Touchstone

A smooth fine-grained, slightly abrasive black stone, usually quartz or jasper, used to assay or test the purity of gold by the streak left when it is rubbed with the metal. After treatment with acids, the colour of the streak can be compared with a standard range of marks produced by standard touch needles of known composition.

The touchstone was the first method of assaying gold and was used at least as early as 500 BC, the earliest touchstones – long strips of slate – having been found in excavations at the ancient city of Taxila in Pakistan with traces of gold streaks still on their surface. Touchstones are still widely used, particularly in the bazaars of India and may be encountered in some other Asian markets. Touchstone testing is not sufficiently accurate for assaying purposes, at best to plus/minus half a carat but is a useful sorting test to indicate approximate caratage. It is not suitable for high carat or white gold.