The Collaborative International Dictionary of English v.0.48:

Touchstone \Touch"stone`\, 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.
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   2. Fig.: Any test or criterion by which the qualities of a
      thing are tried. --Hooker.
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            The foregoing doctrine affords us also a touchstone
            for the trial of spirits.             --South.
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   Irish touchstone (Min.), basalt, the stone which composes
      the Giant's Causeway.
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