Pearls form inside a mollusk which is an invertebrate with a soft body, often protected by a shell such as a clam, oyster or mussel. Any mollusk is capable of producing a pearl, although only those mollusks that have shells lined with nacre produce pearls that are used in the jewellery industry.

What shells are pearls found in?

Pearls are made by marine oysters and freshwater mussels as a natural defence against an irritant such as a parasite entering their shell or damage to their fragile body. The oyster or mussel slowly secretes layers of aragonite and conchiolin, materials that also make up its shell.

Do all seashells have pearls?

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.