Triangle-cut aragonite, 22.19 cts, 22 x 19.3 mm, Czech Republic. © The Gem Trader. Used with permission.
Triangle-cut aragonite, 22.19 cts, 22 x 19.3 mm, Czech Republic. © The Gem Trader. Used with permission.
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| Data | Value |
|---|---|
| Name | Aragonite |
| Crystallography | Orthorhombic. Pseudo hexagonal, crystals often acicular, chisel-shaped, prismatic; also massive columnar, fibrous, stalactitic, coralloidal. Frequently twinned. |
| Crystallographic Forms | |
| Refractive Index | 1.530-1.685 |
| Colors | Colorless, white, yellow, gray, green, blue-green, lavender, reddish, brown. |
| Luster | Vitreous to resinous |
| Hardness | 3.5-4 |
| Fracture | Subconchoidal |
| Specific Gravity | 2.947 (pure). Usually 2.93 – 2.95; up to 3.0 if Pb present. |
| Birefringence | 0.155 |
| Cleavage | Distinct 1 direction |
| Luminescence | See "Identifying Characteristics" below. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, Phosphorescent, UV-Long, UV-Short |
| Transparency | Opaque to transparent |
| Absorption Spectrum | Not diagnostic |
| Formula | CaCO3 + Pb, Sr, rarely Zn. |
| Pleochroism | None |
| Optics | α = 1.530; β = 1.681; γ = 1.685. Biaxial (-), 2V = 18°. Sector twinning observed. |
| Optic Sign | Biaxial - |
| Etymology | After the locality Molina de Aragon, Spain, where the material was first identified. |
| Occurrence | Worldwide, especially in limestone caverns, hot springs, and in the oxidized zone of ore deposits. |
| Inclusions | Usually veil-type inclusions observed. |
Aragonites, spray of crystals on matrix, Podrescany, Lucsenac, Slovakia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Aragonite and calcite are dimorphous or polymorphs. They share the same chemistry but have different crystal habits. While calcite is the most abundant and widespread carbonate mineral on Earth, aragonite is less so. Facetable aragonites are almost always very small, as opposed to calcites, which occur in huge, transparent masses or crystals. (Like calcites, aragonites have a high birefringence and will also show facet doubling).
Be aware that some people describe ammolite as aragonite. However, this mineral only constitutes a part (albeit major) of this fossilized shell gem material.
Aragonite from the Santa Eulalia mine in Chihuahua, Mexico, under daylight (above) and shortwave ultraviolet light (below). Photo by Hadley Paul Garland. Licensed under CC By-SA 2.0.
Heating to 400° C will convert aragonite into calcite. This occurs naturally over millions of years, but no commercial incentive exists to do this in a lab.
Notable gem sources include:
Aragonite, Retamal ravine, Enguidanos, Cuenca, Castile-La Mancha, Spain. Photo by Didier Descouens. Licensed under CC By 4.0.
Faceted gems are usually only a few carats and colorless, but the potential exists for much larger stones. The largest known cut specimen hails from Bilin, Czech Republic: a straw-yellow emerald-cut gem that weighs 110 carats.
Straw-yellow crystals from Horschenz, Germany have yielded stones up to 10 carats.
The Devonian Group in Calgary, Alberta, Canada holds a 7.85-ct stone from Germany.
Aragonite’s hardness is too low for this stone to be worn safely in jewelry. Avoid mechanical cleaning such as steam or ultrasonic processes. Instead, use a soft brush, mild detergent, and warm water for cleaning. Store your aragonites separately from other stones to avoid contact scratches. Consult our gemstone jewelry cleaning guide for more recommendations.
Aragonite: Czech Republic (5.35). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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