touchstone

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.
2. n. 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.

Century Dictionary and Cyclopedia

1. n. A very fine-grained dark-colored variety of schist or jasper, used for trying the quality of alloys of the precious metals. The alloy is rubbed on the stone, and the color of the streak is compared with that of various alloys of known composition prepared for that purpose and called touch-needles. It was formerly extensively used for ascertaining the fineness of gold, but the facility and rapidity with which exact assays are now made have rendered the touchstone a matter of much less importance. It was the “Lydian stone” of the ancients, under which name (Λυδια λίθος) it is mentioned and its nse described by Bacchylidee (about 450 b. c.), while Theophrastus calls it both the Lydian and the Heraclean stone (λίθος (Ηρακλεία). Βασανίτης, βασανιτης λίθος and βάσανος were names given to it by various Greek authors. It was the coticula of Pliny, whose basanites was a dark-colored, very compact igneous rock, probably a variety of basalt, basaltes and basanites having at a very early period become inextricably confused with each other in meaning. By some these words are believed to have been originally different; by others it is thought that basaltes was a corruption of basanites.
2. n. Any test or criterion by which the qualities of a thing are tried: as, money, the touchstone of common honesty.

Wiktionary

1. n. A stone used to test the quality of gold alloys.
2. n. A standard of comparison or evaluation.

GNU Webster's 1913

1. n. (Min.) 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.
2. n. Any test or criterion by which the qualities of a thing are tried.

WordNet 3.0

1. n. a basis for comparison; a reference point against which other things can be evaluated