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touchstone

1. (Science: chemical) 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. Any test or criterion by which the qualities of a thing are tried. The foregoing doctrine affords us also a touchstone for the trial of spirits.

(Science: chemical) (south) irish touchstone, basalt, the stone which composes the Giant's Causeway.