Calcite is one of the most abundant minerals in the Earth's crust. It is a significant rock forming mineral.

Though Calcite is commonly almost pure calcium carbonate, Calcium can be replaced by manganese, magnesium and iron.  Thus Dolomite is a carbonate of both Calcium and Magnesium. Calcite is a major component of limestones, and forms the shell of a large number of marine creatures such as corals, molluscs and sponges.

The picture shows a nice matrix-free group of 'dog's tooth', scalenohedral crystals of Calcite from Minas Gerais, Brazil. It fluoresces a reddish-pink under ultraviolet light.