Natural pearls have always been extremely rare. Only a tiny percentage of mollusks (less than 1 in a thousand) will produce a pearl during their lifetime, and most of these pearls will be unsuitable for jewelry, due to poor size, color or other flaws.
These days natural pearls are even rarer, due to overharvesting and pollution. Most of the natural pearls on the market are reworked from estate jewelry. The vast majority of pearls available today are so-called cultured pearls. Like natural pearls, they are produced by mollusks. But they require the intervention of humans to get the process started.
Pearls can be cultured in both saltwater and freshwater species. A shell bead or a piece of mantle tissue from another individual is inserted into the interior of the animal. This operation must be done very carefully so that the creature not only survives but accepts the "nucleus." If successful, this process induces the animal to form a "pearl sac" whose cells secrete a layer of brownish protein called conchiolin over the irritant. This is followed by the secretion of numerous mineral layers of nacre composed of calcium carbonate in thin overlapping plates.
The composition and structure of this nacre is more or less identical to that which forms under natural conditions. The thin layers create a diffraction phenomenon that is responsible for the surface pearly luster, and if the layers are sufficiently thick and properly aligned, may result in that most prized of all pearl characteristics, an iridescence called "orient."
The culturing process in freshwater pearls takes place from 6 months to 3 years or more, depending on the conditions, the species, and the pearl to be produced. The different species of fresh water mussels are capable of producing a wider range of natural colors than most saltwater mollusks: from white, cream, yellow, gold, silver, blue and brown to gray. They also grow faster and will tolerate multiple tissue nucleations, so that a harvest of 30-40 pearls from a single mollusk in 2 years is possible. The slower growing bead nucleated, saltwater types will generally yield only 1 or 2 pearls per animal. This is why the freshwater cultured pearls are so much less expensive.
Primary sources of production of freshwater cultured pearls are China, Japan, and the United States.