Aragonite and calcite are both forms of calcium carbonate. The two minerals only differ in their crystallization. Calcite is the more common mineral and it forms in trigonal crystals, while aragonite forms orthorhombic crystal. Aragonite and calcite crystals are sometimes too small to be individually determined, and the only possible way to distinguish these two minerals is through optical or x-ray testing. Without complex testing, the real identity of microcrystalline forms of aragonite or calcite may also not be distinguished, which can also cause a confusion between these species.

Large aragonite crystals are mostly twinned growths of three individual crystals that form pseudohexagonal trillings. Though aragonite crystallizes in the orthorhombic system, majority of prismatic crystals have a hexagonal shape due to the twinning. Trillings can be recognized by their multi-directional basal striations from each separate member crystal.

Other minerals may possibly form pseudomorphs after aragonite. In the mineral world, one such oddity is calcite after aragonite, which is a pseudomorph after an existing paramorph. Some aragonite crystals that collectors have access to are actually calcite pseudomorphs after aragonite. A rare but popular pseudomorph is copper after aragonite. There’s also a possibility that aragonite will contain sand inclusions, which give a specimen a brown color.

One such interesting formation of aragonite is as a deposition product of hot springs rich in mineral content. The water releases calcium upon emerging from the spring and then forms growing mounds and thick crusts throughout the springs. When banded, they may be carved and given trade names such as “California Onyx” and “Onyx Marble”.

Pearl, coral and many other organic substances have aragonite as their primary component. The iridescent surface of pearl and mother-of-pearl is in fact a layer of aragonite produced by mollusks and similar invertebrates. Several forms of aragonite, especially the Flos Ferri variety, are brittle and so fragile that they could be easily broken when touched. Such specimens need extra care.

Abraham Gottlob Werner, a German geologist, named aragonite after Molina de Aragón, Spain, the type locality where this mineral was first described.