A white compound, CaCO3, that occurs naturally as marble, chalk, limestone, and calcite. It is used by many marine invertebrates, such as corals and echinoderms, and by protists, such as coccolithophorids, to construct their exoskeletons. Calcium carbonate, in the form of calcite, is also incorporated into the eggshells of amniotes, except for turtles whose eggs are composed of aragonite (CaCO3 + magnesium).