Diamond

Diamond

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A diamond is a precious stone (mineral) that is considered a clear and generally colorless crystalline form of pure carbon, with the same carbon composition as graphite, but with a different structure. It is the hardest of naturally occurring substances. Diamonds are usually found in igneous rock formations and alluvial deposits. Most diamonds are billions of years old. They are often used in industrial and scientific applications. The density of diamond is about 2.0 ounces per cubic inch (3.5 grams per cubic centimeter).

Diamonds, of course, are used as jewelry, especially in engagement and wedding rings. Natural and synthetic diamonds are also commonly used to cut a wide variety of materials. Manufacturers, medical surgeons, and many other groups and organizations use diamond knives, drills, saws, and other diamond-cutting devices to cut and shape their products and materials. Makers of marble, granite, and other natural stone products use diamond blades to cut, grind, and polish their unfinished products.

The word diamond comes from the Greek word adamas, meaning invincible. Diamonds were first found (it is believed) in the sands of India. Alexander the Great (356–323 BC) introduced them to Europe in 327 BC.

French chemist Antoine Laurent Lavoisier (1743– 1794) showed in the eighteenth century that, when air is present, diamonds are combustible, producing carbon dioxide. English chemist Sir Humphrey Davy (1778–1829) demonstrated in 1814 that the sole product of the combustion of diamonds in oxygen is carbon dioxide. He also proved that diamond and charcoal both consist of carbon atoms, so they are chemically identical. This was the first demonstration that two materials with the same chemical composition need not have the same physical properties.

Most people probably believe that diamonds only come from South Africa. However, diamonds are found around the world, except for the continents of Europe and Antarctica. Before the twentieth century, only a few diamond deposits were known. Between the fourth century BC and the sixth century AD, India was the only source of diamonds. Diamonds virtually disappeared from Europe for about one thousand years during the Middle Ages. In 1725, diamonds were discovered in Brazil. Then, in the 1870s, they were also found in South Africa. By this time, the supply of diamonds increased dramatically as the worldwide demand for diamonds also increased.

Today, diamonds are mined in about 25 countries. According to the American Museum of Natural History, as of 2005, about 130 million carats of diamonds are mined annually throughout the world.

Diamonds are a globally traded commodity used for a variety of industrial and artistic purposes. In December 2000, the United Nations General Assembly unanimously adopted a resolution articulating the role of diamonds in fuelling international conflict and dedicated to breaking the link between the illicit transaction of rough diamonds and armed conflict. Two years later, in 2002, the UN approved the Kimberley Process, which further attempts to prevent conflicts in the diamond market. In 2003, U.S. President George W. Bush enacted an executive order imposed trade sanctions against Sierra Leone due to human rights violations with respect to its diamond trade with the United States. These conflict diamonds are facing increasing import-export trade restrictions. Overall, however, the U.S. Government Accountability Office stated in September 2006 that despite sanctions and enforcement conflict diamonds continue to illegally come into the United States in large numbers.

The atoms making up a mineral may be arranged either randomly, or in an orderly pattern, if —as with diamonds—a mineral’s atoms show long-range organization, the mineral is termed a crystalline mineral. The objects commonly called crystals are crystalline minerals of relatively large size that happen to have developed smooth faces. Diamonds are the hardest mineral (10 on the Mohs’ scale), with the highest refractive index of 2.417 among all transparent minerals, and has a high dispersion of 0.044. Diamonds are brittle. Under UV light, the diamond frequently exhibits luminescence with different colors. It has a density of 3.52 g/cm³. The mass of diamonds is measured in carats; 1 carat = 0.2 grams. Diamonds rarely exceed 15 carats. Diamonds are insoluble in acids and alkalis, and may burn in oxygen at high temperatures.

Nitrogen is the main impurity found in diamonds, and influences its physical properties. Diamonds are divided into two types, with type I containing 0.001–0.23% nitrogen, and type II containing no nitrogen. If nitrogen exists as clusters in type I diamonds, it does not affect the color of the stone (type Ia), but if nitrogen substitutes carbon in the crystal lattice, it causes a yellow color (Ib). Stones of type II may not contain impurities (IIa), or may contain boron substituting carbon, producing a blue color and semiconductivity of the diamond.

