" In that case,” the herbalist said to Eragon, “you had best have this, since it seems you and I are to do most of the fighting.” She handed him her short sword, then drew a poniard with a jeweled hilt from within the folds of her dress.
“What is it made of?” Eragon asked as he peered through the transparent blade of the sword, noticing how it caught and reflected the light. The substance reminded him of diamond, but he could not imagine that anyone would make a weapon out of a gemstone; the amount of energy required to keep the stone from breaking with every blow would soon exhaust any normal magician.
“Neither stone nor metal,” said the herbalist. “A word of caution, though. You must take great care when handling it. Never touch the edge or allow anything you cherish to come near it, else you will regret it. Likewise, never lean the sword against something you might need--your leg, for example. "

― Christopher Paolini , Inheritance (The Inheritance Cycle, #4)