A “touchstone” is a figurative standard of value or quality against which something is measured.
The word comes from ancient times when a special stone was used to guard against counterfeit money.
The gold or silver content of coins wasn’t well governed, so phony money was often mixed with other metals and passed off as authentic.
Merchants tested the purity of coins by rubbing them on a hard black stone.
The color of the streak left on the “touchstone” disclosed the coins’ true value.
Ultraviolet scanners provide a kind of touchstone for today’s paper money.
Passing a bill under the scanner gives an instant indication of its authenticity based on a number of security features built into the bill.