As Kadir Munshi says, “pearls have no pedigree”; their beauty is not to be traced to their origin, but exists wholly in the excellence of the surroundings in which they develop.

The pearl-bearing mollusks are luxurious creatures, and for the purpose of protecting their delicate bodies they cover the interior of their shells with a smooth lustrous material, dyed with rainbow hues, and possessing a beautiful but subdued opalescence. No matter how foul, how coral covered, or overgrown with sponges or seaweeds the exterior may be, all is clean and beautiful within. This material is nacre or mother-of-pearl. It consists ordinarily of an accumulation of extremely thin semi-transparent films or laminÆ of a granular organic substance called conchiolin, with the interstices filled with calcareous matter. The nacre decreases in thickness from the hinge toward the lip of the shell, and terminates a short distance from the extreme edge.

Next to the nacre is the middle layer or the shell proper. In species of Margaritifera, this stratum is commonly formed of layers of calcareous prisms arranged vertically to the shell surface. External to this middle or prismatic layer is the epidermis or periostracum, the rough outer coating of varying shades, usually yellow or brown. Where the waves are rough, and the bottom hard and rocky, this covering is thick and heavy, to afford greater protection; but where the waters are smooth and gentle, and the bottom free from rocks, Nature—never working in vain—furnishes only thin sides and slight defense. As is the case with the nacre, the prismatic layer and the periostracum decrease in thickness from the hinge to the edge, and the inside lip of the shell shows the gradual union of the three superimposed layers. The two outer layers are formed by the thick edge of the mantle, the remaining portion—or nearly the entire surface—of this organ secretes the nacral layer.

Not only is the interior of the shell made lustrous and beautiful, but this tendency is exerted toward all objects that come in contact with the soft body of the mollusk, either by intrusion simply within the shell, or deeply within the organs and tissues of the animal itself. All foreign bodies—such as small parasites, diatoms, minute pebbles, etc.,—irritate the tender tissues of the mollusk, and stimulate the pearly formation which in course of time covers them. At first the nacreous covering is very thin; but with added layer after layer the thickness is enhanced, and the size of the object increases as long as it remains undisturbed and the mollusk is in healthful growth.

Chemically considered, aside from the nucleus, the structure of pearls is identical in composition with that of the nacre of the shell in which they are formed. Analyses have shown that those from the fresh-water mussels of England and Scotland, and from the pearl-oysters of Australia and of Ceylon, have nearly identical composition in the proportion of about 5.94 per cent. of organic matter, 2.34 of water, and 91.72 per cent. of carbonate of lime.^[70] The specific gravity ranges from nearly 2 to about 2.75, increasing with the deposit of the nacreous coatings. The following summary by Von Hessling^[71] shows the results of certain determinations of specific gravity:

The distinctive characteristic, the great beauty of a true pearl, is its luster or orient, which is a subdued iridescence, rather than the glittering brilliance of the diamond; and unless the shelly growth be lustrous it does not rank as a gem pearl, no matter how perfect its form or beautiful its color. This luster is due to the structural arrangement of the surface as well as to the quality of the material. The nacreous material forming true pearls, and likewise mother-of-pearl, is commonly deposited in irregular tenuous layers, very thin and very small in area compared with the surface of the pearl. These laminÆ overlap one another, the surfaces are microscopically crumpled and corrugated, and the edges form serrated outlines. The greater the angle which the laminÆ form with the surface, the closer will be these serrated outlines, and where the plane of the exterior lamina is parallel with the plane of the surface the lines are not present. This arrangement causes the waves of light to be reflected from different levels on the surface, just as in a soap bubble, and the minute prisms split the rays up into their colored constituents, producing the chromatic or iridescent effect.

The cause is wholly mechanical, and an impression of the surface made in very fine wax shows a similar iridescence. Also, if a piece of mother-of-pearl be immersed in acid until the surface lime or shelly matter is dissolved, the pellucid membrane shows the iridescence until it is so compressed that the corrugations are reduced. About two score years ago an Englishman invented steel buttons with similar minute corrugations producing pearly effect, but the manufacture was unprofitable, owing, principally, to their liability to tarnish.

In the shells of some mollusks—as the edible oysters (Ostrea) or the giant clam (Tridacna),—there is almost a total absence of the crumpled corrugated laminÆ, and, consequently, there is little luster. In others the nacre is of better quality, resulting in superior orient, and it probably reaches its highest degree of perfection in the pearl-oyster (Margaritifera).

As the curvature of the surface of pearls is greater, and the minute striÆ are more numerous, than in ordinary mother-of-pearl, it follows that the iridescence is likewise greater.

