Touchstone
A piece of black basaltic rock, obtained chiefly from Silesia and used for testing the quality of gold. The piece of gold, or metal to be tested, is drawn upon the surface of the touchstone, and the streak left is treated with nitric acid. Nitric acid eats away the streak, if it is brass or any similar alloy, while if gold only the alloy in the gold is attacked. Testing needles of known alloy are then rubbed on the surface of the touchstone, and treated with the acid, and a comparison made. See Testing Needles.