This is the first time I have seen this in Cassiterite & from an area more known for Ferberite than tin. Delicate layer of 1 mm. black crystals, forming an angular 'hat' that is completely hollow underneath, with large iridescent facets on the back side. Picture is at an angle to try to show a little of each side. From Cerro Tazna, Atocha-Quechisla District, Nor Chichas Province, Potosí Department. Old specimen, originally from Fraziers.