This specimen is a big (protective) mass of rock holding a protected pocket with an OUTSTANDING crystal of the very rare silver species pyrostilpnite. I have seen many of these, probably more than most, in the US-based collections of Bill Pinch, Josef Vajdak, Kay Robinson, and Mark Feinglos, as well as a few on exhibit in Germany. But for sheer visual pizzazz, this one with its robust upright, freestanding crystal, is really one that appeals to me more than most micro-type species do. The crystal is approx 5.5 x 2 mm in size. Note the label dated 1875, from the Sack collection. From MINDAT: originally described as Feuerblende (Feuer = fire, Blende from blenden = to blind, for the adamantine luster), later translated into Pyrostilpnite (from Greek pyr = fire, stilpnos = bright or shining)