Stephanite

Stephanite is a silver antimony sulfide (Ag5SbS4), a soft, brittle, black to lead-gray sulfosalt that has served as an ore of silver. It crystallizes in the orthorhombic system, forming prismatic, tabular, or short crystals with a metallic luster, often in fine combinations with other silver minerals.

Its old name "brittle silver ore" (and German Sprodglaserz) reflects its brittleness compared with the more sectile silver sulfide acanthite. It was named after Archduke Stephan of Austria.

Though not as silver-rich as native silver or acanthite, stephanite occurs in many classic silver districts and was an important contributor to historic silver production.

Stephanite forms as a low- to moderate-temperature hydrothermal mineral in silver-bearing vein deposits, commonly in the primary and enriched portions of the veins. It is often among the later silver minerals to crystallize.

It occurs with acanthite (argentite), pyrargyrite, proustite, native silver, polybasite, galena, and sphalerite in quartz or carbonate gangue. Famous localities include Freiberg in Germany, the Comstock Lode in Nevada, the silver districts of Mexico (such as Guanajuato and Zacatecas), and Sankt Andreasberg.

Look for iron-black to lead-gray crystals or masses with a metallic luster and a black streak; the mineral is soft (Mohs 2-2.5) and notably brittle, which helps separate it from the more cuttable acanthite.

It closely resembles other black silver sulfosalts such as polybasite and acanthite, and reliable separation can require crystal form or analysis: polybasite is often tabular with a reddish internal reflection in thin splinters, while acanthite is sectile (carvable) rather than brittle. Association with other ruby silvers and native silver in a vein setting is a strong contextual clue.

Stephanite is an ore of silver and contributed to production in many historic mining districts, though it is usually a subordinate ore alongside acanthite and native silver. Where abundant, its high silver content made it economically useful.

It is too soft and brittle for any gem or decorative use and has no real metaphysical tradition; well-crystallized specimens are valued by collectors of silver minerals. As an antimony-bearing sulfide it should be handled sensibly, with hands washed after contact.