The above freshwater pearls were cultured in freshwater mussels (although not all the same species).
Here is how it's done. A small piece of mantle tissue taken from a sacrificed donor mussel is implanted in the mantle tissue of the farmed mussel and the mussel is returned to the water. The donated piece of mantle tissue forms a pearl sac and secretes nacre to form a pearl. No bead is implanted with the mantle tissue graft, so the pearls form in baroque shapes instead of round, including the flat backs (which happen when the pearl grows near the shell.)
A great many tissue grafts can be placed on each side of the bivalve mussel, and when the mussel is opened, many pearls are harvested. This is one reason these pearls were so low priced. Cultured, non-bead-nucleated, baroque freshwater pearls are the most common pearls in the world.
Freshwater pearl technology has improved a great deal since those pearls were cultured. These days much rounder pearls can be produced with only a mantle tissue graft (no bead.) Part of that is by using different shells (some cross-bred), and some freshwater pearls are even grafted by placing a bead and tissue graft in the gonad of the mussel.
Almost all pearls are cultured (farmed) at this point, and have been for almost 100 years. Natural (wild) pearls will always have value because of their rarity.
