Found worldwide, with classic specimens from Iceland (the transparent variety known as “Iceland spar”), Mexico, the United States, and Italy, it is one of the most common minerals in the Earth’s crust and the main constituent of limestone and marble. It forms in a wide range of environments—sedimentary (chemical and biogenic precipitation), hydrothermal, and metamorphic—throughout all geological eras. Trigonal system, perfect rhombohedral cleavage in three directions, strong birefringence (double refraction), hardness 3 on the Mohs scale (reference mineral for the Mohs scale), density 2.71 g/cm³, effervescence upon contact with dilute acids.

Widely used as a building material (lime, cement, ornamental marble) and in industry (glass and paper production, agriculture); highly prized by collectors for its enormous variety of crystal forms and colors; historically used in optics for its birefringent properties.