasons include a lack of awareness of the programs or of how to enroll, reluctance due to a perceived stigma associated with public coverage, poor retention of enrollees, and burdensome administrative procedures. In addition, some state programs have enrollment caps.[51]
A study by the Kaiser Family Foundation published in June 2009 found that 45% of low-income adults under age 65 lack health insurance.[52] Almost a third of non-elderly adults are low income, with family incomes below 200% of the federal poverty level.[52] Low-income adults are generally younger, less well educated, and less likely to live in a household with a full-time worker than are higher income adults; these factors contribute to the likelihood of being uninsured.[52] In addition, the chances of being healthy decline with lower income; 19% of adults with incomes below the federal poverty level describe their health as fair or poor.[52]
Consequences

Insurance coverage helps save lives, by encouraging early detection and prevention of dangerous medical conditions. According to a 2014 study, the ACA likely prevented an estimated 50,000 preventable patient deaths from 2010 to 2013.[54] City University public health professors David Himmelstein and Steffie Woolhandler wrote in January 2017 that a rollback of the ACA's Medicaid expansion alone would cause an estimated 43,956 deaths annually.[55]
The Federal Reserve publishes data on premature death rates by county, defined as those dying below age 74.[53] According to the Kaiser Foundation, expanding Medicaid in the remaining 19 states would cover up to 4.5 million persons,[56] thus reducing mortality.[57] Texas, Oklahoma, Mississippi, Alabama, Georgia, Tennessee, Missouri and South Carolina, indicated on the Federal Reserve map (see graph at right) as having many counties with high premature mortality rates,[53] did not expand Medicaid.[56]
A study published in the American Journal of Public Health in 2009 found that lack of health insurance is associated with about 45,000 excess preventable deaths per year.[58] One of the authors characterized the results as "now one dies every 12 minutes."[59] Since then, as the number of uninsured has risen from about 46 million in 2009 to 48.6 million in 2012, the number of preventable deaths due to lack of insurance has grown to about 48,000 per year.[60]
A survey released in 2008 found that being uninsured impacts American consumers' health in the following ways:[61]
More of the uninsured chose not to see a doctor when were sick or hurt (53%) vs 46% of the insured.
Fewer of the uninsured (28%) report currently undergoing treatment or participating in a program to help them manage a chronic condition; 37% of the insured are receiving such treatment.
21% of the uninsured, vs. 16% of the insured, believe their overall health is below average for people in their age group.
Cost shifting
The costs of treating the uninsured must often be absorbed by providers as charity care, passed on to the insured via cost-shifting and higher health insurance premiums, or paid by taxpayers through higher taxes.[62]
On the other hand, the uninsured often subsidize the insured because the uninsured use fewer services[63] and are often billed at a higher rate.[64]  A study found that in 2009, uninsured patients presenting in U.S. emergency departments were less likely to be admitted for inpatient care than those with Medicare, Medicaid, or private insurance.[65] 60 Minutes reported, "Hospitals charge uninsured patients two, three, four or more times what an insurance company would pay for the same treatment."[66]  On average, per capita health care spending on behalf of the uninsured is a bit more than half that for the insured.[67]
Hospitals and other providers are reimbursed for the cost of providing uncompensated care via a federal matching fund program. Each state enacts legislation governing the reimbursement of funds to providers. In Missouri, for example, providers assessments totaling $800million are matched – $2 for each assessed $1 – to create a pool of approximately $2billion. By federal law these funds are transferred to the Missouri Hospital Association for disbursement to hospitals for the costs incurred providing uncompensated care including Disproportionate Share Payments (to hospitals with high quantities of uninsured patients), Medicaid shortfalls, Medicaid managed care payments to insurance companies and other costs incurred by hospitals.[68] In New Hampshire, by statute, reimbursable uncompensated care costs shall include: charity care costs, any portion of Medicaid patient care costs that are unreimbursed by Medicaid payments, and any portion of bad debt costs that the commissioner determines would meet the criteria under 42U.S.C. section1396r-4(g) governing hospital-specific limits on disproportionate share hospital payments under TitleXIX of the Social Security Act.[69]
A study published in August 2008 in Health Affairs found that covering all of the uninsured in the US would increase national spending on health care by $122.6 billion, which would represent a 5% increase in health care spending and 0.8% of GDP. "From society’s perspective, covering the uninsured is still a good investment. Failure to act in the near term will only make it more expensive to cover the uninsured in the future, while adding to the amount of lost productivity from not insuring all Americans," said Professor Jack Hadley, the study's lead author. The impact on government spending could be higher, depending on the details of the plan used to increase coverage and the extent to which new public coverage crowded out existing private coverage.[70]
Over 60% of personal bankruptcies is caused by medical bills. Most of these persons had medical insurance.[71]
