Pearls consist of about 92 percent of calcium carbo|'1ate,or CaCo,,, in the form of aragonite crystals, held together by an organic substance, cohchiolin (about 6 percent), which is identical to the horny outer layer of oyster shells, plus at small quantity of water (about 2 percent). Mother—of-pearl has a similar chemical composition, but with less calcium carbonate (about 66 percent) and more water (about 31
percent) and is used as the nucleus of cultured pearls.

Pearls are undoubtedly the most costly and important of "organic" gems. They have been known since time immemorial in the Orient and were known to the Greeks and Romans, evidently following the conquests of Alexander the Great.

All manner of fantastic explanations for the origins 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 micl sixteenth century that a Dutch scholar by the name of Flondoletius recognized that pearls were pathological formations in pearl oysters.

Crystal system Aragonite crystallizes in the orthorhombic system.

Appearance Pearls 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 apophyges or even sharp crests. The colour is generally much 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 {the latter color 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 (“onion" structure, Fig a). To some extent, the older the pearl, the bigger it is, and the more numerous are the constituent layers. But in cultured pearls, which nowadays tar outnumber the others, the inside consists of a spherical nucleus of mother-of-pearl. often taken from the shell of another mol- lusk. artificially shaped into a bead, but composed of flat. parallel layers (Fig. b), surrounded by a number of concentric layers of nacre deposited around it by the pearl-producing mollusk .

Physical properties Pearls have a hardness of 2.5—4.5; but they are fairly resilient, due to the organic substance they contain and their compact, concentrically layered structure. The density varies from 2.40 to 2.80 g/cm3, but most of those used as gems have a density of about 2.68-274 g/cm3 in the case of natural pearls and 2.73-2.78 g/'cm3 in the case of cultured pearls. The retractive index can only be measured by complex procedures.

Genesis 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. Many mollusks produce pearls. but the most important ones belong to different species of the genus Pinctacle (formerly known as Meleagrina), including P. margarititera P. maxima. P. martensi, P. fucata, and P. vulgaris. These are medium-large bivalve mollusks (about 25 cm) of the Pteriidae family (Filibranchia order). Less valuable pearls are produced by many other mollusks, including some freshwater bivalves and marine gastropods.

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 first serious attempts apparently date from the twelfth century, when the Chinese devised a method according to which variously shaped objects—typically images of Buddha- were placed between the mantle and shell of certain types
of mollusk: after a couple of years it would be covered with a layer of mother-of-pearl. The production of cultured pearls, however, was pioneered in Japan around the turn of the twentieth century. by Miltimoto, who achieved the first positive results in 1893 with the production of blister pearls; by Mise, who set up production in 190?; and finally by Nishlkawa and Mikimoto. to whom we owe the present
method of cultivation, which dates from 1919. Briefly. the procedure consists of cutting a small piece of mantle out of a live oyster, wrapping it around a rnother—of—pearl bead. and inserting it into the living tissue of another oyster. The oysters thus treated are placed in cages suspended from rafts in calm waters, at variable depths depending on the seasons. to ensure a temperature of not less than 50°F.
The depths usually range from 2 to 3 meters in spring—summer and 5 to 6 meters in autumn-winter, thus achieving a balance, over the course of a year, between the effects of shallow water, conducive to rapid growth. and greater depth, which enables the pearls to acquire better color and luster.

The pearls can be harvested alter five to seven years. but better results are achieved with longer periods of up to twelve years. Much shorter periods of growth—even less than halt—are being used in another center of production which is developing alongside the famous Japanese industry, in areas of northern-western Australia, where the average temperatures are higher.

Occurrence The largest quantities of pearls are harvested from Srl Lanka. the Philippines, China. Japan. northern Australia, the Persian Gulf, and the Fted Sea. Pearl production in the Americas is less important and is mainly confinéd to the Gulf of California and the Caribbean.

Natural Pearls
This is the type formed by accident, without human intervention, and was virtually the only one known before the-
beginning of the twentieth century.

