Calcite Crystal

This clear unpolished crystal is calcium carbonate, a compound of calcium and carbon. It exists in three different mineral forms known as aragonite, vaterite and calcite. The most stable of the three is calcite, and that is the form of this crystal. Pure calcite is transparent, however small amounts of different impurities can give shades of gray, red, orange, yellow, green, blue, violet, brown or even black. 

Calcite can occur naturally as very large crystals, the largest on record measuring 6×6×3 metres, weighing in at 250 tonnes. Calcium carbonate is a fundamental ingredient in many biomineralisation processes, giving rise to bones, teeth and shells.