Prosopite cabochon, 4.94 cts, 13.3 x 10.2 mm, Zacatecas, Mexico. © The Gem Trader. Used with permission.
Prosopite is usually a nondescript mineral with no gem significance. However, beautiful blue specimens are sometimes cut for jewelry.
Prosopite cabochon, 4.94 cts, 13.3 x 10.2 mm, Zacatecas, Mexico. © The Gem Trader. Used with permission.
Prosopite is usually a nondescript mineral with no gem significance. However, beautiful blue specimens are sometimes cut for jewelry.
| Data | Value |
|---|---|
| Name | Prosopite |
| Formula | CaAl2(F,OH)8 |
| Colors | Colorless, whitish, grayish, blueish. |
| Fracture | Conchoidal |
| Hardness | 4.5 |
| Cleavage | Perfect; not seen on massive material. |
| Crystallography | Monoclinic; crystals minute, tabular; commonly massive, granular. |
| Refractive Index | 1.501-1.510 |
| Birefringence | 0.009 |
| Dispersion | None |
| Luminescence | UV longwave: blue, blueish white, violet, violet-blue; UV shortwave: orange-yellow, blue, blueish white. |
| Luminescence Present | Yes |
| Absorption Spectrum | Not diagnostic |
| Pleochroism | None |
| Optics | a = 1.501; β = 1.503; γ = 1.510. Biaxial (+). |
| Optic Sign | Biaxial + |
| Luster | Vitreous to dull. |
| Specific Gravity | 2.88 (Mexican prosopite, 2.69-2.85). |
| Transparency | Transparent to translucent. |
| Luminescence Type | Fluorescent, UV-Long, UV-Short |
| Etymology | From a Greek word meaning a mask, in allusion to the deceptive nature of the material when found as pseudomorphs. |
| Occurrence | In tin veins, alkalic pegmatites, and as an alteration product of topaz in volcanic rocks. |
Whitish prosopite crystals, Terry Uranium Prospect, San Mateo Mountains District, Sierra Co., New Mexico, USA. Photo by Jerry Cone. Public Domain.
Prosopites may form as pseudomorphs of topaz. This mineral may form first but then gradually changes chemical structure, becoming prosopite while retaining the external crystal form of the original topaz.
Prosopite, pseudomorph after topaz, 7.1 x 4.7 x 3.4 cm, Santa Rosa Mine, Mazapil, Zecatecas, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
Material from Zacatecas, Mexico has a turquoise-like blue color. These prosopites have an even coloration due to a copper content of approximately 1.4%. They make attractive and convincing turquoise simulants. Of course, they are distinct gem species.
If used as ring stones, place prosopites in protective settings to protect them from bumps and scratches or reserve them for occasional wear. Other uses, such as in pendants or brooches, shouldn’t require any special protections.
Turquoise and Mexican prosopites can show similar colors and also have specific gravity values that overlap. A refractive index reading is the most effective way to distinguish them. (1.501-1.510 for prosopites; 1.590-1.650 for turquoises). Prosopites also have a lower hardness than turquoises, but scratch testing isn’t recommended for finished gemstones.
There are no known enhanced or synthetic prosopites.
Turquoise-like blue prosopite material occurs only in Santa Rosa, Zacatecas, Mexico, in association with azurite.
Other notable sources of crystals include the following:
Prosopites, from the collection of Arthur Montgomery (1972). Field of view, about 2 cm. Mount Rosa, El Paso Co., Colorado, USA. Photo by Rock Currier. Licensed under CC By 3.0.
Avoid cleaning prosopites with mechanical systems, such as steam or ultrasonic processes. Instead, use a soft brush, mild detergent, and warm water. Consult our gemstone jewelry care guide for more recommendations.
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