n. 1. (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. [ 1913 Webster ]

2. Fig.: Any test or criterion by which the qualities of a thing are tried. Hooker. [ 1913 Webster ]

The foregoing doctrine affords us also a touchstone for the trial of spirits. South. [ 1913 Webster ]

Irish touchstone (Min.), basalt, the stone which composes the Giant's Causeway. [ 1913 Webster ]