GeoTech.org - Diamonds Diamonds Diamonds are formed from pure carbon, one of the most abundant elements on planet Earth. Ironically the other form of pure carbon is graphite, which is very soft with a soapy feel and a dull gray color.
Diamonds can be from ten to hundreds times harder than corundum.
The four main optical characteristics of diamonds are transparency, luster, dispersion of light, and color. In its pure carbon form, diamond is completely clear and transparent. As in all natural substances, perfection is nearly impossible to find. Inclusions of other minerals and elements cause varying degrees of opacity. The surface of a diamond can be clouded by natural processes, such as the constant tumbling and scraping in the bed of a river.