This is a most unusual, and attractive for the geometry, pseudomorph from Tsumeb, the mine of so much variety and so many classic and collectible sulfide minerals. This piece consists of one large and a few smaller charcoal-gray tetrahedral crystals on this piece that appear to be Chalcocite pseudomorphs after fairly well-formed Tennantite crystals with the largest crystal measuring two inches across! There is also a 2 cm sidecar crystal on the BAC23k. Both crystals are laced with very fine veinlets of Malachite. Many specimens of this type have been labeled as Tennantite, but the dull and somewhat rough surface texture, as well as disparity in heft, certainly seems to indicate a replacement. The replacement may not be complete, but should at least be majority chalcocite and therefore, when they have this look to them, we have chosen to label them as Chalcocite after Tennantite. This is a complete, display quality specimen, complete all around. The Cooks bought it from the collection of a Colonel GTG Williams, of Cornwall. His prior label notes a purchase date contemporary with when it probably came out, of 1983.