irish touchstone

Webster's Dictionary (GCIDE)

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]