Effects on health of the uninsured
From 2000 to 2004, the Institute of Medicine's Committee on the Consequences of Uninsurance issued a series of six reports that reviewed and reported on the evidence on the effects of the lack of health insurance coverage.[72]
The reports concluded that the committee recommended that the nation should implement a strategy to achieve universal health insurance coverage. As of 2011, a comprehensive national plan to address what universal health plan supporters terms "America's uninsured crisis", has yet to be enacted. A few states have achieved progress towards the goal of universal health insurance coverage, such as Maine, Massachusetts, and Vermont, but other states including California, have failed attempts of reforms.[73]
The six reports created by the Institute of Medicine (IOM) found that the principal consequences of uninsurance were the following: Children and Adults without health insurance did not receive needed medical care; they typically live in poorer health and die earlier than children or adults who have insurance. The financial stability of a whole family can be put at risk if only one person is uninsured and needs treatment for unexpected health care costs. The overall health status of a community can be adversely affected by a higher percentage of uninsured people within the community. The coverage gap between the insured and the uninsured has not decreased even after the recent federal initiatives to extend health insurance coverage.[73]
The last report was published in 2004 and was named Insuring America's Health: Principles and Recommendations. This report recommended the following: The President and Congress need to develop a strategy to achieve universal insurance coverage and establish a firm schedule to reach this goal by the year 2010. The committee also recommended that the federal and state governments provide sufficient resources for Medicaid and the State Children's Health Insurance Program (SCHIP) to cover all persons currently eligible until the universal coverage takes effect. They also warned that the federal and state governments should prevent the erosion of outreach efforts, eligibility, enrollment, and coverage of these specific programs.[73]
Some people think that not having health insurance will have adverse consequences for the health of the uninsured.[74] On the other hand, some people believe that children and adults without health insurance have access to needed health care services at hospital emergency rooms, community health centers, or other safety net facilities offering charity care.[75] Some observers note that there is a solid body of evidence showing that a substantial proportion of U.S. health care expenditures is directed toward care that is not effective and may sometimes even be harmful.[76] At least for the insured population, spending more and using more health care services does not always yield better health outcomes or increase life expectancy.[77]
Children in America are typically perceived as in good health relative to adults, due to the fact that most serious health problems occur later in one's life. Certain conditions including asthma, diabetes, and obesity have become much more prevalent among children in the past few decades.[73] There is also a growing population of vulnerable children with special health care needs that require ongoing medical attention, which would not be accessible without health insurance. More than 10 million children in the United States meet the federal definition of children with special health care needs "who have or are at increased risk for a chronic physical, development, behavioral, or emotional condition and who also require health and related services of a type or amount beyond that required by children generally".[78] These children require health related services of an amount beyond that required by the average children in America. Typically when children acquire health insurance, they are much less likely to experience previously unmet health care needs, this includes the average child in America and children with special health care needs.[73] The Committee on Health Insurance Status and Its Consequences concluded that the effects of health insurance on children's health outcomes: Children with health insurance receive more timely diagnosis of serious health conditions, experience fewer hospitalizations, and miss fewer days of school.
The same committee analyzed the effects of health insurance on adult's health outcomes: adults who do not have health insurance coverage who acquire Medicare coverage at age 65, experience substantially improved health and functional status, particularly those who have cardiovascular disease or diabetes. Adults who have cardiovascular disease or other cardiac risk factors that are uninsured are less likely to be aware of their condition, which leads to worse health outcomes for those individuals. Without health insurance, adults are more likely to be diagnosed with certain cancers that would have been detectable earlier by screening by a clinician if they had regularly visited a doctor. As a consequence, these adults are more likely to die from their diagnosed cancer or suffer poorer health outcomes.[73]
Many towns and cities in the United States have high concentrations of people under the age of 65 who lack health insurance.[79] There are implications of high rates of uninsurance for communities and for insured people in those communities. Institute of Medicine committee warned of the potential problems of high rates of uninsurance for local health care, including reduced access to clinic-based primary care, specialty services, and hospital-based emergency services.[80]
Emergency Medical Treatment and Active Labor Act (EMTALA)

