A pearl is a hard glistening object produced within the soft tissue (the mantle) of a living shelled mollusk. The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can occur.
Natural Pearls occur spontaneously in the wild and are extremely rare. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those currently sold.
Whether natural or cultured, gem quality perals are almost always nacreous and iridescent, like the interior of the shell that produces them.