I already orderd mine so as soon as they come in ill post pics and let you know the quality... they look plastic to me actually. The only thing I think you would have to worry about is using too harsh a wheel cleaner and damage the coating on them.
I believe all the factory wheel centers are plastic.
You might want to try giving then a spray of clear coat before installing them, maybe even stop by you're local body shop and ask if they can give them a spray next time they are clear coating a car?
I have a roll of 3M clear vinyl, the same kind used for bumper/rock guard. I wonder if something like that could be used to protect the caps?