Found worldwide in sedimentary deposits and hydrothermal veins (Morocco, Mexico, and Italy are common sources), it is a variety of calcite whose gray color is due to fine inclusions of clay, organic matter, or graphite- and sulfide-based pigments scattered throughout the crystal lattice during growth. It forms through chemical or biogenic precipitation in a sedimentary environment, or through hydrothermal deposition in veins and geodes associated with limestone. Trigonal system, perfect rhombohedral cleavage in three directions, strong double refraction, hardness 3 on the Mohs scale, density 2.71 g/cm³, effervescence upon contact with dilute acids.

Used as a building and ornamental material just like other calcite varieties; valued by collectors for its crystalline forms (scalenohedra, rhombohedra) rather than for its color, which is generally less sought after than that of transparent or colored varieties; also used in decorative objects with a neutral and understated aesthetic.