Calcitemineral calcite, carbonate of calcium corresponding to the formula CaCO3, is one of the most widely distributed minerals. Its crystals are hexagonal-rhombohedral though actual calcite rhombohedrons are rare as natural crystals. However, they show a remarkable variety of habit including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons. It may be fibrous, granular, lamellar, or compact. The cleavage in three directions parallel to rhombohedron is highly perfect; fracture, conchoidal but difficult to obtain; hardness 3; specific gravity 2.7; lustre is vitreous in crystallized varieties; color is white or colorless, though shades of gray, red, yellow, green, blue, violet, brown, or even black when charged with impurities.
Doubly refracting calcite. The doubling is particularly obvious on the top bar of the letter T. The crystal is 1.5 inches long (4cm)
It is transparent to opaque and may occasionally show phosphorescence or fluorescence. Calcite is perhaps best known because of its power to produce strong double refraction of light such that objects viewed through a clear piece of calcite appear doubled in all of their parts - a phenomenon first described by Rasmus Bartholin. A beautifully transparent variety used for optical purposes comes from Iceland, called Iceland spar. Acute scalenohedral crystals are sometimes referred to as dogtooth spar.
Calcite is a common constituent of sedimentary rocks, limestone in particular. It also occurs as a vein mineral, in deposits from hot springs and in caverns as stalactites and stalagmitess. Calcite is often found in the shells of marine organisms (e.g. plankton).