How about a metal that ignores magic all together, but can never be enchanted? For a weapon, that means it would always bypass magical defenses (short of blink or something where the caster isn't even there), but will pretty much never hurt a monster with DR/Magic?

I don't know what kind of effect armor made of this stuff would have though, maybe granting ac against magic touch attacks-rays-and so on?

Actually, I like this idea alot. How about it?