Many species of shells have a natural nacreous or "pearly" layer of
shell beneath the outermost layer where the colors and pattern are laid
down.  Pearlized shells are made from such species, either by dissolving
away the outer layer with acid, or by physically grinding or buffing it
away until the underlying pearl layer is exposed.  Pearled shells are
worth about nothing as specimens for a serious collector,  but they are
often sold in tourist shops as display items.  They are often priced
rather high because of the work involved in producing them, and because
people who buy them don't know any better.

Paul M.