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.
      [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]

WordNet (r) 3.0 (2006):

touchstone
    n 1: a basis for comparison; a reference point against which
         other things can be evaluated; "the schools comply with
         federal standards"; "they set the measure for all
         subsequent work" [syn: standard, criterion, measure,
         touchstone]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "touchstone":
   acid test, assay, barometer, benchmark, blank determination,
   brouillon, canon, check, criterion, crucial test, crucible, degree,
   demonstration, determination, docimasy, essay, feeling out,
   first draft, gauge, graduated scale, kiteflying, measure, model,
   norm, ordeal, parameter, pattern, probation, proof, quantity,
   reading, readout, reference, rough draft, rough sketch, rule,
   scale, sounding out, standard, test, test case, trial, try, type,
   value, verification, yardstick