Inicialmente, parece ser sua capacidade de se transformar em vários materiais que o protegem, mas depois na segunda temporada:
He has his powers disrupted by the sonic weapon from season 1 and gets turned to stone completely.
No entanto, na terceira temporada:
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.