The Probert Encyclopaedia of Rocks & Minerals

TOUCHSTONE

Touchstone, also known as Indian Stone, is a hard, black variety of quartz. Touchstone is traditionally used for making a rough approximation of the fineness of gold, the gold being rubbed across the touchstone leaves a streak which is then moistened with a mixture pf nitric acid and hydrochloric acid. A comparison of the effects of the mixture on the streak determining the purity of the gold.
Research Touchstone