Pyrargyrite, oval cabochon, 4.81 cts, South Africa (possibly Ehlanzeni). © Rob Lavinsky, www.iRocks.com. Used with permission.
Deep red pyrargyrite occurs in a number of localities in well-formed but small crystals. However, facetable rough is rare and difficult to cut, which makes these gems prized collector’s items.
| Data | Value |
|---|---|
| Name | Pyrargyrite |
| Formula | Ag3SbS3 |
| Colors | Dark red to dark gray, almost black. |
| Fracture | Conchoidal to uneven |
| Hardness | 2.5 |
| Cleavage | Distinct 1 direction |
| Crystallography | Hexagonal (R), trigonal; crystals prismatic, often hemimorphic. Usually massive, compact, disseminated. |
| Refractive Index | 2.88-3.08 |
| Birefringence | 0.200 |
| Luminescence | None reported |
| Luminescence Present | No |
| Absorption Spectrum | Not diagnostic |
| Pleochroism | Shades of red |
| Optics | o = 3.08; e = 2.88. (Note: these reported measurements were made using the Li line at 6710 rather than the customary Na line at 5890). Uniaxial (-). |
| Optic Sign | Uniaxial - |
| Luster | Adamantine |
| Specific Gravity | 5.85 |
| Enhancements | Silver cleaning solution may enhance red color. |
| Transparency | Translucent to opaque, rarely transparent. |
| Phenomena | May lose color and translucence when exposed to light (photochromism); some crystals may have iridescent patinas. |
| Typical Treatments | Surface Coating |
| Etymology | From Greek words for “fire” and “silver,” after its color and composition. |
| Occurrence | In low temperature hydrothermal vein deposits, as an ore of silver. |
When backlit, these pyrargyrites show a glowing, cherry red color. Specimen 3.1 x 3.0 x 2.1 cm (main crystal 2.4 cm), San Genaro Mine, Castrovirreyna District, Huancavelica Department, Peru. © Rob Lavinsky, www.iRocks.com. Used with permission.
Pyrargyrite belongs to the proustite mineral group and forms a series with proustite as the antinomy (Sb) end member. Like proustite, pyrargyrite has been called “ruby silver,” because this silver ore can also show deep red ruby-like color. Most pyrargyrites are opaque with a metallic, dark gray color, but some can show translucency and red color when backlit. Pyrargyrites are seldom transparent, even more rarely than proustites.
With a hardness of only 2.5 and distinct cleavage, rare facetable pyrargyrites are difficult to cut. Furthermore, due to their silver content, these gems are photochromic. They can darken and lose translucency when exposed to light. Thus, they make less than ideal choices for jewelry stones.
Rectangular step-cut pyrargyrite, 3.77 cts, 7.2 x 6.3 mm, Peru. © The Gem Trader. Used with permission.
Despite appearances, pyrargyrites and rubies have very different optical and physical properties. Gemologists can easily distinguish these distinct species. Note that some vendor descriptions of pyrargyrites may refer to ruby cores or zones, but these terms refer strictly to translucent areas of red color.
On the other hand, pyrargyrites and proustites have many identical or overlapping properties. However, pyrargyrites have a higher specific gravity (SG) and a purplish red streak. Proustites have a bright red streak. Note that streak testing is a destructive test, so don’t conduct it on a finished gem and use it only as a last resort for identification.
Cuprite is another rarely faceted gem with properties and colors similar to pyrargyrite. However, it has a higher SG, a brownish red streak, and, as an isometric gem, no birefringence. (Some cuprites may show birefringence anomalously).
Crystal specimen with a 3.5 cm doubly-terminated pyrargyrite on a cluster of quartz matrix. Rob Lavinsky writes that this pyrargyrite, when backlit, “glows red like a cuprite.” Bote Mine, Zacatecas, Zacatecas, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
Pyrargyrite has over-the-limit refractive indices (OTL). Its refractive indices (RI) were measured with refractometers calibrated to the lithium (Li) wavelength line at 6710 rather than the customary sodium (Na) line at 5890.
Some pyrargyrites may have iridescent patinas.
Cluster of deep red, equant pyrargyrites with iridescent patina and a coating of chalcocites, 2.3 x 2.0 x 1.5 cm, Pohla Mine, near Crottendorf, Erzgebirge, Saxony, Germany. © Rob Lavinsky, www.iRocks.com. Used with permission.
Scientists have synthesized pyrargyrite crystals for a variety of projects, including optics research. However, there’s no known jewelry use for this lab-created material.
Silver cleaning solution may enhance red coloration.
Gemmy, transparent, and larger than typical pyrargyrite crystals from Colquechaca, Bolivia and Chañarcillo, Chile have provided a limited amount of cuttable rough.
In the the state of Saxony in Germany, Freiberg, the Harz Mountains, Schlema, and Schneeberg produce fine crystals.
Pyrargyrites, crystals up 0.6 cm across, Freiberg, Saxony, Germany. © Rob Lavinsky, www.iRocks.com. Used with permission.
Other notable sources of crystals include the following locations:
A mass of pyrargyrite with thousands of native silver “wires,” mined in Creede, Colorado during the 19th century. Photo courtesy of liveauctioneers.com and Heritage Auctions.
Faceters have cut pyrargyrites approaching 50 carats. However, gems of such size tend to be too dark to be really attractive.
Store any pyrargyrites away from light sources to prevent loss of color and translucency.
Any pyrargyrite jewelry would need protective settings and be best reserved for occasional evening wear. A copper coin is hard enough to scratch these gems, and household dust, a much more pervasive hazard, has significantly greater hardness than that.
Pyrargyrites contain antimony. Although the health hazards of exposure to this heavy metal may be attributable in part to its frequent association with arsenic and lead, faceters should still take precautions. They should wear protective masks and, ideally, use a glovebox to prevent inhaling or ingesting particles during cutting, polishing, and cleaning. However, wearing or handling finished pyrargyrites should pose no hazards.
Consult our gemstone jewelry cleaning guide for more care recommendations.
Small, red-black pyrargyrites on a matrix of arsenic, Andreasberg, Harz Mountains, Saxony, Germany. © Rob Lavinsky, www.iRocks.com. Used with permission.
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