The current recommendation is to wear the pearls only >20 min after applying any products. And to wipe down the pearls with a clean damp sterile cloth as Jeremy puts it. I never go out without sunscreen, and you can see the blob of sunscreen on the pearl afterwards. I just wipe it down and none of my pearls have been less lustrous than they originally were. I can't say chemicals are beneficial to pearls, but I certainly would not forgo sunscreen in favor of jewellery.
Aspiring ninja. Go Storm Shadow!