a black, smooth, glossy stone, used to examine the purity of metals. The ancients called it lapis Lydus, the Lydian stone, from the name of the country whence it was originally brought. Any piece of pebble or black flint will answer the purposes of the best lapis Lydus. Even a piece of glass made rough with emery is used with success to distinguish true gold from such as is counterfeit, both by the metallic colour and the test of aquafortis. The true touchstone is of a black colour, and is not uncommon in many parts of the world.