EMTALA, enacted by the federal government in 1986, requires that hospital emergency departments treat emergency conditions of all patients regardless of their ability to pay and is considered a critical element in the "safety net" for the uninsured. However, the federal law established no direct payment mechanism for such care. Indirect payments and reimbursements through federal and state government programs have never fully compensated public and private hospitals for the full cost of care mandated by EMTALA. In fact, more than half of all emergency care in the U.S. now goes uncompensated.[81] According to some analyses, EMTALA is an unfunded mandate that has contributed to financial pressures on hospitals in the last 20 years, causing them to consolidate and close facilities, and contributing to emergency room overcrowding. According to the Institute of Medicine, between 1993 and 2003, emergency room visits in the U.S. grew by 26%, while in the same period, the number of emergency departments declined by 425.[82] Hospitals bill uninsured patients directly under the fee-for-service model, often charging much more than insurers would pay,[83] and patients may become bankrupt when hospitals file lawsuits to collect.
Mentally ill patients present a unique challenge for emergency departments and hospitals. In accordance with EMTALA, mentally ill patients who enter emergency rooms are evaluated for emergency medical conditions. Once mentally ill patients are medically stable, regional mental health agencies are contacted to evaluate them. Patients are evaluated as to whether they are a danger to themselves or others. Those meeting this criterion are admitted to a mental health facility to be further evaluated by a psychiatrist. Typically, mentally ill patients can be held for up to 72 hours, after which a court order is required.
Uninsured rates by state

The United States Census Bureau regularly conducts the Current Population Survey (CPS), which includes estimates on health insurance coverage in the United States. The data is published annually in the Annual Social and Economic Supplement (ASEC). The data from 1999 to 2014 are reproduced below.[n 1] As of 2012, the five states with the highest estimated percentage of uninsured are, in order, Texas, Nevada, New Mexico, Florida, and Georgia. The five states/territories with the lowest estimated percentage of uninsured for the same year are, in order, Massachusetts, Vermont, Hawaii, Washington, D.C., and Connecticut. These rankings for each year are highlighted below.[7][84]

Assisting programs for uninsured

People without health insurance in the United States may receive benefits from patient-assistance programs such as Partnership for Prescription Assistance.[85] Uninsured patients can also use a medical bill negotiation service, which can audit the medical bill for overcharges and errors.
See also

Health care reform in the United States
Health insurance
Health insurance in the United States
List of healthcare reform advocacy groups in the United States
Single-payer health care
Universal health care
Patient Protection and Affordable Care Act
Notes

 Institute of Medicine, National Academy of Sciences, 2009.
 contains links to numerous studies and literature about health care-related issues such as the uninsured.
 is a web-based tool that allows users to create customized tables and graphs showing health insurance coverage estimates using U.S. Census surveys between 1987-2009.
Paul Fronstin,  EBRI Issue Brief #376, September 2012, Employee Benefit Research Institute.

What percentage of U.S. children are diagnosed with obesity?


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.