Lydian stone; basanite; -- so called because used to test the purity of gold and silver by the streak which is left upon the stone when it is rubbed by the metal. See Basanite.
- A hard black stone, such as jasper or basalt, formerly used to test the quality of gold or silver by comparing the streak left on the stone by one of these metals with that of a standard alloy.
- An excellent quality or example that is used to test the excellence or genuineness of others: "the qualities of courage and vision that are the touchstones of leadership” ( Henry A. Kissinger). See Synonyms at standard.