touchstone

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.

A very fine-grained dark-colored variety of schist or jasper, used for trying the quality of alloys of the precious metals.

Any test or criterion by which the qualities of a thing are tried: as, money, the touchstone of common honesty.

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.

Any test or criterion by which the qualities of a thing are tried.

basalt, the stone which composes the Giant’s Causeway.

A stone used to test the quality of gold alloys.

A standard of comparison or evaluation.

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