Inledningsvis verkar det vara hans förmåga att förvandlas till olika material som skyddar honom, men senare i säsong 2:
He has his powers disrupted by the sonic weapon from season 1 and gets turned to stone completely.
Men under säsong 3:
it is revealed that he survives this, and that his power gives him a unique resistance not only to the metal but may also provide the key to a vaccine against terrigenesis (episode 12: The Inside Man). The vaccine idea isn't fully fleshed out though, and I suspect we may see more in a later season.