American Freshwater Pearls

(via Wahroongai News, Volume 33, No.8, August 1999)

The freshwater mussels of the rivers and lakes of North America produce a surprising range of both natural and cultural pearls.

Natural pearls
North American Indians, who lived along the rivers and lakes of North America, made use of indigenous freshwater mussels both as a source of food and as a valued source of pearls for ornamental purposes. By the mid 16th century Spanish explorers also had become aware of and greatly appreciated this source of natural pearls.

Following the mid-19th century discovery of pearls in Notch Brook near Paterson, New Jersey, an active trade in these pearls and their shells began in North America. This trade exploded some decades later as pearl shelling also developed along the rivers of Ohio, Wisconsin, Tennessee, and Arkansas. By the end of World War II these fisheries were no longer economically viable, as plastic has supplanted pearl shell for the manufacture of buttons. The pearl shelling industry of North America revived during the early 1950, when Japanese demand for shells for the manufacture of bead nuclei suitable implanting into Akoya oysters became paramount. Today, natural pearls are still recovered from these rivers and lakes; but only as a by-product of the pearl shelling industry.

Many species of bivalve freshwater mollusk, belonging to the family Unionidae, inhabit the sandy-gravely bottoms of fast flowing rivers, and to a lesser extent the more muddy bottoms of gravely lakes of the Mississippi and its tributaries. However, over the last century damming these rivers, increased silting from agriculture and strip mining, and the introduction of competitive predators such the zebra mussel have decimated the native mussel population of North America. Today hardy survivors such as the pigtoe (Pleerobema cordatum) maple leaf (Quadrula quadrula), three ridge (Amblema costata), and washboard (Megiaonaias gigantean) are still surviving with some difficulty.

Natural pearl form as a result of small pieces of mantle tissue dislodged from mussels by the bites of predatory fish, by invasion of the mussel’s body tissues by boring parasites, or by the accidental implantation of fragments of shell or fish scales into mantle tissues. The shape of pearls obtained from these freshwater mussels varies with where they grow within the mussel. For example, round pearls form in or around the posterior adductor muscle, adjacent beak area, or the body of the mollusk. It is hypothesized that the opening and closing of valves of the mussel rotates the forming pearl thus producing an even distribution of nacre. Elongated symmetrical shapes of pearl form between the adductor muscle and the hinge: with ridged barrels probably resulting from the forming pearl rotating against a projection from the hinge. Button shaped pearls and turtle backs (with flattish bottoms) are found near the outer lips of the valves. Flattish teardrop shaped ‘wings’ form in the posterior hinge area, while more angular chunky pearls form in the anterior hinge area. Rare bumpy rosebud pearls either form in the beak area or deep within the body of the mussel. Colors of natural pearls range from common white to attractive pastel shades of pink, rose, lavender and purple. The lusters of these pearls vary widely. Major factors controlling the color and luster of these freshwater pearls include the distribution of color across the shell of various species of mussel, location of the pearl in the mussel, the health of the mussel, and water and environmental conditions under which the pearl grew.

Cultured pearls
In the early 1960s the Tennessee Shell Company—the major supplier of freshwater shell for the manufacture of beads for the Japanese Akoya industry—began experiments in the culture of freshwater pearls in a man-made TVA lake near Lexington, Tennessee. These experiments were initiated by John Latendresse and his Japanese-born wife. Twenty years later and with the assistance of available Japanese freshwater pearl cultivating technology, pearl culture farms had been set up and were operating economically in several unpolluted lakes leased from TVA.

American cultured pearls are produced in the following sequence, based on basic Lake Biwa technology:

- Individual hookah-equipped divers collect mature mother mussels from the Mississippi River and its tributaries; care being taken that younger mussels are left to continue breeding.
- The harvested mussels are placed into pockets of ‘kangaroo’ nets that hold up to 18 shells. Shells are then transported to the pearl farm where, following inspection and sorting, the mussels are suspended vertically from rafts made from sealed polythene pipes that are so arranged as to leave sufficient space between both mussels and nets to allow the bivalves to recuperate (for at least a month) to feed, to grow, and indeed to spawn. ‘Mother shells’ are kept separated from ‘sacrifice’ mussels.
- Nucleation occurs at the farm using American-trained technologists and American implant technology based in traditional Japanese methods. Once cleaned, the ‘mother shells’ are held in troughs located in the implant lab.
- Because of their large size American freshwater mussels are multi-nucleated using MOP beads of various shapes.
- Three types of pearls are produced: hemispherical blister pearls, bead nucleated whole pearls of various shapes, keshis.
- Following recuperation under the controlled conditions of laboratory-based ‘ponds’, the implanted mussels are returned to the pearl farm for periods ranging from 1½ (for blister pears) to 3-5 years (for whole pearls) depending on the nature of the implant.
- For blister pearls the shells are first cleaned, their cultured blister pearls sawn from the shell, and the sawn edges shaped and polished either to free from or calibrated sizes.
- Fancy shape free pearls, cultivated in the body of the mussel, are covered by killing the mussel at harvest. Due to shaped nuclei these pearls are available in a range of interesting shapes that include marquise, teardrop, bar, marquise, disc, triangle etc.,--with and without bumps, circles, and nodelles (fish-tails).
- Factors determining the grade of these pearls include their luster, orient, color, shape and color.
- Over the years mortality rates for American freshwater cultured pearls have been reduced to less than 5%.