Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate CaCO3. Aragonite and vaterite the other two forms which are very unstable. Calcite measures 3 on Mohs scale of hardness. Its luster is vitreous in crystallized varieties. Color may be white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with certain impurities.

Calcite is transparent to opaque and may occasionally exhibit Phosphorescence and fluorescence. Iceland spar, a transparent variety is used for optical purposes. The largest documented single crystals of calcite originated from Iceland, measured 7×7×2 m and 6×6×3 m and weighed about 250 tons. Calcite is found in spectacular form in the Snowy River cave of New Mexico, where microorganisms are credited with natural formations.