touchstone

touchstone is a Noun

Definition:

[1] 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] Fig.:

[3] Any test or criterion by which the qualities of a thing are tried. Hooker. The foregoing doctrine affords us also a touchstone for the trial of spirits. South. Irish touchstone (Min.), basalt, the stone which composes the Giant`s Causeway.