Many deposits of limestone are altered to some extent into dolomite rock. The details are still a subject of research. Dolomite also occurs in some bodies of serpentinite, which are rich in magnesium. It forms at the Earth’s surface in a few very unusual places marked by high salinity and extreme alkaline conditions.
Dolomite is harder than calcite (Mohs hardness 4). It often has a light pinkish color, and if it forms crystals these often have a curved shape. It commonly has a pearly luster. The crystal shape and luster may reflect the atomic structure of the mineral, in which two cations of very different sizes—magnesium and calcium—place stress on the crystal lattice. However, commonly the two minerals appear so much alike that the acid test is the only quick way to distinguish them. You can see the rhombohedral cleavage of dolomite in the center of this specimen, which is typical of carbonate minerals.
Rock that is primarily dolomite is sometimes called dolostone, but “dolomite” or “dolomite rock” are preferred names. In fact, the rock dolomite was named before the mineral that composes it.