The member of the mollusk family that produce freshwater pearls are more commonly called mussels. Freshwater mussels can accept up to 50 “irritants” at a time, which can yield up to 40 pearls. Compared to the Akoya oyster, which might yield 5 pearls at a time, and the Tahitian and south sea oysters, which can only yield 1 pearl per oyster, this is a huge benefit. The numbers of Freshwater pearls that can be produced dwarfs the amount of saltwater pearls by a fair margin, and the result is a price advantage.
The formation of freshwater and saltwater pearls has many similarities. In both cases, the oyster and mussel are nucleated with an irritant(s), which entices the oyster/mussel to create a pearl. But there are also many differences. In Saltwater oysters, usually a piece of mantle tissue and a bead are placed into the gonad of the oyster. While in freshwater mussels just the mantle tissue is implanted. No bead is required to nucleate a freshwater mussel, and the mantle tissue is placed in the mantle of the mussel, not the gonad. Freshwater mollusks must be raised with fish, either wild or farm-raised. On the other hand, saltwater oysters can produce pearls without the need for any other species.
There is a huge difference in the quantity of nacre between freshwater and saltwater pearls as well. Because saltwater pearls are nucleated with a bead, their nacre thickness typically ranges from 0.2mm to 3.0mm. Freshwater pearls don’t require a bead for nucleation, and therefore they are almost completely composed of nacre. So if the pearl is 8mm then the nacre thickness is 8mm. This makes freshwater pearl jewelry such as pearl necklaces more durable.