Webster's English Dictionary

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touchstone

Cross references:
  1. standard              
 
touch.stone \'t*ch-.sto-n\ n 1: a black siliceous stone related to flint 
   and formerly used to test the p urity of gold and silver by the streak left 
   on the stone when rubbed by the metal 2: a test or criterion for 
   determining the quality or genuineness of a thin g