I love the many faces of Caesar. 
What exactly happens to Luffy when his arms turn black?
He used haki to make it as hard as steal, haven't you been watching the anime?
I have watched an episode here and there, but not in order. So I've only tidbits of OP knowledge from the past year. I understand he has the haki to impose his presence on someone and make them faint (like in the auction house on the island with the grooves), but I didn't know it could be used to turn himself into steel...