Thrandumin

Or Thrandium, depending on your proclivities.

Is a black metal, in both ore and bolt. It is unique in that when refined the metal loses its rigidity, and has only been successfully mined by dwarves of Thrandun, the Black Mountain. The refined metal is then cut into threads and spun on spools. The metal itself is incredibly hard, couple with it's incredible tensile strength it makes for wonderfully protective clothing that excels at shock absorption. It is also used in the manufacturing of high quality firearms, industrial equipment, and various cutting tools. As well as being a sign of incredible wealth. Only five full coats have ever been made of this material, and each were given to the kings of each mountain. Only three are known to remain in existence today.

This metal is also particularly difficult to enchant, requiring a circle of many, many mages in order to even try to enchant the metal, but even with overwhelming arcane power, the enchantment may fail and even if it does take, it fades much quicker than with other more arcanically stable materials.