Newberyite

Single crystal of this rare phosphate mineral derived from bat guano and only found at this one locality. This is a rare to find specimen, only the second I've seen. The Skipton Caves were a favoured roosting and breeding site for bats, and at one time contained columns of guano rising up to 12 feet high. The guano was harvested for fertilizer and exhausted by 1895, and the bats are reportedly now gone, and collecting or removal of any material from the caves is prohibited. If you want a rare and unusual thumbnail, here you go.