Diamonds form only at extremely high pressure (over 45000 atmospheres) and temperatures over 2012°F (1100°C) from liquid ultrabasic magmas or peridotites. Diamonds, therefore, form at great depths in the Earth’s crust. They are delivered to the surface by explosive volcanic phenomena with rapid cooling rates, which preserve the diamonds from transformation. This process happens in kimberlites (a peridotitic type of breccia), which constitutes the infill of diamond-bearing pipes. Also found with diamonds are olivine, serpentine, carbonates, pyroxenes, pyrope garnet, magnetite, hematite, graphite and ilmenite. Near the surface, kimberlite weathers, producing yellow loose mass called yellow ground, while deeper in Earth, it changes to more dense blue ground. Diamonds are extremely resistive to corrosion, so they can be found in a variety of secondary deposits where they arrived after several cycles of erosion and sedimentation (alluvial diamond deposits, for example). Even in diamond-bearing rock, the diamond concentration is one g in 8 to 30 tons of rock.

Most diamonds are used for technical purposes due to their hardness. Gem quality diamonds are found in over 20 counties, mainly in Africa. The biggest diamond producer is South Africa, followed by Russia. Usually, diamonds appear as isolated octahedron crystals. Sometimes they may have rounded corners and slightly curved faces. Microcrystalline diamonds with irregular or globular appearance are called Bort (or boart), while carbonado are roughly octahedral, cubic or rhombic dodecahedral, blackish, irregular microcrystalline aggregates. Both are valued for industrial applications because they are not as brittle as diamond crystals. Frequently, diamonds have inclusions of olivine, sulfides, chrome-diopside, chrome-spinels, zircon, rutile, disthene, biotite, pyrope garnet and ilmenite. Transparent crystals are usually colorless, but sometimes may have various yellowish tints. Rarely, diamonds may be bright yellow, blue, pale green, pink, violet, and even reddish. Some diamonds are covered by translucent skin with a stronger color. Diamonds become green and radioactive after neutron irradiation, and yellow after further heating. They become blue after irradiation with fast electrons. Diamonds have different hardnesses along their different faces. Diamonds from different deposits also have different hardnesses. This quality allows for the polishing of faceted diamonds by diamond powder.

Most diamond gems are faceted into brilliant cuts. Due to the high reflective index, all light passing through the face of such facetted diamonds is reflected back from the back facets, so light is not passing through the stone. This can be used as a diagnostic property, because most simulants (except cubic zirconia) do not have this property. Diamonds do have many simulants, including zircon, corundum, phena-kite, tourmaline, topaz, beryl, quartz, scheelite, sphalerite, and also synthetic gemstones such as cubic zirconia, yttrium-aluminum garnet, strontium titanate, rutile, spinel, and litium niobate. Diamonds have high thermal conductivity, which allows it to be readily and positively distinguished from all simulated gemstones. The most expensive diamonds are those with perfect structure and absolutely colorless or slightly bluish-white color. Yellow tint reduces the price of the diamond significantly. Bright colored diamonds are extremely rare, and have exceptionally high prices.

In January 2003, a number of international concerns came to a preliminary consensus on the Kimberley Process Certification Scheme to curtail international trade in what are termed conflict diamonds.

As of early 2003, nearly 50 countries agreed to use and require standardized, tamper-proof packaging and official certificates attesting to the source of the enclosed diamonds when shipping rough uncut diamonds. Such controls are designed to stem illegal trade in diamonds and to reduce the ability of despotic regimes to exploit diamond trade to perpetuate their political and or military power (e.g., the protocols prohibit trade in contraband diamonds from rebel sources in Sierra Leone). Without proper certification, many nations and industrial sources have agreed not to import or purchase diamonds.

See also Mineralogy.

Resources

BOOKS

Hart, Matthew. Diamond: A Journey to the Heart of an Obsession. New York: Walker & Co., 2001.

Klein, Cornelis. Manual of Mineral Science, 22nd ed. New York: John Wiley & Sons, 2001.

O’Donaghue, Michael, ed. Gems: Their Sources, Descriptions, and Identification. Oxford, UK, and Burlington, MA: Butterworth-Heinemann, 2006.

Paterson, Vicky. Diamonds. Richmond Hill, Ontario, Canada: Firefly Books, 2005.

Yavor Shopov