Whether wild or cultured, gem-quality pearls are almost always nacreous and iridescent, like the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls (technically "calcareous concretions") of lesser shine or less spherical shape. Although these may also be legitimately referred to as "pearls" by gemological labs and also under U.S. Federal Trade Commission rules, and are formed in the same way, most of them have no value except as curiosities.
whether wild or cultured gem-quality pearls are almost always nacreous and iridescent like the interior of the shell that produces them however almost all species of shelled mollusks are capable of producing pearls (technically "calcareous concretions") of lesser shine or less spherical shape although these may also be legitimately referred to as "pearls" by gemological labs and also under us federal trade commission rules and are formed in the same way most of them have no value except as curiosities