Proustite

Proustite — “light ruby silver” — is a silver-arsenic sulfosalt celebrated for its translucent ruby-red to scarlet color. Together with pyrargyrite (“dark ruby silver”), it forms one of the most beautiful and historically important silver-mineral pairs.

Properties

• Formula: Ag₃AsS₃
• Crystal system: Trigonal
• Hardness: 2 – 2.5
• Color: Scarlet to vermilion red, darkening on light exposure
• Streak: Brick-red
• Luster: Adamantine
• Cleavage: Distinct on {1011}
• Density: 5.6 g/cm³

Occurrence

Classic localities: Chañarcillo (Chile) — among the finest 19th-century specimens; Sainte-Marie-aux-Mines (France); Schneeberg (Germany); Cobalt (Ontario). Proustite remains scarce in Chinese deposits.

Identification

Translucent scarlet-red prismatic trigonal crystals + adamantine luster + softness. Distinguish from pyrargyrite (darker, deeper red bordering on black-red) by lighter color when held to strong light. WARNING: proustite is light-sensitive — fine crystals must be stored in darkness or they tarnish to dull black.

Collector Notes

Top-grade Chañarcillo proustite crystals are among the most valuable historical silver minerals — six-figure prices for the best pieces. Always store away from sunlight.