For South Sea pearls... not all pearls can have the luster improved. Unfortunately, I don't know the exact method used, just that it involves some machine/tool but I guess it's basically just applying strong buff on the pearl. I got the info from a fellow who sometimes got request to do the job from pearl company. Few things he told me:
1. It can't be done to freshwater pearl as he tried it once and caused the pearl to lose luster instead; and
2. The process is also thinning the nacre, but not much.
Well, I'm planning to visit him some day to see the process myself.