Touchstone
TOUCHSTONE, noun 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.
Irish touchstone is the basalt, the stone which composes the Giant's causey. This is said also to be an excellent touchstone