These are cassiterite crystal twins from the Pingwu Beryl Mine in the Huya Township, Mt Xuebaoding, Pingwu Co., Mianyang Prefecture, Sichuan Province, China. This mine is well known for its production of world class mineral specimens like this cassiterite. Cassiterite is an oxide mineral with a chemical formula of Sn O. It is in the rutile group of minerals. This piece consists of a mound-like cluster of silver mica crystals that hosts well defined lustrous cassiterite crystals. They are modified twinned and tetragonal. The crystals are intact and complete on all sides. This piece measures 3 x 2 1/8 x 1 3/4 inches or 76 x 60 x 45 mm.