Aragonite

Aragonite is a calcium carbonate mineral (CaCO3) and the orthorhombic polymorph of calcite, forming distinctive tabular, needle-like, or pseudohexagonal twinned crystals rather than calcite's classic rhombohedra. Its multiple twinning by the {110} planes produces the pseudohexagonal "trilling" prisms that make aragonite so recognizable, while a separate branching habit known as flos ferri ("flowers of iron") forms delicate coral-like or tree-like sprays.

The mineral's name comes from its type locality of Molina de Aragón in Guadalajara, Spain, where lustrous doubly-terminated twins are still found today, alongside similar material from Enguídanos and Minglanilla. Sicily's Racalmuto, Cianciana, and Agrigento localities have produced classic fine crystals for centuries, and Tsumeb in Namibia is famed for its rare colored aragonite varieties, tarnowitzite and nicholsonite. Today, however, the market is dominated by exceptional pseudohexagonal clusters from Tazouta, near Sefrou in Morocco's Middle Atlas Mountains, prized for their orange-red color, sharp terminations, and abundance, alongside striking fluorescent stalactites from Oujda.

Collectors seek aragonite for its elegant twinning, wide color range from colorless and white to green, blue, and purple, and its close relationship to biogenic carbonate found in mollusk shells, coral, and pearls. Fine, undamaged twinned specimens and flos ferri sprays remain especially desirable for both mineral and crystallography collections.