Several samples of natural black pebbles from various locations, selected for their suitability for gold assaying purposes when used as touchstones: Two black pebbles (with explanatory letter) collected from the ground surface at the tumulus cemetery near Gure (or Guve) about 150 miles E of Sardis in Turkey, by Prof Andrew Oddy.

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.