A standard or criterion by which something is judged or evaluated; a test or trial of the genuineness or quality of something. Originally a dark stone used to test the purity of gold and silver.
From 'touch' + 'stone,' referring to a dark siliceous stone (basanite) used by goldsmiths to test precious metals by observing the color of the streak left when the metal was rubbed against it.
Medieval goldsmiths could tell if gold was pure by rubbing it on a special black stone - real gold left a specific color streak! So a touchstone literally 'touched' gold to reveal its true nature, just like how we use standards to reveal true quality.
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