Well, Oxyclean doesn't seem to have any warnings against using on marble, but you might want to call the company and talk to them. Call the company that makes CLR and ask them about brass. Bar keeper's friend might not be good for the brass, and I'm not sure if it's too abrasive for marble or not. Baking soda might be okay for brass, but it's still an abrasive.
Sorry, I tend to clean with white vinegar. (It's not poisonous, but still kills germs on food surfaces.) I have a stone tile, and I haven't noticed any damage on that.