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mikimoto_pearlsWhat is a Pearl?

A pearl is a hard, rounded object produced by certain animals, primarily mollusks such as oysters. Pearls can be used in jewelry and also crushed in cosmetics or paint formulations. The pearl is highly valued as a gemstone and is cultivated or harvested mostly for jewelry.

Pearls are formed inside the shell of certain bivalve mollusks. As a response to an irritant inside its shell, the mollusk will deposit layers of calcium carbonate (CaCO3) in the form of the minerals aragonite or calcite (both crystalline forms of calcium carbonate) held together by an organic horn-like compound called conchiolin. This combination of calcium carbonate and conchiolin is called nacre, or as most know it, 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 a cut piece of the mantle epithelium, together with processed shell beads, the combination of which the animal accepts into its body.

The unique lustre of pearls depends upon the reflection and refraction of light from the translucent layers and is finer in proportion as the layers become thinner and more numerous. The iridescence that some pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface. Pearls are usually white, sometimes with a creamy or pinkish tinge, but may be tinted with yellow, green, blue, brown, purple, or black. Black pearls, frequently referred to as Black Tahitian Pearls, are highly valued because of their rarity; the culturing process for them dictates a smaller volume output and can never be mass produced. This is due to bad health and/or non-survival of the process, rejection of the nucleus (the small object such as a tiny fish, grain of sand or crab that slips naturally inside an oyster's shell or inserted by a human), and their sensitivity to changing climatic and ocean conditions.

Before the days of cultured pearls, black pearls were rare and highly valued for the simple reason that white pearl oysters rarely produced natural black pearls, and black pearl oysters rarely produced any natural pearls at all. Since pearl culture technology, the black pearl oyster found in Tahiti and many other Pacific Island area has been extensively used for producing cultured pearls. The rarity of the black cultured pearl is now a "comparative" issue. The black cultured pearl is rare when compared to Chinese freshwater cultured pearls, and Japanese and Chinese Akoya cultured pearls, and is more valuable than these pearls. However, it is more abundant than the south sea pearl, which is more valuable than the black cultured pearl. This is simply due to the fact that the black pearl oyster Pinctada Margaritifera is far more abundant than the elusive, rare, and larger south sea pearl oyster - Pinctada Maxima, which cannot be found in lagoons, but which must be dived for in a rare number of deep ocean habitats.

Black cultured pearls from the black pearl oyster - pinctada margaritifera - are NOT south sea pearls, although they are often mistakenly described as black south sea pearls. In the absence of an official definition for the pearl from the black oyster, these pearls are usually referred to as "black Tahitian pearls" The correct definition of a south sea pearl - as described by CIBJO and the GIA - is a pearl produced by the pinctada maxima pearl oyster. South sea pearls are the colour of their host pinctada maxima oyster - and can be white, silver, pink, gold, cream, and any combination of these basic colors, including overtones of the various colors of the rainbow displayed in the pearl nacre of the oyster body itself.

The largest pearl ever found so far, came from the Philippines in 1934. It weighed 14 lbs (6.36 kgs) when it was discovered by an anonymous Muslim Filipino diver off the island of Palawan. Later, a Palawan chieftain gave the pearl to Wilbur Dowell Cobb in 1936 as gift for having saved the life of his son. It was first called the Pearl of Allah and is now officially named the Pearl of Lao-Tzu.

Natural pearls are 100% nacre. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or grain of sand enters an oyster (mollusk) and settles inside the shell. The oyster, being irritated by the intruder, secretes the pearl substance called nacre to cover the irritant. This process is repeated for many years, thus producing a pearl.

'Cultured' pearls (nucleated and non-nucleated or tissue nucleated cultured pearls) 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 nucleated cultured pearl is X-rayed it will reveal a different structure to that of a natural pearl. It exhibits a solid center with no concentric growth rings, compared to a solid center with growth rings. A natural pearl however is solid nacre or 100% pearl. Also it is an all-natural shape, round being the most rare formation.

So basically, what makes a pearl a pearl?

They must have an outer nacre (mostly aragonite) layer to be considered a true pearl so only pearls from mollusks with a nacreous mother of pearl lining are considered "true" pearls.

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