Superior nacre is more or less translucent, depending on its quality; and to the iridescence of the outer laminÆ is added that of many interior ones, so that the luster is vastly increased. The position of the pearl within the shell may greatly affect the quality of the material and, consequently, the orient. The choicest are commonly found within the soft parts of the animal, and those of poorer quality are at the edges of the mantle, or within the fibers of the adductor muscle of bivalves.

The structure of pearls may be studied by examining thin cross sections under the microscope, or by transmitted polarized light. It appears that ordinarily a pearl is made up of many independent laminÆ superimposed one upon another “like the layers of an onion,” or, rather, resembling the leaves near the upper part of a well formed cabbage. When subjected to sufficient heat, the laminÆ separate from each other, as do shells of edible oysters and similar mollusks under like conditions. When broken by a hammer, a pearl may exhibit this laminated formation. If not split directly through the center, the central section may retain the spherical form; and as this commonly remains attached to one of the parts, its concave impression appears in the other portion of the broken pearl. The outer laminÆ of many pearls may be removed with a fair prospect of finding a good subjacent surface, and this may be continued until the size is greatly reduced. These laminÆ are not always similar in color or luster.

However, not all pearls are laminated in this manner. Instead of superimposed layers, some of them exhibit a crystalline form, composed of beautiful prismatic crystals radiating from the center to the circumference. In at least one oriental pearl examined, these crystals were in well defined arcs, and were further separated into concentric rings of different degrees of thickness, depth of color, and distance apart. Another specimen—a Scotch pearl—combined in separate layers both the laminated form and the crystalline structure.

Dr. Harley points out that some crystalline pearls apparently originate in mere coalescences of mineral particles, rather than in well defined nuclei.^[72] Microscopic sections of crystalline pearls convey the idea that the prisms branch and interlace with one another, and also that in some instances they are of fusiform shape. However, these appearances seem to be due simply to the cross sections having cut the prisms at different angles.

Pearls showing these types were exhibited at a meeting of the Royal Society of London, June 8, 1887. That exhibit also contained a section of a west Australian pearl of curiously complex crystalline formation; instead of one central starting-point, it had more than a dozen scattered about, from which the crystalline prisms radiated in all directions.

Since the three superimposed layers of the shell are secreted by separate parts of the mantle, viz., the nacre by the general surface, the prismatic layer by the inner edge, and the epidermis by the outer edge, it follows that if a pearl in course of formation is moved from one of these distinctive portions of the palial organ to another, the nature of its laminÆ changes. Thus, if a pearl formed on the broad surface of the mantle is moved in some way to the inner edge of that organ, it may be covered with a prismatic layer; if then moved to the outer edge it may receive a lamina of epidermis, and then by changing again to the broad surface of the mantle it receives further coats of nacre.

The structure of pearls from univalve mollusks, such as the conch, the abalone, etc., as well as those from some bivalves, as the Pinna, for instance, differs from that of the true pearls formed in species of Margaritifera. Instead of the alternate layers of conchiolin and of carbonate of lime, many of these have an alveolar structure. When greatly magnified, the surface of a Pinna pearl appears to be formed of very small polygones, which, as decalcification shows, are the bases of small pyramids radiating from the nucleus. The walls of these pyramids are formed of conchiolin, and they are filled with carbonate of lime of a prismatic crystalline structure. This is simply a modification of the parallel laminÆ in the Margaritifera pearls, for, as Dubois points out, in some sections we can see portions where the alveolar formation has proceeded for a time coincidentally with the lamellar form.

Pearls are affected by acids and fetid gases, and may be calcined on exposure to heat. Their solubility in vinegar was referred to by the Roman architect Vitruvius (“De Architectura,” L. viii. c. 3) and also by Pausanias, a Greek geographer in the second century (“Hellados Periegesis,” L. viii, c. 18); but it seems that there could be little foundation for Pliny’s well-known anecdote in which Cleopatra is credited with dissolving a magnificent pearl in vinegar and drinking it—“the ransom of a kingdom at a draught”—to the health of her lover Antony.^[73] It is no more easy to dissolve a pearl in vinegar than it is to dissolve a pearl-button—for the composition is similar, and one may easily experiment for himself as to the difficulty in doing this. Not only does it take many days to dissolve in cold vinegar the mineral elements of a pearl of fair size, but even with boiling vinegar it requires several hours to extract the mineral matter from one four or five grains in weight, the acid penetrating to the interior very slowly. And in neither case can the pearl be made to disappear, for even after the carbonate of lime has dissolved, the organic matrix of animal matter—which is insoluble in vinegar—retains almost the identical shape, size, and appearance as before. If the pearl is first pulverized, it becomes readily soluble in vinegar, and might be thus drunk as a lover’s potion, but it would scarcely prove a bonne bouche.

