A touchstone is the thing you measure other things against — the standard you keep coming back to when you want to judge whether something is any good, or true, or genuine. 'Fairness is the touchstone of a good law.' 'For many readers, that novel remains the touchstone.' It takes 'of' or 'for'. The word feels serious and a little literary; you meet it in essays, reviews and speeches rather than in chat. What makes it different from plain 'standard' or 'test' is that a touchstone is usually one concrete example rather than a rule — a particular performance, a particular case — and applying it is quick and revealing, one contact and you know. Originally it was a real object: a piece of dark stone that jewellers rubbed gold against, because the colour of the streak left behind told them how pure the gold was. Everything the figurative word means is already in that: a fast test, honest, hard to fake.