Black siliceous pebbles suitable for assaying use as touchstones, 1995
Several samples of natural black pebbles from various locations, selected for their suitability for gold assaying purposes when used as touchstones: Two black pebbles from the beach at Staithes, Yorkshire August 1995.
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.