touchstone

Meanings

1. 1
   noun

   a standard, principle, or criterion by which the quality or truth of something is tested

   Reproducibility became the touchstone for evaluating the experimental claim.

2. 2
   noun

   a fine-grained dark stone on which a streak of precious metal is rubbed to test its purity

   The goldsmith compared the streak on the touchstone with marks made by known alloys.

From touch + stone. Goldsmiths tested precious metal by touching or rubbing it against a dark stone and comparing the streak; the figurative sense turns that physical test into a decisive standard of judgment.

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