Proustite

Proustite is an interesting mineral that contains silver in its chemical structure. It is one of the few silver-bearing minerals that can exhibit transparency. Proustite is usually transparent, with deep-red crystals, but may also be a darker, more metallic-looking form. However, even darker, more metallic Proustite will be visibly red and transparent when backlit.

Proustite is light sensitive. Prolonged exposure to bright light will darken its transparency and cause it to become darker. Exposure also may cause a dark, dull film to form on crystal faces; this film can be removed by brushing a specimen with soap and water.

Proustite is very similar to Pyrargyrite, and forms a series with it. Proustite is the arsenic-rich member, and Pyrargyrite is the antimony-rich member. It is often not possible to visually distinguish these two minerals from each other, though Proustite is usually lighter in color. Most good material in collections today are from closed, historical localities.

Proustite is named in honor of Joseph Louis Proust (1754-1826), a French chemist famous for defining the law of definite proportions, also known as Proust's Law.

Several classic European localities have produced highly desirable Proustite specimens. Relatively large crystals have come from the Erzgebirge in Germany at Freiberg, Schlema, and the Schneeberg Districts. Across the border in the Czech Republic, some of the earliest sources Proustite have come from Jáchymov, Krušné Hory Mts, Bohemia. Small Proustite crystals, often associated with Quartz, were once found in the Ste Marie-aux-Mines, Haut-Rhin, Alsace, France.

A more recent producer of good Proustite crystals is Morocco, at the Imiter and Bou Azzer mines. In South America, some of the best examples of this mineral have come from Chañarcillo, Copiapo Province, Chile; and the Uchucchacua Mine, Oyon, Lima Department, Peru. In Canada, good crystal clusters and crusts of Proustites have come from the Cobalt region, Timiskaming District, Ontario.

Pyrargyrite - Often has a darker color and steeper crystals than Proustite; otherwise indistinguishable without complex analysis.
Cuprite,  Sphalerite, and Rutile - Form in different crystal types, have greater hardness.
Rhodochrosite - Different crystal habits, usually lighter in color.
Realgar - Different crystal habits, usually lighter in color, different mode of occurrence.