FWP looking like SSP are no rare, about lustre, but shapes make sure the difference, except for round pearls. Anyway, I think it's just a matter of subjectivity, to value so more south sea pearls than freshwater, that for exactly similar aspects: they are all cultured pearls. Only specific kind of SSP, as gold, for exemple, would have to be so high valued. Of course, I talk about first graded appearence, which is suffisant for most people which don't look at the thickness of nacre.