adamantium, n., D&D-ism: in fantasy RPGs, esp. D&D, an imaginary metal that is harder than diamond; from adamant, an actual stone that was believed in the Middle Ages to be impenetrable.
Ex: GM: "The fireball kills you." Player: "But I'm wearing adamantium plate armor!" GM: "Okay, you're roasted alive, but your armor's intact. Roll up a new character!" [GJ1]
Addendum::For those not into Middle Ages references, a modern reference for the stuff comes from Marvel Comics' character , the mutant whose bones were originally laced with the ultra-hard metal. Without giving explanation how he'd be able to move around with all that stuff in him, of course.