Calcium carbonate is a chemical compound that is used for shells of marine organisms, snails, and eggshells. It is also commonly found in limestone. The chemical formula is CaCO3; the compound appears to have a white chalky consistency that is also odorless. This compound’s structure is trigonal making the main shapes dipyramids, rhombohedron, and scalenohedron. Calcium carbonate is found naturally in marble, limestone, chalk, and aragonite.