The fossils that make up limestone—marine shells and skeletons—consist of calcium carbonate, a chemical compound that occurs as one of two minerals: calcite or aragonite, depending on the animal that produced it. Most mollusks, such as clams, build their shells of aragonite, but some corals, including H. percarinata, produced their structures from calcite. Both are common minerals made up of the elements calcium, carbon, and oxygen; the key difference between them lies in their crystal structure. The molecular structure of aragonite is less stable than that of calcite, and as a result, aragonite slowly alters, or transforms, into calcite (but usually with little to no visual change). Limestone can also contain minor amounts of other minerals, namely dolomite, quartz, clay minerals, and even other rock fragments, such as sand.