Some more information about pearls

I found some more interesting information about pearls 

A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate  in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries, and because of this, the word pearl has become a metaphor for something very rare, fine, admirable, and valuable.

The most valuable pearls occur spontaneously in the wild, but they are extremely rare. Cultured or farmed pearls from pearl oysters make up the majority of those that are currently sold. Pearls from the sea are valued more highly than freshwater pearls. Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor, and generally speaking, artificial pearls are easily distinguished from genuine pearls. Pearls have been harvested and cultivated primarily for use in jewelry, but in the past they were also stitched onto lavish clothing. Pearls have also been crushed and used in cosmetics, medicines, and in paint formulations.

Pearls that are considered to be of gemstone quality are almost always nacreous and iridescent, wild or cultured, like the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls of lesser shine or less spherical shape. Although these may also be legitimately referred to as "pearls" by gemological labs and also under U.S. Federal Trade Commission rules, and are formed in the same way, most of them have no value, except as curios.

Creation of a pearl

The difference between wild and cultured pearls focuses on whether the pearl was created spontaneously by nature or with human aid. Pearls are formed inside the shell of certain mollusks as a defense mechanism against a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. The mollusk creates a pearl sac to seal off the irritation.

The mantle of the mollusk deposits layers of calcium carbonate in the form of the mineral aragonite or a mixture of aragonite and calcite held together by an organic horn-like compound called conchiolin. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk's body. These small particles or organisms gain entry when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically an introduced piece of the mantle epithelium, together or without a spherical bead.

Natural pearls are nearly 100% calcium carbonate and conchiolin. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.

Typically, the build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate and a yellowish to white outer zone consisting of nacre. The presence of columnar calcium carbonate rich in organic material indicates juvenile mantle tissue that formed during the early stage of pearl development. Displaced living cells with a well-defined task may continue to perform their function in their new location, often resulting in a cyst. Such displacement may occur via an injury. The fragile rim of the shell is exposed and is prone to damage and injury. Crabs, other predators and parasites such as worm larvae may produce traumatic attacks and cause injuries in which some external mantle tissue cells are disconnected from their layer. Embedded in the conjunctive tissue of the mantle, these cells may survive and form a small pocket in which they continue to secrete their natural product: calcium carbonate. The pocket is called a pearl sack, and grows with time by cell division; in this way the pearl grows also. The juvenile mantle tissue cells, according to their stage of growth, produce columnar calcium carbonate, which is secreted from the inner surface of the pearl sack. With ongoing time the external mantle cells of the pearl sack proceed to the formation of tabular aragonite. When the transition to nacre secretion occurs, the brown pebble becomes covered with a nacreous coating. As this process progresses, the shell itself grows, and the pearl sack seems to travel into the shell. However, it actually stays in its original relative position within the mantle tissue. After a couple of years, a pearl will have formed and the shell might be found by a lucky pearl fisher.

Cultured pearls are the response of the shell on a tissue implant. A tiny piece of mantle tissue of a donor shell is transplanted into a recipient shell. This graft will form a pearl sac and the tissue will precipitate calcium carbonate into this pocket. There are a number of options for producing cultured pearls: use freshwater or seawater shells, transplant the graft into the mantle or into the gonad, add a spherical bead or do it non-beaded. The large majority of saltwater cultured pearls are grown with beads, the trade name of the cultured pearls are Akoya, white or golden South sea, black Tahiti. The majority of beadless cultured pearls are mantle-grown in freshwater shells, tradename Chinese cultured pearls.

Cultured pearls (beadless or beaded) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often 'pre-formed' as they tend to follow the shape of the implanted shell bead nucleus. Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.

When a cultured pearl with bead is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A beadless cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.

Original/unedited content may be found at Wikipedia.org