touchstone

Etymology

touch + stone

noun

1. A stone used to check the quality of gold alloys by rubbing them to leave a visible trace.
2. (figurative, by extension) A standard of comparison or evaluation.
   "Expectation of life" is an elastic term: the touchstone here is rather the aggregate amount of work that the locomotive does during its working life. 1950 January, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 12
   In the print firestorm that followed the publication of the royal couple's letters, the generalissima was one polemical touchstone. The Annotations to The Kings Cabinet Opened itself depicted the queen as an enemy to king and country: […] 2012, Ann Baynes Coiro, Thomas Fulton, Rethinking Historicism from Shakespeare to Milton, page 243