Natural freshwater pearls occur in mussels for the same reason that saltwater pearls occur in oysters. Foreign material, usually a sharp object or parasite, enters a mussel and cannot be expelled.

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 ...

Yet the oyster deaths continued long after the ... author of a book on pearls (see History of Pearls and Culture of Freshwater Pearls). "I would never attribute it to an unidentified mystery ...

Most pearls come from oysters, which thrive in both freshwater and saltwater environments. Oysters are bivalves that means their shell is composed of two valves connected by an elastic ligament.