touchstone (n.) late 15c., from touch (v.) + stone (n.). Black quartz, used for testing the quality of gold and silver alloys by the color of the streak made by rubbing them on it. Cf. also basalt. Figurative sense is from 1530s. – Online Etymology Dictionary
The method of using soft stones to test metals was known in ancient Greece, and the touchstone played an important role in the evolution of gold economies – it tests the veracity of metallic values, and looks for gold and silver in alloys. As an art historian, I like the idea that such mark-making can reveal secret identities – like a magic palimpsest that carries traces of gold and silver truths.
What’s interesting about this etymologically is that the term, when it was invented in the fifteenth century, was used immediately in both the literal and figurative senses – most words need time to settle into the metaphoric realm of referential word use. Thus it can also be used as a means of indicating that one is trying to get to the relative merits of a concept – a touchstone signifying the intellectual means by which to test the merits or value of ideas or concepts.
By the time Shakespeare wrote As You Like It in 1599 or 1600, the idea had become so widely known that he embodied it in the character of Touchstone, one of ‘wise fools’, by whom the “truth will out.”
The more pity, that fools may not speak wisely what
wise men do foolishly (1.2.7). – Touchstone