Emily Dickinson Lexicon

TOUCH-STONE, n. [tuch'stone; touch and stone.]

1. A stone by which metals are examined; a black, smooth glossy stone. The touchstone of the ancients was called lapis Lydius, from Lydia in Asia Minor, where it was found.
2. Any test or criterion by which the qualities of a thing are tried; as, money, the touchstone of common honesty. L'Estrange. Irish touchstone, is the basalt, the stone which composes the Giant's causey. This is said also to be an excellent touchstone.