Pearl is a mineral with a hardness of 3 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of calcium carbonate, their full chemical compound being CaCO3,C3H18N9O13.nH2O.

Pearl is one of four main ornamental materials that are classified as "organic" gems, the other three being coral, ivory, and amber.

About 92 percent of pearl is calcium carbonate, in the form of aragonite crystals, held together by an organic substance (concholin), which is identical to the horny outer layer of oyster shells, plus a small quantity of water. Mother-of-pearl has a similar chemical composition, but with less calcium carbonate, and more water, and is used as the nucleus of cultured pearls.

Pearls are undoubtedly the most costly and important of organic gems. They are globular, usually almost spherical cysts, which form inside the tissues of the mollusk. Sometimes they are pear-, egg-, or bean-shaped, or display more pronounced irregularities consisting of roundish outgrowths or even sharp crests.

The color is generally the same as that of the inside of the oyster shell. Most pearls, therefore, are white with a touch of gray to yellowish gray-white, but they may be grayish, blackish, or iridescent from gray to green-blue-violet, and pink (applies to rare pearls produced by marine gastropod mollusks of the Haliotis and Strombus genera).

Pearls are composed of numerous thin, concentric layers, which are deposited successively by the mollusk in an onionlike structure. To some extent, the older the pearl, the bigger it is, and the more numerous are the constituent layers. But in cultured pearls, the inside consists of a spherical nucleus of mother-of-pearl, often taken from the shell of another mollusk, artificially shaped into a bead, but composed of flat, parallel layers, surrounded by a number of concentric layers of nacre deposited around it by the pearl-producing mollusk.

Many mollusks produce pearls, but the most important ones belong to a different species of the genus Pinctada (formerly known as Meleagrina). Less valuable pearls are produced by many other mollusks, including some freshwater bivalves and marine gastropods.

The largest quantities of pearls are harvested from Sri Lanka, the Philippines, China, Japan, northern Australia, the Persian Gulf, and the Red Sea.

Most natural pearls used in jewelry are roughly spherical, even tending to a cylindrical shape, which are most suitable for necklaces. But they may also be irregular in shape. If they have rounded, not too obvious projections, they are known as baroque pearls. These are also pierced and threaded, while larger specimens are used as parts of designs. Pearls may also be pear-shaped, in which case they are normally used as pendants, or they may be flattened at one pole, or both, in which case they are generally rested on a piece of jewelry. Pearls may lose their luster and wear away its nacre due to the dulling effect of acid perspiration on the calcium carbonate.

Cultured pearls are more spherical in shape than natural ones. Their luster and iridescence are not noticeably different from those of natural pearls. They often have highly translucent outer layers, which makes them more liable to deteriorate.

Freshwater pearls, also called nonnucleated cultured pearls, are a product of experiments done in the early twentieth century to improve methods of pearl culture. The pearl produced has no artificial nucleus, but only a small cavity after the fragment of implanted foreign tissue in the mollusk (Hyriopsis schlegeli) has decomposed. These pearls are somewhat egg-shaped, with one pole more pointed than the other, sometimes slightly compressed, almost in the shape of a bean. Their luster is generally very good.

The specific gravity [?] for Pearl is 2.71, its refractive index [?] is 1.53-1.68, and its double refraction [?] is None.

History

Pearls have been known since time immemorial in the Orient and were known to the Greeks and Romans, following the conquests of Alexander the Great.


All manner of fantastic explanations for the origin of pearls were advanced in earlier times, some of them highly poetic. There is, for example, the old eastern legend quoted by Pliny, according to which, oysters rose to the surface of the sea beneath the moon's rays, opened their shells and were fertilized by drops of dew. It was not until the mid-sixteenth century that a Dutch scholar, Rondoletius, recognized that pearls were pathological formations in pearl oysters.


Pearls form when a foreign body such as a grain of sand, or more often a small parasite, finds its way into a pearl oyster which, in self-defense, surrounds the intruder with a cyst, and goes on depositing layer after layer of pearl over it, even when the intruder has been completely encapsulated and rendered incapable of doing any harm.

Industrial Usages

All are considered alongside the more prestigious "precious stones" on account of their very pleasing appearance, which make them invaluable for preparation of items of personal adornment such as rings, bracelets, necklaces, cameos, and to an even greater extent, decorative objects vases and figurines.


Ever since antiquity, efforts have been made to speed up the natural processes of pearl formation and obtain more, bigger, and better-shaped pearls from oysters.


The production of cultured pearls, however, was pioneered in Japan around the turn of the twentieth century by Mikimoto, who achieved the first positive results in 1893 with the production of blister pearls; by Mise, who set up production in 1907; and finally by Nishikawa and Mikimoto, to whom we owe the present method of cultivation, in which the pearls can be harvested after five to seven years, but better results are achieved with longer periods of up to twelve years.


It is often difficult to distinguish natural pearls from cultured pearls by its outside appearance, but one way is by observing the inside of a hole of a pierced pearl with a strong lens or binocular microscope. A succession of concentric layers is characteristic of natural pearls, while a compact, almost waxy-looking nucleus, with a single, clearly different layer around it is characteristic of cultured pearls.


Natural pearls are evaluated according to size, color, luster, regularity of form and compactness. The more watery, translucent ones are less durable, therefore less valuable. A string of pearls of equal diameter is worth much more than one consisting of larger pearls at the center and smaller at the end, because numerous pearls of a uniform size are harder to find. Even a pair of matching pearls is worth more than double the price of a single pearl because of the quantities that have to be sorted out to find two that are identical.


There is a big difference between the value of natural pearls and that of cultured pearls, which on average cost at least ten times less.