1. (pearlspar) Widely distributed rock-forming mineral, CaMg(CO₃)₂; sp. gr. 2.8–2.9; hardness 3.5–4.0; trigonal; usually white or colourless, but can be yellowish and brown; white streak; vitreous lustre; crystals are usually rhombohedral with curved, composite faces, also occurs massive and granular; cleavage perfect rhombohedral {1011}; usually secondary, having formed by the action of magnesium-bearing solutions on limestones (dolomitization), also occurs as a gangue mineral in hydrothermal veins particularly associated with galena and sphalerite. It dissolves very slowly in cold, dilute acid, but effervesces very readily when warmed. It is used as a building stone and in the manufacture of bricks for furnaces.

2. (dolostone) A sedimentary rock type, usually formed by the dolomitization of limestones, and commonly occurring interbedded with them. Most limestones contain some magnesium carbonate and strictly the term ‘dolomite’ refers to rocks containing 90% or more of the mineral dolomite (see 1). Dolomite that has formed soon after deposition tends to be fine-grained and to have preserved the original sedimentary structures, whereas recrystallization in late-diagenetic dolomites produces a coarser-grained rock, a loss of sedimentary structures, and an increases in porosity. See also ankerite.