Like their marine cousins, many freshwater mollusks can produce pearls. However, this rarely occurs in nature. Today, the majority of pearls on the market are actually cultured freshwater pearls, and they make very popular and affordable jewelry stones.
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Prices of pearls are based on several quality factors. The values provided in this guide are for round cultured pearls. Baroque pearls are about 25-35% the cost of round pearls. Natural pearls are extremely rare, and largely limited to auction and collector’s markets. These can be worth 10 to 20 times an equivalent Akoya cultured pearl.
Many freshwater mussels of the mollusk order Unionoida can produce freshwater pearls, also known as river pearls.
Freshwater and saltwater pearls form in the same manner. An irritant trapped in a mollusk stimulates the production of nacre and the formation of the pearl. Although both varieties share most of their chemical and physical properties, natural freshwater pearls have poorer luster and more irregular, baroque shapes. However, cultured freshwater pearls can now match saltwater pearls in appearance because of improved farming techniques.
Of course, unusual shapes and textures can have their own appeal. Cultivators can also intentionally grow freshwater pearls into large and complex shapes, which offer unique options for the traditional June birthstone.
Multi-armed cultured freshwater pearl
Cultured Freshwater Pearls
Freshwater farming has some advantages over the saltwater process. Freshwater pearls can be seeded with just mantle tissue instead of a mother-of-pearl bead or other material. Cultivators place the foreign mantle tissue in the mantles of the host mollusk. Depending on the species, up to thirty of these fleshy “seeds” can be implanted.
A newly opened freshwater mussel, Hyriopsis cumingii, showing rows of cultured pearls inside. Photo by Istara. Licensed under CC By 3.0.
After one to two years, the pearls can be removed, wrapped in new mantle tissue, and reinserted into the mollusk. This technique makes the pearls grow rounder. With care, the mollusks can survive the removal of pearls, and farmers can use them again to grow more product. The pearls grown in this manner are called beadless. All perliculture is a complicated process, but freshwater cultivation generally has a higher success rate than saltwater.
Commercial freshwater pearl farming originated in Lake Biwa, Japan at the end of the 1920s. Cultivators can insert up to thirty seeds at a time into a single Biwa pearl mussel (Hyriopsis schlegeli). Production time for these pearls is just three years. Although people often use the term “Biwa pearls” to mean any cultivated freshwater pearls, the term strictly refers to those pearls from Lake Biwa.
Unfortunately, problems such as pollution and disease have severely hindered freshwater farming in Japan, despite efforts to restore ecosystems and the use of more resistant hybrid mollusks.
Today, China is the premier source of cultivated freshwater pearls due to advances in farming techniques. Additionally, marketing has made the Chinese freshwater pearl a much-sought gem. The triangle mussel (Hyriopsis cumingii) is the predominant source of Chinese pearls. It produces pearls with smooth shapes in sizes ranging from 4 mm to over 10 mm. The body colors are white to cream, orange, and purple.
The wrinkle shell or cockscomb pearl mussel (Cristaria plicata) lives in lakes and rivers in Asia. In China, Japan, South Korea, and Vietnam, cultivators use it to produce “rice krispie” pearls, so-called due to their irregular shapes and surfaces similar in appearance to the well-known breakfast cereal.
In the United States, the Tennessee River Freshwater Pearl Farm produces pearls from the washboard mussel (Megalonaias nervosa). This mollusk produces pearls in sizes and colors similar to those of the triangle mussel.
Cultivated freshwater pearl colors range from white to tan and gray, depending on the mollusk species. Treatments are very common. Unless a seller specifically states otherwise, assume a pearl has been at least bleached. This process removes dark spots of conchiolin that show through the nacre. More dramatic techniques, such as dyes or radiation, produce pearls with exotic colors such as green, rose, and lavender.
Today, pearl-producing freshwater mollusks are rarely found in the wild due to pollution and other environmental disruptions. Once widespread in Europe, Asia, North America, and South America, these creatures are endangered almost everywhere they can still be found.
Sometimes, jewelers measure individual pearls in grains. One grain equals 0.25 carats. Please note: a grain as a unit of measurement has other values when used for other materials, such as gold.
Some well-known freshwater pearls include the following:
The largest known natural freshwater pearl is the 361.4 grains (90.35 carats) pink colored Survival Pearl. This baroque pearl formed in the shape of a snail that evidently entered a freshwater mollusk in Tennessee. The mollusk survived for 50 to 70 years after that intrusion, creating the unusual shape.
Discovered near Paterson, New Jersey in 1857, the Queen Pearl is round, pink, and translucent. It weighs 93 grains (23.25 carats).
The Abernathy Pearl, 44 grains (11 carats), was discovered in 1967 in the River Tay in Scotland.