Proustite Identification Guide
Identify proustite, the scarlet 'ruby silver' sulfosalt, by its red color, vermilion streak, high density, and light sensitivity.
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What Proustite Looks Like
Proustite (Ag3AsS3), called "light ruby silver," is a silver arsenic sulfosalt prized for its intense scarlet to vermilion-red color. Fresh crystals are transparent to translucent with a brilliant adamantine to submetallic luster. A critical caution: proustite darkens to a dull grayish-black on prolonged light exposure, so museum specimens are kept in the dark.
It forms prismatic to rhombohedral hexagonal crystals with steep terminations, as well as massive and crusty aggregates in silver veins.
Step-by-Step Field ID Checklist
- Color: Look for a bright scarlet-red mineral, often with darkened surfaces if exposed.
- Luster: Note the gemmy adamantine sparkle, unlike dull cinnabar.
- Streak test: Scratch on unglazed porcelain — proustite gives a scarlet/vermilion streak.
- Heft: Feel the high density (it is noticeably heavy).
- Setting: Confirm association with silver, arsenic, and cobalt-nickel minerals in hydrothermal veins.
Key Diagnostic Tests
- Mohs hardness: 2–2.5 (very soft; a copper coin scratches it).
- Streak: scarlet to vermilion-red.
- Cleavage: distinct rhombohedral; brittle with conchoidal fracture.
- Specific gravity: ~5.55–5.65 (heavy).
- Light sensitivity: surfaces tarnish dark on exposure — a useful, almost diagnostic trait among red minerals.
Common Look-Alikes and How to Tell Them Apart
- Pyrargyrite (Ag3SbS3, "dark ruby silver"): deeper, darker red with a purplish-red streak and slightly higher SG; proustite is brighter scarlet with a vermilion streak. They are otherwise nearly identical and often intergrown.
- Cinnabar (HgS): also red with a scarlet streak but much denser (SG ~8.1), with no rhombohedral silver-vein association.
- Cuprite: red but with a brownish-red streak and a copper-mineral association.
- Realgar: more orange-red, softer, and associated with arsenic/sulfur, not silver.
The best separation among red minerals: proustite's bright scarlet vermilion streak, hardness ~2, SG ~5.6, rhombohedral cleavage, and darkening in light.
Where It Is Found
Proustite occurs in the oxidized and primary zones of silver hydrothermal veins. Classic localities include Chañarcillo, Chile, the Erzgebirge of Saxony, Germany, Jáchymov, Czech Republic, Mexico, and the Cobalt district of Ontario, Canada. Store specimens away from light.
Frequently asked questions
How can you tell if it's real proustite?
Look for a scarlet-red, gemmy adamantine mineral that is very soft (Mohs 2–2.5), heavy (SG ~5.6), gives a vermilion streak, has rhombohedral cleavage, and darkens when left in light.
What is the difference between proustite and pyrargyrite?
Proustite (light ruby silver) is brighter scarlet with a vermilion streak, while pyrargyrite (dark ruby silver) is a deeper red with a purplish-red streak and slightly higher density. They are otherwise very similar.
Why does proustite turn dark?
Proustite is light-sensitive; prolonged exposure to light alters the surface and dulls the bright scarlet color to grayish-black, so specimens should be stored in the dark.
Proustite vs cinnabar — how to tell them apart?
Both are red with red streaks, but cinnabar is far denser (SG ~8.1) and is a mercury sulfide from low-temperature deposits, while proustite is a lighter silver mineral (SG ~5.6) from silver veins.
Proustite identified by the community
Recent Proustite specimens identified with Rock Identifier.
