Aragonite
Aragonite is calcium carbonate, CaCO3, crystallizing in the orthorhombic system as a metastable polymorph of the more common mineral calcite. It readily forms in low-temperature settings such as caves, hot springs, and the shells of many marine organisms, and it slowly converts to calcite over geologic time. Twinned crystal clusters resembling six-rayed stars are among its most recognizable forms.
Aragonite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | CaCO3 |
| Mohs hardness | 3.5–4 |
| Crystal system | Orthorhombic |
| Luster | Vitreous to resinous |
| Streak | White |
| Cleavage | Distinct, two directions (pseudo-hexagonal) |
| Fracture | Subconchoidal |
| Specific gravity | 2.93–2.95 |
| Transparency | Transparent to translucent |
| Common colors | Colorless, White, Yellow, Gray, Blue, Pink |
| Rarity | common |
Geology & background
Aragonite precipitates directly from calcium-carbonate-saturated water in evaporating lagoons, cave systems, and hot spring terraces, and it is also secreted biologically by mollusks, corals, and the organisms that build pearl nacre. In caves it forms delicate frostwork and helictites alongside calcite, while hydrothermal veins can host the striking sputnik-like clusters of pseudohexagonal twins famous from Molina de Aragon, Spain, the type locality that gave the mineral its name. Morocco has become the leading modern source of these star-shaped twinned specimens sold to collectors worldwide.
Because aragonite is thermodynamically less stable than calcite, older specimens and ancient fossil shells often show partial recrystallization, a process collectors should keep in mind when judging the age or preservation of a piece. Hand-sized twinned clusters from Morocco remain inexpensive and widely available, making the species an approachable entry point for anyone starting a systematic carbonate mineral collection.
How to identify Aragonite
Aragonite shows a vitreous to slightly resinous luster, a white streak, and colors ranging from colorless and white to pale yellow, blue, or green depending on trace impurities. Its hardness of 3.5 to 4 is close to calcite's, but its crystal habit differs: aragonite favors elongated prisms, needle-like sprays, or pseudohexagonal twins rather than calcite's classic rhombohedra. Like calcite, it effervesces readily in dilute hydrochloric acid, so acid testing alone cannot separate the two minerals; crystal shape and slightly higher density are the more reliable clues.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Aragonite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Aragonite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Most pearl nacre and much of the mother-of-pearl lining mollusk shells is biologically produced aragonite.
- The distinctive six-pointed star clusters sold as aragonite are actually cyclic twins of three interpenetrating crystals.
- Aragonite is thermodynamically unstable at Earth's surface and slowly inverts to calcite over long geologic timescales.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
.jpg?width=320)
.jpg?width=320)