Mettle is a quality of temperament. It refers to courage, fortitude and disposition. Mettle can also mean the “stuff of which a person is made.” The word is a variant spelling of metal, and its origin comes exactly from the allusion to the temper of a sword.
But it’s only after a stumble or failure that a manager or leader shows his or her true mettle. (Business Week)
But several have come upon a new way to test their virtuosic mettle: Amid the latest violin disks, most of which are conventional studio productions, there are several recordings of live performances, issues without cosmetic editing. (NY Times)