Pearls are formed by a few species of mollusks (oysters, clams, mussels) and even a couple of snails. It starts when an irritant gets into the mollusk. If the irritant or parasite cannot be ejected it is slowly surrounded by a substance called nacre (pronounced NAY-ker). The nacre is produced by the mantle tissue which produces two substances a protein and aragonite a six sided flat crystal. Together these are called nacre. It coats the inside of the shell and any other foreign object in the shell. The nacre accumulates in layers and builds up over time. This is how the foreign object becomes a pearl.
That is the readers digest version of pearl creation. Actually itís a lot more complex. Sometimes the foreign object is a living parasite that kills the oyster before it can be ejected. Other times the object may attach to the inside of the shell and nacre formation cements it to the shell creating a 'blister or button pearl. Most mollusks will produce a concretion that does not contain nacre and that has no gem value, they are commonly dark brown or purplish.
All in all the creation of a fine natural pearl is extremely rare. Only 1 in 10,000 oysters will produce a pearl. Most natural pearls are referred to as seed pearls due to their small size and irregular appearance. Truly fine large pearls are remarkably rare. They command enormous prices and were worth a hundred times their weight in rubies or diamonds before the advent of cultured pearls.
The Chinese knew that pearls could be made by the 1300s but did not expand on this for commerce, rather they made the occasional pearl buddha. A small limestone carving of a buddha would be placed in an oyster and allowed to grow for several years. The oyster would then be harvested and the pearl buddha collected as a miracle.
Cultured pearls were first developed for commercial purposes in Japan about 100 years ago by the Mikimoto Company. Kokichi Mikimoto spent many years developing the techniques beginning in 1893 and this became commercially viable by 1905. By 1912 European Pearl sellers asked the courts in London and Paris to ban the sale of Japanese cultured pearls, but scientist proved that their formation is essentially identical to natural pearls.
Cultured pearls are given a helping hand. The oysters are raised on farms and after 5 or six years of growth they are removed from their beds. Skilled workers, mostly women, carefully open each oyster and insert a mother of pearl bead and a piece of mantle tissue into the shell. With luck one oyster in three will survive the procedure and produce a cultured pearl. Nacre accumulates quite slowly in Japanese waters and it takes about 3 years for a millimeters worth of nacre to accumulate and form a cultured pearl. Some pearl farms in the southern waters from Burma to Australia can boast of a faster nacre buildup, in some cases up to 3 millimeters a year.
Freshwater pearls are produced by yet another method. Several species of mussels in Lake Biwa in Japan are farm raised. After a yearís growth they are notched in many places along the edges of the shell, a piece of mantle tissue is placed in each notch and the mussel is returned to the lake. The mantle tissue forms a 'pearl sac' and the resulting pearl is harvested in about 3 years. Each mussel produces many pearls. The mussel can also be harvested a second time 2 or 3 years later.
Each species of oyster or mussel will create a characteristic form of pearl and the quality of the pearls depends on a variety of factors, most important of these are the cleanliness of the water and its temperature