calcite

Colourless, white, or light-coloured common rock-forming mineral, calcium carbonate, CaCO₃. It is the main constituent of limestone and marble and forms many types of invertebrate shell.

Calcite often forms stalactites and stalagmites in caves and is also found deposited in veins through many rocks because of the ease with which it is dissolved and transported by groundwater; oolite is a rock consisting of spheroidal calcite grains. It rates 3 on the Mohs scale of hardness. Large crystals up to 1 m/3 ft have been found in Oklahoma and Missouri, USA. Iceland spar is a transparent form of calcite used in the optical industry; as limestone it is used in the building industry.