From one of the best worldwide localities for chalcocite is this gorgeous geometric miniature from the Bristol copper mine in Connecticut. A large, twinned chalcocite crystal serves as the base of another smaller, upright, prismatic and distinctly twinned chalcocite. The pseudo-orthorhombic, matte black chalcocites are twinned on the {110} and these are textbook! Commercial mining began at the Bristol mine in 1837 and carried on until 1953 (according to Mindat.org) - this mine was a very important US copper resource through the Civil War period and beyond, and collectors lament its closing. This is a large thumbnail ("toenail) or small miniature specimen, and is exquisitely formed and dramatically three-dimensional in its display.