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Diamond

diamonds mineral yellow type

Diamond is a mineral with the same carbon composition as graphite, but with different structure.

Diamonds are formed in the compression of coal. Photograph by Rick Gayle. Stock Market. Reproduced by permission.

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. These "conflict diamonds" are facing increasing import-export trade restrictions.

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 fond 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 1 g in 8–30 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, phenakite, 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 are agreed to import or purchase contraband diamonds.

See also Mineralogy.

Yavor Shopov

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.

Schumann, Walter. Gemstones of the World. London: Sterling Publications, 2000.

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