This is a very large and relatively thick sheet of natural silver, sturdy and not wimpy and thin, that is still attached to a chunk of bornite ore. Both the ore and the silver have a beautiful, colorful iridescence (did not show up well in the photo) from the bornite. This was likely a thin vein of silver filling a crack in the matrix ore that was then exposed by the miners, who retained some of the ore with the silver sheet. From the classic Mexican mining locality of Zacatecas.