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.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Hexagonal (R). Crystals prismatic, rhormbohedral; massive, compact.|
|Colors||Deep red, scarlet to vermilion red.|
|Luster||Adamantine to submetallic.|
|Fracture||Conchoidal to uneven. Brittle.|
|Specific Gravity||5.51 - 5.64|
|Cleavage||Distinct 1 direction.|
|Transparency||Opaque to translucent, some crystals transparent.|
|Pleochroism||Strong in shades of red.|
|Optics||o = 3.088; e = 2.792. 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.|
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.
This mineral has been called “ruby silver” and is a source of ore for this metal. The presence of silver in proustite causes a photochromic effect. Exposure to light over time turns it black.
This material has a bright red streak. Please note: don’t conduct streak testing on finished gems. Test material in inconspicuous spots as a last resort only.
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:
- Cobalt district, Ontario, Canada: small crystals.
- Batopilas, Chihuahua, Mexico: small crystals.
- United States: California; Colorado; Idaho; Nevada.
- Czech Republic; Peru; Sarrabus, Sardinia.
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.
Due to its photochromism, don’t display this gem material under strong light. Furthermore, the stone’s very low hardness makes it a very impractical choice for a jewelry piece. (A coin could scratch it). Consult our gemstone jewelry cleaning guide for more recommendations.