Aragonite is a polymorph of the mineral calcite, both having the chemical composition CaCO3. Its structure differs from calcite and leads to a different crystal shape, an orthorhombic system with acicular crystals. By repeated twinning pseudo-hexagonal forms result. It may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri (flowers of iron) from their association with the ores at the Carthinian iron mines. The type location for aragonite is Molina de AragÃ³n (Guadalajara, Spain), 25 km outside Aragon.
It exists naturally in the shells of chiton and other mollusks. The fossil shells of some extinct ammonites form a mineral called ammolite, which is primarily aragonite with impurities that make it iridescent and valuable as a gemstone.