Calcium is a very reactive and does not occur as a free element in Nature. It is present in the earth's crust in the forms of carbonate, sulfate, fluoride, silicate and borate. The calcium carbonate occurs in marble, chalk, limestone and calcite. Calcium sulfate (CaSO4) occurs in anhydrite and gypsum, calcium fluoride in fluorspar or fluorite (CaF2) and calcium phosphate occurs in apatite. Calcium also occurs in numerous silicates and aluminosilicates.
Almost all natural waters, including seawater, contain either or both calcium carbonate and calcium sulfate. Many organisms concentrate calcium compounds in their shells or skeletons. For example calcium carbonate is formed in the shells of oysters and in the skeletons of coral.