the word 'touchstone' is a compound formed from two old english elements, 'touch' and 'stone', with a literal meaning of "a stone used for touching." its modern abstract sense as a criterion or standard derives from a specific historical practice in metallurgy. a touchstone was a fine-grained black stone, such as basalt or jasper, upon which a streak made by a piece of gold or silver could be compared to streaks made by alloys of known purity. this tangible, physical test provided a reliable benchmark for assessing quality. by metaphorical extension, any reliable test or standard for judging the quality or genuineness of something else came to be called a touchstone.

Structure: touch + stone touch: from middle english 'touchen', from old french 'tochier', of uncertain origin, possibly imitative. functional role: conveys the action of making contact. stone: from old english 'stān', from proto-germanic 'stainaz'. functional role: denotes the material object.