Proustite

Mineral group is sulfosalt; crystal system is trigonal; luster adamantine to greasy.

Silver arsenic sulfide compound, rare today. Many found in early centuries of silver mining. A bright red known as ruby silver. Pyrargyrite is similar, a silver antimony sulfide, and is a darker red, still found in silver mines in Mexico.

Both darken with age, and are heat and light sensitive. Darkening can happen regardless of protection from light - it occurs due to internal processes and cannot be reversed.

Under strong light proustite can appear to glow from within. Sources include Saxony & Harz Mountains, Germany; Charnarcillo, Chile; Joachimsthal, Czech Republic; \Colorado, USA.