Touchstone from Ugui Village, Higashi-Muro Districe, Wakayama Prefecture, Japan. ‘Nachi Black’ is the popular name of this stone, used for ink stones and touchstones. Previous identifier: 1990-2

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.