Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). Pure limestone is almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestone exhibit different colours, especially on weathered surfaces. Limestone may be crystalline, caustic, granular, or massive, depending on the method of formation. Crystals of calcite, quartz, dolomite or barite may line small cavities in the rock