That's why Goblins couldn't destroy it in Rage Fire Chasm
Just talk to the random goblin that spawns in the slums with the blue royal apothecary thingy on his back
pay him 100g and he will tell you that He got screw'd on money (pretty much like the ones in heart of pandaria)
He said that it is smelted from Destroyer's Hide... So I assume he is refering to Deathwing