Pearls assume an almost infinite variety of forms, due largely to the shapes of the nuclei, and also to their positions within the mollusk. The most usual—and, fortunately, also the most valuable—is the spherical, resulting from a very minute or a round body as a nucleus and the uniform addition of nacre on all sides. Of course, spherical pearls can result only where they are quite free from other hard substances; consequently they originate only in the soft parts of the mollusk and not by the fixation of some nucleus to the interior surface of the shell.

The perfectly spherical pearls range in weight from a small fraction of a grain to three hundred grains or more, but it is very, very rare that one of choice luster weighs more than one hundred grains. The largest of which we have any specific information was that among the French crown jewels as early as the time of Napoleon, an egg-shaped pearl, weighing 337 grains. The largest pearl known to Pliny in the first century A.D. weighed “half a Roman ounce and one scruple over,” or 234½ grains Troy. These very large ones, weighing in excess of one hundred grains, are called “paragons.” The small pearls—weighing less than half a grain each—are known as “seed-pearls.” The very small ones, weighing less than ¹/₂₅ of a grain, are called “dust-pearls.” These are too small to be of economic value as ornaments.

Slight departures from the perfect sphere, result in egg shapes, pear shapes, drop shapes, pendeloque, button shapes, etc. Some of these are valued quite as highly at the present time as the spherical pearls, and many of the most highly prized pearls in the world are of other than spherical form. Indeed, pearls of this kind are found of larger size than the perfectly round pearls. The egg-shaped pearl,^[74] called “la RÉgente,”—one of the French crown jewels sold in May, 1887—weighed, as stated above, 337 grains. The great pear pearl described by Tavernier—“the largest ever discovered”—weighed about 500 grains. A button pearl received from Panama in 1906 weighed 216 grains.

Wider departures from the spherical form result in cylindrical, conical, top-shaped, etc. Some pearls present the appearance of having been turned in a lathe with intricate tooling. Remarkable examples of these “turned pearls” have been found, competing in their circular perfection with the best work of a jeweler’s lathe.

Many standard varieties of non-spherical, but normally shaped pearls, are recognized by the fishermen and the jewelers. For instance, in the nomenclature of the American fishermen, bouton, or button pearls are divided into “haystacks” and “turtle-backs,” according to the height of the projection. Also, certain imperfections result in distinguishing names: “bird’s-eye” refers to a pearl having a little imperfection on the best surface; “ring-arounds” have a dark or discolored ring about them; and “strawberries” have numerous minute projections on the surface.

During its growth, a spherical pearl may come in contact with a foreign body, such as grit or a vegetable film, and the additional nacral layers envelop the adjacent matter until it is entirely concealed within the pearl, its position being recognized only by the excrescence on one side, and, with continued increase in size, even this may be almost overcome.

Sometimes double, triple, or multiple pearls are formed; each of these may have a separate nucleus and grow independently for a time until they adjoin each other; continuing to grow, they become so united as to form a connected mass. The “Southern Cross” is a remarkable example of this. It appears to consist of seven nearly spherical pearls attached to one another in a straight line, and one projecting from each side of the second in the row, thus forming a Roman cross.^[75]

A few years ago, near Sharks Bay, on the coast of western Australia, a cluster was found containing about 150 pearls closely compacted. This cluster measured about one and a half inches in length, three quarters of an inch in breadth, and half an inch in thickness.

When a growing pearl is very near to the nacreous lining of the shell, the pressure between the two hard substances results in a rupture of the pearl-forming sac and the epithelial layer of the shell, and the pearl comes in actual contact with the nacre. The pearl gradually becomes attached to the shell, and the under portion is prevented from growing further; the upper or exposed surface receives other layers, resulting in the formation of a bouton. As the shell around the pearl continues to grow, it gradually closes about, and almost wholly conceals the pearl. Since it is constantly wasting away on the exterior surface as it grows on the interior, it follows that in time the shell passes the pearl quite through to the outside, where it rapidly decays. Thus the oyster virtually forces the annoying intruder directly through the wall of its house instead of by way of the open door, and magically closes the breach with its marvelous masonry.

These embedded pearls are generally faulty and of diminished luster, but in the aggregate, large quantities of imperfect ones, and especially half and quarter pearls, are secured in this manner. Sometimes—particularly in the Australian fisheries—large pearls are thus found, weighing twenty, forty, sixty, and even eighty grains; and when the faulty outside layers of nacre are removed, a subjacent surface of fine luster may possibly be revealed. In bivalves, these adherent pearls are commonly in the deep or lower valve, except in those unusual cases where the mollusks have been lying in a reverse position. At the fisheries, the surfaces of the shells are carefully inspected for evidence of pearly nodules, and these are broken open in search for encysted objects. Cutters of mother-of-pearl occasionally find embedded pearls of this kind which have escaped the vigilant eyes of the fishermen.

