Diamonds. The hardest metal known to man!
Yeah, but that doesn't count magical metals because.... because diamonds are totally a metal ;;;; Uh, a carbon metal maybe? ... uh... well, I'm sure in a world of magic they can figure out how to mix metal and diamonds to create something as hard as (or harder than) diamonds but as tough and durable as metal (since diamond armor wouldn't work with how britel it is)
I'm down with the use of materials for high end stuff. I think that would be an interesting dynamic to the magic item creation process that would come from the master of magic game. Like certain magical items or effects can only be created with the right materials available perhaps? The catch I see is balancing what could happen if there is too much monopolizing on certain busted materials.