Diamond

Diamond, mineral allotrope (molecular form) of carbon forming colorless cubic crystals (the other forms being graphite and the recently discovered fullerene). Diamond is the hardest known substance, with a Mohs hardness of 10, which varies slightly with the orientation of the crystal. Thus diamonds can be cut only by other diamonds. They do not conduct electricity, but conduct heat extremely well. Diamonds occur naturally in the mineral kimberlite, notably in South Africa (Orange Free State and Transvaal), Tanzania, and in the United States at Murfreesboro, Ark. They are also mined from secondary (alluvial) deposits, especially in Brazil, Zaire, Sierra Leone, and India. The diamonds are separated by mechanical panning, and those of gem quality are cleaved (or sawn), cut, and polished. Inferior, or industrial, diamonds are used for cutting, drilling, and grinding. Synthetic industrial diamonds are made by subjecting graphite to very high temperatures and pressures, sometimes with fused metals as solvents.

See also: Carbon.