touch·stone / ˈtəchˌstōn/ • n. a piece of fine-grained dark schist or jasper formerly used for testing alloys of gold by observing the color of the mark that they made on it. ∎  a standard or criterion by which something is judged or recognized: they tend to regard grammar as the touchstone of all language performance.

touchstone a piece of fine-grained dark schist or jasper formerly used for testing alloys of gold by observing the colour of the mark which they made on it; in figurative usage, something which acts as a test of genuineness, a criterion.

Touchstone is also the name of the fool in Shakespeare's As You Like It, who loyally accompanies Rosalind and Celia into exile.