Did You Know?
Our example sentence uses "touchstone" in its most recently acquired sense: "a fundamental part." The earlier sense, "a criterion for quality" (as in "I Love Lucy is often seen as a touchstone for comparison with today's TV comedy shows"), provides a better clue to the original meaning of "touchstone," however. The "criterion" sense, which goes back more than 470 years, alludes to a method of testing the purity of a sample of gold. The sample was rubbed on a piece of dark quartz or jasper -- the "touchstone." The mark it made was compared to adjacent rubbings on the touchstone from gold of known purity. This method has proved accurate enough that touchstones are still used by jewelers today.