Appearance Mostly natural pearls used in jewelry are roughly spherical. and this is the most suitable shape for ordinary necklaces. Pearls may, however. be somewhat irregular in shape. If they have rounded, not too obvious protections, they are known as baroque pearls. These are also pierced and threaded. especially if medium-sized or small while larger specimens are used as parts of designs. for instance. as the head or body of an animal, a human face or a fruit. 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. such as a ring, brooch, or earring, When examined under a 10x or 20x lens, pearls often display small. superficial irregularities, roughly conical protuberances. barely visible furrows arranged in "parallel." or tiny flaws like miniature craters on the moon. sometimes with a cornetlike tail on one side. At higher magnifications. the normally smooth and shining surface displays close set. minute, sinuous lines, evenly distributed throughout The color varies from white with a hint of gray to white with a yellow tinge. but can also be silvery gray or more noticeably yellow. In strong light. pearls have characteristic "pearly " or "nacreous" luster and may also be iridescent. with the emphasis on pink or other colors. which give a very pleasing effect. Alternatively, they can be slightly translucent. revealing faint speckles or marks on the inside, this generally being due to the presence of abnormally large quantities of the organic component. Conchiolin, and water, which makes the pearl more liable to
deteriorate.

Many antique pearls look badly damaged. Sometimes part of the outer surface of nacre has been worn away or there may be loss of luster, caused by dehydration of the organic component or by the dulling effect of acid perspiration on the mineral component (calcium carbonate). Therefore, one often finds that some of the pearls in an antique piece of jewelry have been replaced, usually by modern, cultured
pearls.

Distinctive features Pearls can generally be distinguished by their surface appearance-—which is lustrous but with microscopic. discontinuous wavy lines—from glass imitations with a very different surface or from other imitations covered by a special, minutely granular varnish made from ground fish scales. A much harder problem is distinguishing natural from cultured pearls. Individual pierced pearls can be distinguished by observing the inside of the hole with a strong lens or binocular microscope. A succession of concentric layers (possibly with a dark center, it the pearl is slightly grayish) 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. although it is impossible to judge a single unpierced Dfiarl by its outward appearance. In the case of a string of. say. a few dozen to a hundred or so pearls, simple observation of their outward appearance can be conclusive. In fact. although natural pearls are almost spherical, if one looks closely at a string of natural pearls. they nearly always appear to be bodies revolving about an axis (along which the hole is drilled]. slightly flattened at one or both poles. perhaps even tending to a cylindrical shape. On the other hand. most. although not all, cultured pearls are more or less spherical, even when they have superficial irregularities and protuberances. This is because they consist of few layers of nacre on a strictly spherical support. Nonspherical cultured pearls, which are very similar in shape to most natural pearls, do occur. but are not common and, as a rule, there will be very few on a string.

The physical properties of pearls are not easily measured {except for the density) and are not normally used for recognition; but. on average, natural pearls have a slightly lower density (2.71 g/cm“) than cultured pearls (2.73-2.74 g/'cm3). Grayish natural pearls have a still lower density (2.61 -2.69 g/cm3) due to an excess of conchiolin. To distinguish natural from cultured pearls with any certainty. specialist laboratories use both radiography and the X-ray diffraction method. which give precise information on the arrangement of the internal layers of nacre and the prismatic crystals of aragonite of which they are composed.

Occurrence Most of the few natural pearls harvested nowadays come from the Persian Gulf. Sri Lenka, the Red Sea. and the Philippines; still smaller quantities are collected from the sea off the coast of Venezuela and from the Gulf of California. In other places, such as the seas of Japan and along the northwest coast of Australia. the industry for cultured pearls has now developed to such an extent that the possibility of finding natural pearls as well is disregarded.

Value One of the most valuable gems in antiquity. pearlsare still highly valued today, although not to the same extent. They are evaluated according to size. color. luster. regularity of form. compactness (the more watery. translucent ones are less durable, therefore less valuable). In the case of a number of pearls in a piece of jewelry, much depends on their uniformity of color or. at any rate. how well matched they are. A string of pearls of equal diameter is worth much more than one consisting of pearls of graduated diameters (larger at the center. smaller at the ends. 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 to find two that are identical. But many natural pearls are old or antique and when they are in a poor state of repair. dehydrated or cracked. brittle or yellowed with age. their value is greatly reduced.

