Although the traditional source of pearls has been oysters which live in saltwater, mollusks which live in freshwater lakes and rivers can also produce pearls. China has harvested freshwater pearls for many a millennia. The first record mentioning pearls in China was from 2206 BC. The United States was also a major source of freshwater pearls from the discovery of the New World up through the 19th century, when over-harvesting and increasing pollution significantly reduced the number of available pearl-forming mussels.
Freshwater pearls are often somewhat less lustrous than their salt water counterparts. However, they appear in a wide variety of shapes and colors, and they tend to be less expensive than saltwater pearls, making them quite popular. Freshwater pearls are also quite durable, resisting chipping, wear, and degeneration.
Freshwater pearls differ from other cultured pearls in that they are not bead-nucleated. Freshwater mollusks are nucleated by creating a small incision in the fleshy mantle tissue and inserting a piece of mantle tissue from another oyster. This process may be completed 25 times on either side of the mantle, producing up to 50 pearls at a time. The mollusks are then returned to their freshwater environment where they are tended for 2-6 years. The resulting pearls are of solid nacre, but without a bead nucleus to guide the growth process, the pearls are rarely round.
In recent years the Chinese have been able to take the art of culturing freshwater pearls to new levels. In the last decade the quality of pearls produced have become so high that many pearls in the top percentage of a harvest are nearly indistinguishable of their saltwater relatives. Gone are the rice-shape seed pearls as they are now being replaced with round, lustrous pearls of sizes as large as 16mm, mimicking large South Sea pearls. This has created a renewed interest in freshwater pearls as an affordable alternative to the higher priced saltwater.
The Japanese have a distinguished history of culturing freshwater pearls as well. Lake Biwa was once world renowned for producing high-quality freshwater pearls. However, in the mid 1970´s pearl farming all but came to a halt due to pollution in this lake that was once synonymous with freshwater pearls. Today the Japanese are trying once again to farm freshwater pearls in Lake Kasumigaura, utilizing a bead-nucleated hybrid mussel ( Hyriopsis Schlegeli anadonata/plicata hybrid mussels). The resulting pearls have been quite large and unique. But due to the high prices of such pearls the market remains a niche for collectors.
A natural pearl forms when an irritant, such as a parasite or a piece of sand, accidentally enters the body of a particular species of oyster, mussel or clam and cannot be expelled. As a defense mechanism, the mollusk secretes a smooth, crystalline fluid, called "nacre" (the same secretion it uses for shell-building, composed mainly of carbonated calcium), to coat the intruder. As long as the irritant is present, the mollusk continues to add layer upon layer of nacre on the irritant until a lustrous pearl is formed.
A cultured freshwater pearl, which is grown in a clam undergoes exactly the same process. The only difference is that the irritant is a surgically implanted piece of mantle tissue. To culture freshwater pearls, skilled technicians slightly open a host clam´s shell up to 1cm - 1.2cm, cut small slits into the mantle tissue inside both shells, insert a small piece of epithelial membrane (the lip of mantle tissue) from another clam into those slits, and using a fine needle to shape it into round. After implanting, it takes five to seven days for a host clam to cover an irritant with its own tissue and 10 days later it begins producing centric layers of nacre.
Tendering pearl-bearing clams is a continuous process aiming at smoothly developing the pearls to minimize blemishes. The pains-taking process can last four to five years long. Apparently, in freshwater clams, the insertion alone is sufficient to start nacre production. No artificial nucleating bead is needed. Therefore cultured freshwater pearls are composed entirely of pure nacre, making them "real" pearl, just like their natural freshwater and natural saltwater counterparts.