touchstone

/ˈtəch-ˌstōn/

noun

  1. a fundamental or quintessential part or feature : basis

  2. a test or criterion for determining the quality or genuineness of a thing

  3. a black siliceous stone related to flint that is used to test the purity of gold and formerly silver by the streak left on the stone when rubbed by the metal

a touchstone film of that decade

standard criterion gauge yardstick touchstone mean a means of determining what a thing should be. standard applies to any definite rule, principle, or measure established by authority. criterion may apply to anything used as a test of quality whether formulated as a rule or principle or not.