Pearls are unique among gemstones, being the only ones found within a living creature and the only ones that requires no fashioning, (cutting or polishing,) before use. Another distinctive feature is its near exclusive use by one gender. Although some efforts have been made to market pearl jewelry to males in recent years, pearls remain the most "feminine" of all gemstones. Designated officially as the June Birthstone, it is, unofficially, a near requirement for brides.
... Cultured pearls are those that form in certain mollusks, (oysters and mussels,) at the intervention of man. A piece of mantle tissue, or a shell bead, is inserted into the interior of the animal. This causes it to secrete a layer of organic material, (conchiolin,) over the irritant, followed by layers of nacre (nay-ker.) The composition and structure of this nacre is essentially identical to that which forms under natural conditions. The thin layers of nacre create a kind of diffraction grating through which light must pass and are responsible for the surface iridescence, called orient, so admired in pearls. Pearls have both a body color and "overtones" of rainbow hues created by the orient. The culturing process takes place over a period of one to three years, depending on the conditions, the species and the desired outcome.
... The commercial process for raising freshwater pearls originated in Lake Biwa, Japan at the end of the 1920's. Various problems, such as pollution and viral diseases, have hampered production in recent years. Progress is being made in restoring the ecosystems and breeding resistant mollusks, so we should see a return of Japanese pearls to a prominent place in the market in the future. At present, however, the premier source is China.
... Although once thought of as an inferior product, advances in technique and marketing practices have made today's Chinese freshwater pearl a true gem. The US continues to command a share of the market with the pearls produced from areas in the South, especially the Tennessee River. Culturing pearls is a delicate process, not assured of success - only 25 - 50% of the altered mollusks produce pearls and generally, only a small percentage of the pearls harvested are of gem quality.
 ... The natural range of colors in freshwater pearls is from white to tan to gray, depending primarily on the species that is used in production. Enhancements are so common that unless it is specifically stated by the seller, you should assume a pearl has been at least bleached to remove dark spots of conchiolin that show through the nacre. More dramatic techniques, such as dying or irradiation, produce pearls with exotic colors such as green, rose and lavender.
 ... Pearls are generally named by their shape; so we have baroques that can be any shape, stick pearls, button pearls, seed or rice pearls, rounds and drops. There are blister pearls, which are created by attaching a bead or other nucleus to the shell of the mollusk and then cutting it out after it has become nacreous. Mabe pearls are assembled from blister pearls, which are filled and glued to a shell base. The term "Keshi" has come to be used for just about any baroque pearl, but in its strictest sense refers to a pearl that spontaneously forms in the oyster during the culturing process, without mantle tissue or bead.
... Although pearls are delicate, they have been successfully used in jewelry for thousands of years. As they are sensitive to heat, chemicals and abrasion, they should be stored in a cloth bag or their own box away from contact with other materials. They should be protected from chemicals such as hairspray and perfume. Wiping them with a damp cloth after wearing and occasional cleaning in mild soapy water is all that's required. Under no circumstances should they be placed in an ultrasonic or steam cleaner. Jewelry settings in rings and bracelets should be protective, or if not, (as in many pearl rings,) the piece should be considered for occasional use only, rather than daily wear.
... Faux pearls have been around for a long time and can consist of a variety of materials such as glass, plastic or shell with various surface treatments meant to simulate the pearls luster. With cultured pearl prices at historic lows, there is little incentive to buy or wear imitations. A rule of thumb when testing a suspect pearl is to rub it across the surface of your teeth. Real pearls will feel slightly gritty, most imitatons will feel smooth.

... The value of a pearl is most related to the thickness and quality of the nacre. Other factors include size, (especially in rounds) shape, and color. In general, the highest prices will be paid for large, round, well colored, unenhanced gems. Factors that influence value in pearl jewelry pieces would add to these general considerations, quality of stringing and degree of matching in size and color.

Salt Water Pearls

Pearls are one of our most ancient gems with records of commercial harvesting going back 2500 years. Their natural occurrence is very rare, with only one in several million shellfish ever producing a pearl. Oysters are the best-known source, but clams, mussels, and abalone also produce pearls.
... For a pearl to form an irritant, (usually a grain of sand,) must get deep enough inside a shell, that the animal cannot expel it. The creature's shell producing system begins coating the irritant with nacre, the shiny substance that you find on the interior of most shells.
... Nacre is composed of the mineral aragonite, with an organic binder called conchiolin. The aragonite forms minute crystals with a radial oriented, concentric structure. You can feel the edges of these crystals by rubbing a pearl against your teeth. The tooth test has long been used for pearls. Most of their imitations will feel smooth, rather than gritty, when rubbed against a tooth.
   

Natural pearls almost disappeared from the market in the late 1800's due to excessive harvesting. Even today, they are extremely rare and demand a huge ransom. Necessity is the mother of invention and, as pearl sources began to dry up, several enterprising Japanese developed methods of culturing pearls in oyster farms.
... Cultured pearl production began in Japan around 1910. Not only does the culturing process provide more pearls, it also allows larger pearls to be produced.
... The process involves inserting a mother of pearl seed, along with a piece of tissue, known as the mantel, into the oyster. After surgery, the oysters convalesce in a "hospital" for four to six weeks. They are then transferred to cages between seven and ten feet under water. Here, they are allowed to grow for three to six years.
... Freshwater pearl farms began in Lake Biwa, Japan around 1928. They have the advantages that up to thirty seeds can be inserted in a single clam and the production time is just three years. The term "Biwa" is nearly synonymous with freshwater pearls, but one should be careful about using it if the source is unknown.
... Freshwater pearl shapes were originally quite irregular, but steady improvements have been made. Today, freshwater pearls rival their saltwater cousins in shape and luster. When grading pearls, it makes no difference if they are from fresh or salt water.
... Unfortunately, pollution and illness have greatly reduced the Japanese saltwater farms in recent years. However, freshwater farming is on the rise. Freshwater pearls are being grown all around the world and the quality is constantly improving.

Facts about Pearls

CHEMISTRY CaCO₃ (aragonite, the outer layer) about 82 - 86%, conchiolin 10 - 14%, water 2%.
CRYSTALLOGRAPHY Aragonite is orthorhombic, with crystals radially oriented.
REFRACTIVE INDEX 1.53 - 1.69
HARDNESS 2.5 - 4.5
SPECIFIC GRAVITY 2.6 - 2.78.
CLEAVAGE None
HEAT SENSITIVE Yes

WEARABILITY* Good
SPECIAL CARE INSTRUCTIONS Avoid heat and all chemicals, including perfume and other cosmetics.

ENHANCEMENTS Dying, common