Sirnulants Cultured pearls are not really imitations. but something much better. We shall, therefore. be discussing these lully below. Pearls have been imitated. at least since the mid-seventeenth century; hollow spheres of thin glass.coated on the inside with a special varnish made from fish scales. and usually filled with paraffin or wax, were used- For this reason, pearls were once tested between the teeth: if they broke. they were clearly false. They wet, subsequently imitated by (solid or hollow) spheres of glass or mother-of-pearl, varnished on the outside in the same" way. All these imitations are very easily recognizable if observed under a magnifying glass, which shows the typical features of glass fusion around the hole, the paraffin wax filling, and the translucent outer layer in one case. and, in the other, the minutely granular appearance of the special varnish used. From a distance, however, they look Very much like real pearls, and there is a thriving industry for them in both Japan and Majorca.

Cultured Pearls
The cultivation of pearls, which now accounts for the vast majority of pearls issued on the market each year, involves, as mentioned, introducing mother-of-pearl beads into the tissues of certain types of mollusks, which deposit concentric layers of nacre around the foreign body. These spheres are quite large in relation to the final volume of the pearl. in fact, depending on the size of the mollusks. beads from 1 to 3-4 millimeters in diameter are used (nuclei of 6—7 and even 8 millimeters are only used for Australian pearls).

Appearance Cultured pearls are generally more spherical in shape than natural ones. Less regular, baroque or pear-shaped pearls are also found, or specimens slightlyflattened at one or both poles, which are inevitably of more limited use. The most common types of minor irregularities are either a small, almost conical protuberance or small. barely noticeable cavities with a white base, resembling lunar craters. which are also found, though much less often, in natural pearls. Protuberances are partially removed with a file, and the site is used for the hole in pierced pearls. when magnified twenty or more times, cul- tured pearls also display a close pattern of minute. sinuous lines. As with natural pearls. the normal color is gray-white to yellowish gray-white, but cultured pearls sometimes display a rather unattractive greenish tinge in strong light. Their luster and iridescence are not noticeably different from those of natural pearls. One does not normally see the faint gray, dark-centered coloration of natural pearls, but cultured pearls may be a definite gray color, even in the layers near the surface: this coloration is always artificial and due to treatment. Cultured pearls often have highly translucent outer layers because of an excess of water and conchiolin, which makes them more liable to deteriorate.

Australian cultured pearls are a special case. Being produced by large mollusks, they are often 10-12 millimeters in diameter, and even over 15 millimeters. with striking luster great compactness and a very white. almost silvery color, probably due to lack of bleaching, but no iridescence. They may be perfectly spherical, but sometimes have a series of small furrows arranged like the parallels of a globe, or are slightly flattened at the poles, or tend to a biconical shape.

Distinctive features in pierced cultured pearls. the junction between the external layers of nacre and the uniform nucleus at the center may be quite clearly visible through the drill hole, not always easy to see with a lens, but certainly visible with a good binocular microscope of 15x or 20>: magnification. It is most easily recognizable when the hole that has been drilled is at the site of a small protuberance, which has been filed down and which generally has a small cavity bounded on the inside by the convex surface of the spherical nucleus.

Sometimes, when a cultured pearl whose external layers are either too thin or translucent is viewed against a strong light, horizontal lines can be seen. These are the flat, parallel layers of the artificial nucleus and have nothing to do with the concentric layers—never visible, incidentally—characteristic of natural pearls. Even simple observation ot the external shape can provide information useful for purposes of distinction, when one is dealing with whole strings of pearls rather than individual pearls. If the pearls are all perfectly spherical in shape or have only very faint protuberances, rather than slight but extensive spherical defects, it is very probable that there is a turned nucleus inside, forming a sizeable, rigid infrastructure onto which the oyster has been forced to deposit the nacre in a spherical shape. This factor, combined with the two preceding ones (viewing against the light and observation of the drill hole) should almost always make it possible to identify a string of mainly cultured pearls. Australian cultured pearls are unique. These, in fact, have a highly characteristic appearance (and size), which makes them quite easy to identify.

As already mentioned in the discussion of natural pearls. the physical characteristics of this gem are not easy to measure, except for the density. This is usually slightly higher in cultured pearls than natural pearls and is about 2.74 g/cma. When cultured and natural pearls cannot be distinguished by the types of direct observation just described, radiography and X-ray diffraction techniques are used, as they are with natural pearls, and these will positively identify even unpierced specimens.

