Proustite

A terminated, highly lustrous, 5 x 3 x 3 mm Proustite crystal nestles in a shallow vug within a white Calcite and Dolomite matrix from the Příbram mining district in the Czech Republic.  The Proustite, a silver-arsenic sulphosalt, appears metallic jet-black, but once strongly backlit releases flashes of deep claret-red.  A slightly smaller, more granular patch of Proustite sits adjacent to this and on the base, Proustite can be seen disseminated throughout the sulphide-rich matrix together with sub-mm crystals of Pyrite.