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Diamond is one of the natural allotropes of carbon (the main one being graphite). Sometimes known as adamant, it is the hardest known naturally occurring material, scoring 10 on the old Mohs hardness scale. The only other material which is as hard is boron nitride in a form structurally identical to diamond; a currently hypothetical material, beta carbon nitride, may also be as hard or harder in one form.

Diamond is a transparent crystal of high refractive index consisting of tetrahedrally bonded carbon atoms. It is a good electrical insulator, but unlike most insulators, is a good conductor of heat due to the strong bonding within the molecule.

The diamond derives its name from the Greek Adamas, "uncorrupted." Diamonds occur in a variety of colours - steel, white, blue, yellow, orange, red, green, pink and black. Coloured diamonds contain impurities that cause the coloration, pure diamonds are always translucent and colourless.

In the late 18th century, diamonds were demonstrated to be made of carbon by the rather expensive experiment of igniting a diamond (by means of a burning-glass) in an oxygen atmosphere and showing that carbonic acid gas (carbon dioxide) was the product of the combustion. The fact that diamonds are combustible bears further examination because it is related to an interesting fact about diamonds. Diamonds are carbon Crystals that form deep within the Earth under high temperatures and extreme pressures. At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favourable (ΔH = -2KJmol-1). So, despite De Beers' ad campaign, diamonds are definitely not forever. However, owing to a very large kinetic energy barrier, diamonds will not decay into graphite under normal conditions.

Due to their high refractive index, diamonds have been prized as a constituent of jewellery, and a large trade in gemstone-class diamonds exists, mostly controlled by the De Beers company, who use their influence to artificially inflate the gemstones' value. Diamonds are valued according to the four C's of diamond grading, namely color, clarity, cut, and carat.

Poorer quality (discoloured, less transparent) diamonds are used as industrial diamonds, where their extreme hardness is useful in cutting and grinding otherwise intractable materials (including other diamonds). Lately, gas-phase deposition processes have been devised that allow thin diamond films to be grown on some surfaces, greatly increasing the durability of some machine tools.

Diamonds typically have cubic symmetry. A second form called lonsdaleite with hexagonal symmetry is also found. The local environment of each atom is identical in the two structures.

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