Found worldwide, with classic specimens from Iceland (the transparent “Iceland spar” variety), Mexico, the United States, and Italy , this is one of the most common minerals of the Earth’s crust and the main constituent of limestone and marble. It forms across 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 Mohs (the reference mineral for the Mohs scale), density 2.71 g/cm³, effervesces with dilute acids.

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