Aragonite

Aragonite is a polymorph of calcite, and shares its chemical composition CaCO₃. Its structure, however, differs from calcite and leads to a different crystal shape. The type location[?] for aragonite is Aragon, Spain.

It exists naturally in chitons' shells.

See also: list of minerals.

External link: Aragonite information (http://mineral.galleries.com/minerals/carbonat/aragonit/aragonit.htm)