touchstone (tuch stohn)
1: a test or criterion for determining the quality or genuineness of a thing
2: a fundamental or quintessential part or feature 3: A hard black stone, such
as jasper or basalt, formerly used to test the quality of gold or silver by
comparing the streak left on the stone by one of these metals with that of a
standard alloy.
The first sense of "touchstone", " a criterion for quality" (as in "I Love Lucy is often seen as a touchstone for comparison to today's TV comedy shows"), provides an excellent clue to its original meaning. This sense, which goes back more than 450 years, alludes to a method of teesting the purity of a sample of gold. The sample was rubbed on a peice of dark quartz or jasper, i.e., the touchstone, and the mark it made was then compared to adjacent rubbings from gold of known purity. This method has proved accurate enough that touchstones are still used by jewelers today.
Time said of Albert Einstein, "The touchstones of the era -- the Bomb, the Big Bang, quantum physics and electronics -- all bear his imprint."