touchstone \TECH-ston\ (noun) - A smooth, black stone (basanite) used to test the quality of gold and silver by the color of the streak produced by rubbing it across the precious metal; any test of genuineness or excellence.
"Creativity is the touchstone of an excellent member of the company team."
Today's compound comes from touch + stone, a calque (loan translation) of Old French "touchepierre," modern day "pierre de touche" (see also Spanish "piedra de toque"). French toucher "touch" (cf. "Touche!" in sportive or verbal fencing) shares an origin with Italian toccare "to touch," whose participle "toccata" refers to a musical piece emphasizing a variety of keyboard touches. Bach's 'Toccata and Fugue in D minor' is a majestic example. "Stone" is Germanic, related to German Stein "stone" and, more distantly, to Russian stena "wall" and Greek stia "pebble"