Some metal can stain flesh as well, like those cheap metal mood rings. If you wear them long enough they will color what they are touching green. Gold and such would be fine, but cheaper metals will probably cause problems. I do not know if sealing would help, but if you want to have them wear such jewelry, I suggest only having them wear it for a photoshoot, then having them take it off afterwards.
I think we had a similar question in a thread a while ago. I'll have to search.
Edit:
Here is the thread.
viewtopic.php?f=2&t=4796