Pearlsofnature - definition of a pearl

Natural pearls:

A “natural pearl” is one that forms without any human intervention, in the wild, and is very rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. A cultured pearl, on the other hand, is one that has been formed with human intervention on a pearl farm. The vast majority of pearls on the market today are cultured pearls.

One family of nacreous pearl bivalves, the pearl oysters, lives in the sea while the other, very different group of bivalves live in freshwater; these are the river mussels such as the freshwater pearl mussel. Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls grow within certain species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae.

The unique lustre of pearls depends upon the reflection, refraction, and diffraction of light from the translucent layers. The thinner and more numerous the layers in the pearl, the finer the lustre.

The iridescence that pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface. In addition, pearls (especially cultured freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black.

Freshwater and saltwater pearls may sometimes look quite similar, but they come from different sources.

Natural freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland. However, most freshwater cultured pearls sold today come from China.

Saltwater pearls grow within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.
The difference between natural and cultured pearls focuses on whether the pearl was created spontaneously by nature – without human intervention – or with human aid. Pearls are formed inside the shell of certain molluscs: as a defence mechanism to a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. The mollusc creates a pearl sac to seal off the irritation.

The mantle of the mollusc deposits layers of calcium carbonate (CaCO3) 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 animal’s body. These small particles or organisms enter the animal 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 mollusc, and settles inside the shell. The mollusc, 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 (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). In a pearl cross section such as in Fig. 6 these two different materials can be seen. The presence of columnar calcium carbonate rich in organic material indicates juvenile mantel tissue that formed during the early stage of pearl development. We know, e.g. from human medicine, that displaced cells with a well-defined task, continue to perform their function. The term “cyst” is applied to such situations. The displacement may occur with 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 mantel, 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 a lucky pearl fisherman might find the shell.

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