Freshwater cultured pearl means cultured pearls produced in mollusks in freshwater. Normally, freshwater pearls are referred to as “freshwater cultured pearls” in commerce.
Freshwater cultured pearls come from freshwater mussels and are produced by Japan,China, and the US. The most famous type of freshwater cultured pearl is the Biwa (pronounced bee-wah) pearl which use to come from mussels grown in Lake Biwa, Japan’s largest freshwater lake. Because of pollution, production has stopped. Freshwater shell and pearl mussels are from the family Unionidae, from which about 20 different species are commercially harvested. The tones of the freshwater cultured pearls are dictated by the mother shell. White is the most common, followed by pink. Other colors depend on the type of mussels. Big washboard mussels (Megalonaias nervosa) usually have pink pearls, as do the wartyback mussel (Quadrula nodulata). Threeridge mussels (Amblema plicata) have pearls in shades of blue-green and lavender. Muckets (Actinonaias ligamentina) produce fine pink pearls, and sand mussel (Lasmigona costata) have salmon-pink pearls. Other mussels used to produce freshwater cultured pearls include the ebony, heelsplitter, pimple back, elephant ear, mapleleaf, three-ridge pigtoe, pistol grip, and butterfly.
For non-nucleated cultured pearls, the grafting process begins by selecting a suitable donor mussel and cutting a strip of tissue from the mantle. This strip of tissue is then cut into 3mm squares. These square are delivered to a technician who performs the operation. Unlike saltwater bead nucleation, this process is not considered difficult, and technicians need only minimum training to perform the operation. The technician creates small incisions on the upper value and inserts the tissue piece. A small twist of the tissue upon insertion is believed to create a higher ratio of round pearls. After the maximum number of grafts had been performed, the mussel is flipped, and the procedure is performed once again on the reverse side of the value.
The production of freshwater pearls with a nucleus is difficult. The workers at the farm are trained at a high level, which is constantly being improved. It is now possible to implant up to nine nuclei of an average size of 5 to 7 mm into the connective tissue of the mantle, together with a tiny square of mantle tissue from a donor mussel. The mussel is used at the same time for the production of non-nucleated pearls, and 10 to 20 tiny pieces of tissue are implanted in the mantle. Only one nucleus of over 7 mm in size is implanted into the gonad.
After the implantation procedure and after two weeks of intensive care, the mussels are placed foot-up into labeled plastic nets or baskets, which are hung from ropes just below the surface, the water depth rarely exceeds two meters.
The density should not exceed 150 to 225 mussels per one hundred square meters, but the smaller farms probably do not adhere to this rule.
Cleaning take place twice a year, but only the larger farms use cleaning machines. Water quality is monitored constantly, at least on the larger farms. When water temperatures increase, the farmers will add cooler, flowing water from neighboring rivers and canals, but they also use ground water.
the mussels are quite strong and they can survive several days in the dry without damaging the quality of the pearls they hold. They are also able to endure a certain amount of water pollution. However, the farmers do everything to protect them during the rain season. Moreover, nets at the entrance of each pond protect the mussels from their natural enemies.
The farmers determine the end of the growth period in accordance with their financial requirement. Harvests may take place each month, especially on the small farms. But October and November, the late autumn months, are considered the best time as the growth rate slows down due to the cooler water temperatures. The smaller size of the growing aragonite platelets will have a favorable influence on the luster of the pearls.
The skilled workers open mussels and gather the pearls quickly. It took only a few minutes until a colorful mixture of pearls of all possible sizes, shapes and colors appeared on the bottom of the tubes. They were immediately collected and seemed to disappear quickly into sheds where they were weighted and packed without undergoing any further treatment.
The result of the harvests depends on the size of the farm and may range from several kilograms to several tons. It’s usually thought that one hundred mussels produce about 500 to 1,000 grams of pearls. Only about 10 to 20 percent of the marketable pearls are of good quality, and only a tiny amount is of very good quality.
After harvest the pearls are delivered to a first stage factory. This factory is responsible for cleaning and sorting the pearls by size and shape. After this process has been completed, the pearls are considered ready material for processing factories.
The pearls are pre-treated in a warm and cold chemical solution and then bleached. The pearls that exhibit strong coloration will only go through the pre-treated.
After the pearls are bleached they are drilled and then polished with a mixture of cornmeal and wax.
The rounded shapes are sometimes dyed artificially in loud blue, green, yellow, pink, dark grey and black. The dyed pearls are all in the lower price ranges.