Aragonite and calcite are both composed of calcium carbonate, but the two minerals have different crystallographic structures. Aragonite is the substance of pearls, while calcite typically makes up the shells of mollusks. Aragonite is extremely common and widespread, and forms very attractive crystals that are often grown together in forms known as twins. Twinning in crystals may result in forms with symmetry that appears different from that of individual crystals. This is evident in the material from Tazouta, Morocco.