Calcite is a calcium carbonate (CaCO3). Its crystals are trigonal-rhombohedral, however, they show a remarkable variety of habits including acute to obtuse rhombohedron, tabular forms, prisms, or various scalenohedral. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.
It has a defining Mohs hardness of 3. Its luster is vitreous in crystallized varieties. Color is white or colorless, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur due to impurities. Masses of Calcite also tend to be light colored, but as with the transparent crystals, the presence of even minor amounts of impurities can create a wide range of colors.