Aragonite
Aragonite is a polymorph of calcite — same composition (CaCO₃), different crystal structure. It is the mineral in pearls and most mollusk shells. Aragonite forms distinctive "flos ferri" (flowers of iron) branching cave formations and also produces the remarkable twin crystals resembling pseudo-hexagonal prisms. Pearls, mother-of-pearl, and coral skeletons are all composed of aragonite.
Physical Properties
Chemical Formula
CaCO₃
Hardness (Mohs)
3.5–4
Color
White, colorless, yellow, orange, brown
Luster
Vitreous to resinous
Crystal System
Orthorhombic
Streak
White
Cleavage
Distinct in one direction
Specific Gravity
2.93–2.95
🔍 Field Identification Tips
Distinguished from calcite by slightly higher SG (2.93 vs 2.71), lower tendency to form perfect rhombohedral cleavage, and different crystal habit (orthorhombic vs hexagonal). Both fizz in cold HCl acid. Aragonite is less stable at surface conditions — many aragonite specimens slowly convert to calcite over geological time. Pearl test: aragonite feels gritty against teeth; glass does not.
Varieties & Forms
📍 Where to Find Aragonite in the US
Uses & Applications
Collectors specimens, historical source of lime, pearl formation (shells and pearls are aragonite).
💰 Collector Value Guide
Common white clusters: $10–$50. Spanish twin crystals: $20–$200. Flos ferri specimens: $50–$500.
Find Aragonite With Our Tools
Use the We Lick Rocks app to identify specimens, find dig sites, and connect with local rockhounds.