A great thumbnail of this rare iron arsenic sulfide.  This species is known from a number of localities world wide, but the only ones that I have ever seen for sale (including this one) are from old (pre-1950) finds in the Cerro de Pasco area.  This is a particularly good thumbnail, The ball of gratonite is positioned  nicely on its matrix, and I can see only one missing crystal in the whole cluster.

This is a beautiful thumbnail of one of the rarer South American sulfides.