Biwa pearls are small, unusually shaped cultured pearls from the freshwater mussels of Lake Biwa, Japan. First produced in the 1930s, their quality rivaled that of both natural and cultured saltwater pearls, and they were far less expensive to own. For many years any freshwater pearl was called a Biwa, regardless of its provenance.

Certain bivalve (two-shelled) mollusks, like some mussels and oysters, create pearls as a reaction to an irritant that has found its way inside the shell. In nature, this irritant may be microscopic, such as a parasite. The mollusk secretes nacre, the same material used in shell-building, and over time the nacre builds up to create a pearl. Natural pearls may not necessarily be perfectly round. The longer the mollusk works to create the pearl, the greater the chances of its being misshapen or pitted. Both saltwater and freshwater mollusks make pearls.

When technicians introduce an irritant into a mollusk's shell, it protects itself with nacre as usual, and the result is a “real” pearl. However, the irritant is much bigger than a grain of sand or a parasite. Saltwater mollusks are seeded with a small round bead of shell material (mother-of-pearl), plus a piece of mantle tissue from the mollusk itself, which carries the cells that prompt nacre production. Cultured saltwater pearls therefore, start out larger, have a non-nacre center, are uniformly rounder and are ready for harvest sooner than a natural pearl.

Kokichi Mikimoto is the man most credited with perfecting the techniques of freshwater pearl culturing. He and his associates, experimenting at Lake Biwa, seeded mussels only with soft mantle tissue. This resulted in an all-nacre pearl of good luster and unusual shape—the rice-grain shape was typical. Biwa pearls also emerged in previously unseen colors, and they could be mass-produced. Technicians could plant many bits of mantle tissue in one mussel, and harvest 15 or 20 small pearls from each. From the 1930s on, Biwa set the standard for freshwater cultured pearl quality, and made pearls more affordable than they had ever been.

Biwa pearls came on the market just when the natural, saltwater pearl fishing industry was going into serious decline. Overfishing and pollution had damaged mollusk beds, especially in the Persian Gulf. By the 1930s, the oil industry there also tempted workers away from the great dangers of pearl diving to safer jobs on land. In less than 50 years, however, Biwa pearl production also declined, thanks to similar factors—pollution at Lake Biwa and fresh competition from abroad, especially China.

Today, Biwa’s pearl production is negligible. China’s huge natural and labor resources have put it at the forefront of cultured pearl production, and “Biwa” pearls sold from Japan today sometimes are produced in China. Legally in the U.S., however, no pearl may be sold as Biwa unless it comes from Lake Biwa. Older examples remain the best.