Limestone is a sedimentary rock and is composed of calcium carbonate (calcite) it also contains small quantities of other minerals and impurities. Most grains in limestone are skeletal fragments of marine organisms (such as molluscs that are rich in calcium carbonate). Some limestones do not consist of grains at all, and are formed completely by the chemical precipitation of calcite. Secondary calcite may be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems, such as stalagmites and stalactites. Limestone makes up about 10% of the total volume of all sedimentary rocks. Limestone appears in numerous varieties with different grain textures from fine to coarse. Pure limestone is white but due to impurities such as clay, iron oxide and iron can have colours of creams, reds and grays.