Faceted proustite, 2.58 cts. Photo © Joel E. Arem, PhD, FGA. Used with permission.
Proustite crystals have magnificent red colors and good brilliance. Although facetable, they’re too soft for jewelry use but highly desired as collector’s gems.
Faceted proustite, 2.58 cts. Photo © Joel E. Arem, PhD, FGA. Used with permission.
Proustite crystals have magnificent red colors and good brilliance. Although facetable, they’re too soft for jewelry use but highly desired as collector’s gems.
| Data | Value |
|---|---|
| Name | Proustite |
| Crystallography | Hexagonal (R) (trigonal). Crystals prismatic, rhombohedral; massive, compact. |
| Crystallographic Forms | |
| Refractive Index | 2.792-3.088 |
| Colors | Deep red, scarlet to vermilion red. |
| Luster | Adamantine to submetallic |
| Hardness | 2-2.5 |
| Fracture | Conchoidal to uneven |
| Specific Gravity | 5.51-5.64 |
| Birefringence | 0.296 |
| Cleavage | Distinct 1 direction |
| Dispersion | Weak |
| Luminescence | None |
| Luminescence Present | No |
| Transparency | Opaque to translucent, some crystals transparent. |
| Absorption Spectrum | Not diagnostic |
| Phenomena | Proustite with silver may be photochromic. |
| Formula | Ag3AsS3 |
| Pleochroism | Strong in shades of red. Blood red/scarlet. |
| Optics | o = 3.088; e = 2.792. Uniaxial (-). |
| Optic Sign | Uniaxial - |
| Etymology | After Joseph L. Proust, the French chemist who identified this mineral. |
| Occurrence | Low-temperature ore deposits or the upper portions of vein deposits. |
Cluster of cherry-red proustites, Jachymov, (St Joachimsthal), Krusne Hory Mtns (Erzgebirge), Karlovy Vary Region, Bohemia, Czech Republic. © Rob Lavinsky, www.iRocks.com. Used with permission.
One of the rarest of all the better-looking collector gems, faceted proustite can display deep red colors with a beautiful and distinctive metallic surface luster. Sometimes called “ruby silver,” proustite is also a source of ore for this metal. (Of course, “ruby” here refers strictly to proustite’s color, since these minerals are unrelated).
Proustites and xanthoconites are dimorphous. They share a chemical formula, but proustites have hexagonal (trigonal) crystal habits, while xanthoconites have monoclinic habits. Proustite also forms a series as the arsenic (As) analogue to pyrargyrite, the antimony (Sb) analogue.
Custom shield-cut proustite, 5.46 cts, 10.5 x 8.7 mm, Chanarcillo, Chile. © The Gem Trader. Used with permission.
Proustites are seldom cut into gemstones and even more rarely used as jewelry stones. A number of factors make proustites very impractical for wear. Most notably, the presence of silver in proustite can lead to an irreversible photochromic effect. Exposure to light over time turns it black. Reserve any proustite jewelry strictly for evening wear, and don’t display this gemstone under strong light.
Proustites also have a very low hardness (2-2.5). A copper coin (hardness 3) could scratch them, and household dust (hardness 7), a much more pervasive hazard, could scratch them even more easily.
Proustite does contain arsenic. When cutting this gem, faceters should take particular care not to accidentally ingest dust or particles from it. For more information, consult our articles on general safety tips for lapidaries and advice for handling toxic gem materials. However, wearing or touching finished proustites should pose no health problems.
Never place a proustite in your mouth or swallow one in any form.
Proustites are best treated as collector’s gemstones for display only.
Proustite: Germany (9.53). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Proustite is one of the few gemstones that has a bright red streak. However, don’t conduct streak testing on finished gems. Test this material in inconspicuous spots as a last resort for identification purposes only.
Scientists have synthesized crystal proustites for research into optic mixing and lasers. This lab-created material may appear faceted in collections of rare and unusual gems.
Synthetic proustite, 6.98 cts. © Joel E. Arem, PhD, FGA. Used with permission.
There are no known treatments or enhancements for proustites.
Chile produces the world’s finest proustite, particularly from the Dolores Mine in Chanarcillo. Deep red, often transparent crystals occur here in sizes up to 6 inches long and very thick. The occurrence here is unique. Most cuttable gem material comes from Chile.
Freiberg, Germany also produces fine crystals. Other German localities sometimes yield very fine, large, cuttable crystals and, occasionally, transparent fragments.
Other notable gem sources include the following:
Proustites with calcite, Chanarcillo, Chile. © Rob Lavinsky, www.iRocks.com. Used with permission.
Faceters could cut gems weighing several hundred carats from crystals displayed in various museums and private collections. However, that likely won’t happen. The National History Museum in London holds some of the finest specimens known. The Smithsonian Institution holds a 9.9-ct red stone from Germany.
Spray of proustites, Marienberg District, Erzgebirge, Saxony, Germany, crystals 3 cm in height. 19th century East European museums commonly used wax and wood display pedestals like the one shown here. © Rob Lavinsky, www.iRocks.com. Used with permission.
Clean proustites only with a warm damp cloth, mild detergent, and soft brush. Consult our gemstone jewelry cleaning guide for more recommendations.
Proustite: Germany (7.53). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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