Calcite, calcium carbonate,
Formula CaCO3.

Crystal system: Trigonal (many crystal forms, prismatic, rhombohedral, scalenohedral)
Hardness: 3
Density: 2.7 gms/cm3
Colour: Colourless (impurities can cause Calcite to take on many colours).
Streak: White.

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.

Dog-tooth Calcite, Minas Gerais, Brazil