Several samples of natural black stones from various locations, selected for their suitability for gold assaying purposes when used as touchstones: Four samples of ‘Lydian stone’ or ‘basanite’ from a source near Prague, Czechoslovakia, together with a replica of a medieval touchstone (marked 1978-2) made from this basanite. There are modern gold streaks on the surface.

Touchstones are fine-grained siliceous stones, on whose abrasive surface a gold sample undergoing checks for purity would be rubbed. A streak of gold would be left on the stone’s surface, which could be compared against gold of known quality, to establish the sample’s purity. Touchstones were the main means of assaying the quality of gold throughout history, but remain in use today.