Dolomite, DOL uh myt, is a mineral that serves as the chief source of magnesium obtained from the earth's crust. It is fairly hard and brittle and consists of calcium carbonate and magnesium carbonate. Pure dolomite may appear white or yellow. Impurities, such as manganese or iron, may make dolomite pink, brown, or some other color. Dolomite and a mineral called calcite, which consists only of calcium carbonate, often look alike.