All About Pearls

Pearls are formed when an irritant, such as a piece of sand or shell, enters the oyster and forces it to produce a pearl sac in order to protect itself. Though pearls can be found in numerous species of mollusks, including clams and mussels, only one type is harvested for use in jewelry: the pearl oyster. The production process begins with removing a small object from inside the oyster's mantle by inserting an instrument called an awl through its shell. A thread is looped through one end of the object and then tied around a needle-like object called an "eye." The eye is then inserted into one end of the oyster's body. This process is repeated several times until the internal organs of the oyster are exposed. The diver then inserts a piece of mantle tissue into each of the wounds, which causes the shell to close over it.

Exposure to air irritates the oyster, causing it to secrete nacre, or mother-of-pearl, onto the inserted tissue and into its interior. Contrary to popular belief, a pearl is not formed when an irritant becomes trapped in an oyster's shell. The oyster secretes nacre onto any foreign object inside its body and coats it in layers of nacreous material until it becomes a pearl. The object is nearly invisible at first, but the nacre builds up around it until the pearl becomes large enough to see. The oyster's mantle ultimately forms a pearl sac, which may be several centimetres in length.

The size of a cultured pearl depends on several factors, including the size of the adult oyster and the number of times it is "pearled." The more times an oyster is pearled, the larger its pearl sac will be; however, multiple pearlings can weaken an oyster's body and shorten its life span. In addition to increasing size, each time an oyster is pearled it produces a finer quality pearl. The best quality pearls come from adult oysters that are frequently pearled, but not over-pearled; however, these ideal conditions are rarely achieved in commercial pearl cultivation. Oyster farmers must balance the number of times they should pearl their stock with the pearl's size and quality.

The rate at which a pearl is produced also depends on numerous factors, including the size of the original irritant, the thickness of the nacreous layer and how frequently a divers inserts tissue into each hole. The size of a finished cultured pearl ranges from less than one millimetre to more than ten centimetres in diameter if it is an Akoya pearl. The largest pearl ever found was a pearl that weighed 64.55 carats and was 8.25 centimetres in diameter. It was discovered in 2004 and is believed to have originally been a marine snail host shell.

Source: http://somedayumykids.blogspot.com/2007/08/pearls-are-produced-when-oyster...

Conclusion:

Pearls are only found in natural oysters and mussels. There is not a single case of a pearl being found in man-made creations such as humans or other species. The fluid that the oyster secretes to prevent it from getting infected is called nacre or mother-of-pearl, which forms into a layer around the particle that entered the oyster, eventually forming into one solid pearl. The oyster does not know whether the particle is something that should be in its body, or if it is harmful to it.