touch ‧ stone /ˈtʌtʃstəʊn $ -stoʊn/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Origin: touchstone 'stone used for testing the quality of gold and silver' (16-21 centuries) ]
something used as a test or standard
touchstone of
Pupil behaviour was seen as ‘the touchstone of quality’ of the school system.