Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg(CO₃)₂. The word dolomite is also used to describe the sedimentary carbonate rock, which is composed predominantly of the mineral dolomite (otherwise known as dolostone).

The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, gray to pink, crystals. Dolomite is a double carbonate, having a different structural arrangement, and it does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid. Crystal twinning is common. A solid solution series exists between dolomite and iron rich ankerite. Small amounts of iron in the structure give the crystals a yellow to brown tint. Manganese substitutes in the structure also up to about three percent MnO. A high manganese content gives the crystals a rosy pink color noted in the image above. A series with the manganese rich kutnohorite may exist. Lead and zinc also substitute in the structure for magnesium. The mineral dolomite is closely related to huntite Mg₃Ca(CO₃)₄.

Because dolomite can be dissolved by slightly acidic water, areas of dolomite are important as aquifers and contribute to karst terrain formation.