Touchstone
Etymology
From touch + stone. A touchstone was a dark, fine-grained stone jewelers used to test gold and silver: rubbing the metal on it left a streak whose color revealed its purity. From that literal test of genuineness came the sense of any standard for judging quality.
noun · /TUCH-stohn/
A standard or criterion by which the quality or genuineness of things is judged.
What it names
The reference you test against — the fixed standard you hold a new case up to in order to gauge its worth or authenticity.
Example
For her, honesty was the touchstone; every decision got measured against whether she could say it plainly.
Related idea
A touchstone tests authenticity or essential quality; a benchmark is a measured level of performance to match or beat.
noun · /TUCH-stohn/
A hard, dark stone once used to test the purity of gold or silver by the color of the streak they left on it.
What it names
the assayer’s testing stone
Example
Jewelers rubbed the metal on a touchstone to judge its purity.
Related idea
The figurative “standard for judging quality” comes directly from this physical test; compare benchmark.