This is an excellent, and unusual, example of the Bournonites from Yaogangxian, which have frequently been world-class. What is unusual, rather than having the cogwheel habit, is that this is a set of three stacked stereotypical crystals, of which the top one is actually doubly-terminated and the middle one very nearly so! The Bournonites have superb luster (so bright and metallic), and a very slight surface oxidation that creates a wonderful blue, gold, and purple iridescence. I would say that for the quality and size (the upper crystals are about 1.8 cm each), it would be hard to do better in this size even unless you found an equivalent cogwheel. The coating makes all the difference. Note there is a natural crack running through part of the top crystal but it is not repaired or broken. At the very top of its game, this mine's Bournonites are, to me and most others, among the most beautiful in the world.