This is an oddball acanthite! It looks exactly like a spinel law twin silver crystal to me, but it is acanthite. I am not sure if acanthite can form spinel law twins as well or if this is a silver crystal that has been replaced by acanthite. Maybe one of the mineralogists out there can clear this up for me? In either case, it is a wonderful specimen that I got over 10 years ago. No damage and 4.3 x 0.9 x 1 cm.