This is an interesting, "old time". It appears to be a flattened rhombohedron of white calcite that has either grown onto or has adhering to it a bizarre, scalenohedral growth of the same mineral, although more lustrous and more clear. Kind of like a giant thumbtack. Some tiny pyrite crystals sprinkled here and there. Old labels (see pic) accompany the specimen and identify it as from Pibram in 1940. Really interesting calcite specimen!
