I read somewhere that washing pearls on the silk thread will stretch the thread. I tried this on my newly restrung pearls which I felt were strung too tight. (The strand kinked, they were so tight.) I hung them to dry rather than laying them out. They did smooth out.
Also I used Zeide's suggestion of buffing a scratched pearl with a sunshine cloth. I rubbed for a good 20 minutes on an eBay find and it worked like magic plus the shine is incredibly better. I don't know how long the result lasts. Is this just a temporary fix? Can it be done often or is this just a last ditch fix? Dare I try this on old cultured pearls?