Calcite (CaCO₃), the most common simple salt at the Earth's surface, is present in almost all geological environments. Depending on the nature and extent of impurities, calcite is usually white or light in color, and its physical and optical properties make it attractive when polished. It is not a very hard mineral (3 out of maximal 10 on the Mohs hardness scale), so rocks composed of calcite are relatively easy to quarry, carve, and polish. Calcite-bearing rocks have, therefore, been favored as building stones for centuries, and include marble, limestone, and calcite-cemented sandstone.