Touchstone

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.

a basis for comparison; a reference point against which other things can be evaluated; "the schools comply with federal standards"; "they set the measure for all subsequent work"