Proustite

Proustite is a soft sulfosalt mineral, silver arsenic sulfide (Ag3AsS3), prized by collectors for its brilliant scarlet to vermilion-red color and adamantine luster. It is one of the two classic "ruby silvers," the arsenic-bearing counterpart to antimony-rich pyrargyrite.

Fresh crystals are translucent and glow a deep blood-red when backlit, but the mineral is notoriously light-sensitive: prolonged exposure to light darkens the surface to a dull grey-black, which is why fine specimens are stored in the dark.

Named after the French chemist Joseph Louis Proust, it is a minor but historically important silver ore found in the oxidized and enriched zones of silver veins.

Proustite forms as a low-temperature hydrothermal mineral in silver-bearing vein deposits, typically in the enriched (secondary) zones where descending solutions redeposit silver. It crystallizes in the trigonal system, often as prismatic or rhombohedral crystals, and is commonly among the last sulfosalts to form in a vein.

It occurs alongside other silver minerals such as pyrargyrite, acanthite (argentite), native silver, and stephanite, plus gangue minerals like calcite, quartz, and dolomite. Classic localities include Chanarcillo in Chile, Sankt Andreasberg and Freiberg in Germany, Jachymov in the Czech Republic, and several Mexican silver districts such as Guanajuato.

Look for a deep scarlet to cochineal-red color with a brilliant adamantine to sub-metallic luster on fresh surfaces; the streak is a distinctive vermilion to scarlet-red. It is very soft (Mohs 2-2.5) and relatively heavy (specific gravity about 5.6).

The surest field clue is light sensitivity: cut or freshly broken faces dull and blacken over hours to days in light. Its main look-alike is pyrargyrite (dark ruby silver), which is darker, more purplish-red, slightly harder, and contains antimony rather than arsenic; cinnabar is softer and has a brighter scarlet streak but lacks the silver association. Cuprite is harder and copper-related.

Proustite is an ore of silver, though it is far more valued as a collector's mineral than as an industrial source because good crystals are rare and fragile. Transparent, gemmy specimens are among the most coveted of all silver minerals.

Because of its softness and light sensitivity, it is essentially never used in jewelry; faceted stones exist only as fragile curiosities for collectors. In metaphysical circles it is sometimes associated with transformation and inner light, but its primary significance is scientific and aesthetic. Specimens must be kept in dark storage to preserve their color.