Diamond is ideally a pure carbon mineral. It is the only mineral wherein the bonding is completely covalent. Each carbon is in sp3 orbital configuration and is bonded to 4 other carbons with similar electronic configurations. Crystals are commonly Isometric, mostly octahedrons; less commonly they are dodecahedral, rarely cubic. Crystals usually have curved and striated faces, but they may also occur as rounded or irregular grains. Granular to cryptocrystalline forms are called bort, and black compact forms are called carbonado.
Diamond is the hardest mineral known, rating a value of 10 on the Mohs scale. It is most easily confused with quartz crystals; the luster and hardness are two criteria used to distinguish one from the other. Diamonds may contain small inclusions of graphite, magnetite, garnet, and other minerals.
As a primary mineral, diamond occurs only in olivine-rich peridotites, notably kimberlite, where it is associated with olivines, magnetite, and phlogopite. Substantial amounts of diamond are mined from both recent and ancient (consolidated conglomerates) placer deposits. Earliest sources of diamond were in India, Brazil and South Africa.
Gem varieties of diamond include colorless (which may be completely colorless or slightly tinted, commonly with yellow, brown and sometimes green or blue) and fancies, diamonds with a definite tint to them. Fancy diamonds are most commonly brown; orange, violet, strong yellow, and yellowish green colors are less common; and red, blue, and deep pure green are the rarest colors known. Black diamonds are rarely used in jewelry, because they are aggregates of minute crystals (micro- or cryptocrystalline). They are used only as abrasives and in diamond saws. Principal sources of gem diamond are Zaire, South Africa, and Brazil. Some diamonds have been found in Arkansas and California (USA).
Chesterman, C.W. 1978. The Audubon Society Field Guide to North American Rocks and Minerals. A.A. Knopf, Chanticleer Press Inc., New York. USA.
Phillis, W. R. and D.T. Griffen. 1981. Optical Mineralogy: The nonopaque Minerals. Freeman &Company, San Fransisco, USA. pp: 677.