Occurrence The main center of production for cultured pearls is Japan. Other, less important centers are chiefly along the northwest coast of Australia. More recently, cultivation of pearls has begun along the coasts of lndia.

Value 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. The same criteria are used to evaluate them as to evaluate natural pearls, i.e., color, luster, regularity of form and compactness. When dealing with a string of many pearls, great importance is attached to their general homogeneity. Furthermore. a string of cultured pearls of uniform diameter is worth more than one of graduated diameters and a pair of identical pearls is worth more than two different ones. The thickness of the pearly layer surrounding the nucleus is especially important in evaluating cultured pearls. A thicker layer in relation to the diameter of the nucleus takes longer to produce (the average period of cultivation is four to five years, but sometimes it is extended to six to seven years) and therefore costs more. Furthermore. a very thin layer, sometimes seen in poor quality pearls, is much less durable. Lastly, a thick layer of nacre usually, if not always, gives an appearance closer to that of natural pearls. Despite their rather color and lack of iridescence, Australian cultured pearls are worth much more than the others partly because of their size. Sometimes they have a thick layer of nacre, bacause one of the many qualities of the mollusks used to produce them is the ability to complete their task with considerable speed. Nowadays, however, the tendency is to restrict culture to only three years, more pearls in a shorter time being preferred to pearls with thicker layers.

Simulants These are the same as already mentioned for natural pearls. Majorca is now famous for the production of imitation pearls, which look very convincing at a distance.

Nonnucfeated Cultured Pearls
Around 1935, during experiments to improve methods of pearl culture, it was discovered that pearls could be obtained simply by grafting a piece of soft tissue from another mollusk. In this case. the pearl produced has no artificial nucleus inside, but only the small cavity left after the fragment of foreign tissue has decomposed. Experiments showed that an elongated freshwater mollusk. Hyriopsis schlegeli, was most suitable for the purpose, and the culture was set up at Lake Biwa in Japan, where it has developed considerably over the last two decades. Initially, pearls were obtained with a fairly large internal cavity (half the total diameter), but as the method has been improved. it has become possible to produce pearls with minimal cavities, known as "scars." Further attempts have recently been made in both India and China, in freshwater, and in Australia. where the same type of salt-water mollusks are used as with traditional methods. Because of the environment in which most of these pearls are produced, they are known in the trade as “freshwater pearls," without specifying that they are cultured. But noncultured freshwater pearls are very uncommon.

Appearance Non nucleated cultured pearls are generally somewhat egg-shaped. with one pole more pointed than the other. sometimes being slightly compressed, almost in the shape of a bean. On other occasions, the shape is nearly spherical, but still one pole will be more pointed than the other. Much less typical forms are a flat, button shape. or still more rarely, a shape like a demijohn with no neck.
i.e. rounded bodies with a vaguely pentagonal main crosssection, and, of course. rounded edges. When these pearls are used as part of a string, they are normally drilled crossways to their axes of rotation, except for those that are button-shaped. Their luster is generally very good, almost metallic at the acute pole, diminishing slightly further away’ from it. They have a very compact but translucent appearance. The color is usually silvery white. almost like that of Australian cultured pearls, but a bit warmer. They are often small, like a rather chubby grain of rice and in any case, never largo, the rnaxiniurn size being about 6 or 7 millimeters.

Distinctive features The features mentioned above are characteristic and distinctive, and with a little experience one can recognize a string of nonnucleated, culture pearls by simple examination under a lens, and usually even by eye. But it may be hard to pronounce on a single peart, in which the typical features are not very clear. As always, in case ot doubt, radiography can be used, as car. diffraction diagrams. because when freshwater pearls are exposed to X rays, they exhibit particularly strong fluorescence and phosphorescence.

Occurrence Most of these pearls come from Lake Biwa in Japan; but this type of freshwater culture has also started recently in India and China. The Australian salt water variety is extremely rare at present.

Value Thanks to their luster, compactness, pleasing color, and absence of foreign or artificial components, and despite the handicap of their shape, these pearls are worth at least as much as cultured pearls with a nucleus; in tact, well-shaped specimens are worth quite a bit more, although their price is still of the same order of magnitude.

Simulants Glass imitations, closely’ resembling the small grain-of-rioe—shapect pearls, are made for use in cheap jewelry.