Freshwater mussels, whether in ponds, rivers, or lakes, can produce Pearls
in a similar fashion to their mollusk
cousins in the sea. Almost all freshwater Pearls are cultured, meaning they are made with the assistance of a human. Multiple pieces of donor tissue – up to twenty per valve, or shell - are grafted into the mantle on either side of an opened mussel.
Each small square of donor tissue is given a twist, which is believed to lead to rounder Pearls. However, most freshwater Pearls are not perfectly round, since the Pearls do not start off with a round bead to grow around. The mussels are tended by Pearl farmers for two to seven years while they grow Pearls.
In the beginning of the cultured Pearl industry in China, which is the main producer of freshwater Pearls, a different cockscomb mussel (Cristaria plicata
) was used that allowed up to twenty-five grafts per side, or fifty in all. These Pearls were plentiful, but the quality was not good and they were called "Rice Krispie Pearls" because of their bumpy ovoid shapes.
In the last twenty years, the triangle shell mussel (Hyriopsis cumingii
) has become the preferred choice, and though their Pearls are of better quality, the mussel can accept only twelve to sixteen grafts per valve.
The Biwa mussel (Hyriopsis schlegeli
) in the Biwa Lake in Japan is the origin of the cultured freshwater process. However, the lake became too polluted to sustain the mussel populations, whereby the bulk of freshwater Pearl production was taken over by China. Interestingly, though, the mussels are usually delivered to processors outside of China – many of them in Japan – who open the mussels, and sort and clean the Pearls.
If freshwater Pearls are not of a top grade, they may be further treated in a chemical solution and bleached before dying. Pearls that exhibit the favored typical pearly luminescence
are only bathed in a hot and cold solution (named maeshori in Japan) before being drilled and polished, then matched onto temporary strands, bunched in hanks, and sold wholesale to Pearl dealers and jewelers.
Freshwater Pearls are not as lustrous as saltwater Pearls, but they are less expensive and have a wider variety of shapes and colors. Because they don’t have a bead nucleus, they are solid nacre
and hold up better against chipping and other wear. Freshwater Pearls are varied in color, shape and size. Predominant in the shapes are baroque, or not round-shaped pearls. The round or near-round freshwater Pearl makes up only a small percentage of all freshwater Pearls produced. The colors may range from black and dark purple to pale pink or white. The sizes can run from six to ten millimeters.