Touchstone

wiktionary

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touch +‎  stone

etymonline

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touchstone (n.)

late 15c., from touch (v.) in the Middle English sense "to test" (metal) + stone (n.). Fine-grained black quartz, used for testing the quality of gold and silver alloys by the color of the streak made by rubbing them on it. Also see basalt. Figurative sense is from 1530s.