We read of an instance in an important paper treating of the jeweling trade of Birmingham: “A few years since [the paper was written in 1866] a small lot of shells was brought to Birmingham, which either from ignorance or mistake had not been cleared of the pearls at the fishery. A considerable number were found and sold, and one especially was sold by the man who had bought the shell for working into buttons, for £40. The purchaser, we believe, resold the same for a profit of £160; and we have heard that it was afterward held in Paris for sale at £800.”

A choice gem which was found in New York, in October, 1905, in an Australian shell, sold finally for $1200.

The intrusion and continued presence of grains of sand or similar material between the mantle and the shell causes the formation of nacre over the foreign body, resulting in a chicot (blister pearl), or possibly a quarter or a half-pearl. The growth of a chicot sometimes results from the mollusk covering a choice pearl which has become loosened from the soft tissues and adheres to the shell, as above cited. Hence, it is sometimes desirable to break a chicot to secure its more valuable inclosure. In the account of his interesting pearling experiences on the Australian coast, Henry Taunton states: “During the first season’s shelling at Roebuck Bay, we came across an old worm-eaten shell containing a large blister, which was removed in the usual manner by punching a ring of minute holes around its base; a slight tap was then sufficient to detach it. For many weeks it was untouched, no one caring to risk opening it, for if filled with black ooze, which is frequently the case, it would be of little value. At last, baffled in his attempt to solve the problem, and emboldened by an overdose of ‘square face,’ the skipper gave it a smart blow with a hammer, which cracked it open, and out rolled a huge pearl, nearly perfect, and weighing eighty grains. A few specks and discolorations were removed by a skilful ‘pearl-faker,’ and it was sold in London for £1500.”^[76]

Blister pearls are also caused by the defensive or protective action of the mollusk in resisting the intrusion of some animal, as a boring sponge or a burrowing worm, which has begun to penetrate the outer layers of the shell. This stimulation causes the mollusk to pile nacreous material upon the spot, thus making a substantial mound closely resembling a segment of a large pearl. This walling-out of intruders is not the result of intelligent forethought or of instinct, analogous to the repairing of a damaged web by a spider, or the retunneling of a collapsed gallery by ants; it is a pathological rather than an intelligent action.

When the nucleus of a pearl is large and very irregular, it necessarily follows that the deposited nacre roughly assumes the irregular outline of the inclosed object. This is strikingly shown in pearls covering a minute fish, a crayfish, or a small crab. Several specimens have been found in which the species could be identified by examination through the nacreous coating.

In the American Unios there is a strong tendency to produce elongated pearls near the hinge of the shell, which are consequently known as “hinge pearls.” The occurrence and form of these suggest that their origin may not be due to nuclei, but that they result from an excess of carbonate of lime in the water, and that the animal stores a surplus of nacre in this convenient form. There are several standard forms of these hinge pearls. Many are elongated or dog-toothed, some are hammer-shaped, others resemble the wings of birds, the petals of flowers, the bodies of fish, and various other objects. A large percentage of the pearls found in Unios of the Mississippi Valley are of these types.

Some irregular pearls or baroques are very large, weighing an ounce or more. A well-known example is the Hope pearl, described on page 463, which weighs three ounces. These monster pearls sometimes assume odd shapes, such as clasped hands, the body of a man, lion, or other animal, etc.

Although baroques may have a pearly luster, they are not highly prized unless unusually attractive, and they have little permanent value, apart from their estimation in the eyes of admirers of the curious and unique. They are used largely in l’art nouveau, and in forming odd and fanciful objects of jewelry, the designer taking advantage of the resemblance which they bear to common objects of every-day life, and by additions of gold and other ornaments completing the form which nature had merely suggested.

Some remarkable examples of baroque mountings have been produced, and a few are to be found in most of the large pearl collections. In a single case in the Imperial Treasury at Vienna are baroques forming the principal parts or figures of a horse, stag, lamb, tortoise, lizard, cock, dragon, butterfly, gondola, hippopotamus, female bust, and three mermaids. Other well-known collections are those of the royal family of Saxony in the GrÜne GewÖlbe at Dresden; those in the Palace of Rosenberg at Copenhagen; in the Waddesden (Rothschild) collection of the British Museum; among the jewels in the Louvre in Paris; with the treasures of the Basilica of St. Mark in Venice; and in the museum of the University of Moscow.

A remarkable pearl-like ornament more common in Asia than in the Occident, is the coque de perle, which is an oval section of the globose whorl of the Indian nautilus. The exterior or convex surface is highly lustrous, but the material is very thin. It is commonly provided with a suitable filling or backing of putty or cement to impart solidity, and is used like a blister pearl. Sometimes two perfectly matched coques de perle are filled and cemented together, giving the appearance of an abnormally large oblong or nearly spherical pearl.

The color of pearls has no connection with the luster. In general it is the same as that of the shell in which they are formed. Black pearls are found in the black shells of Mexico, and pink pearls in the pink-hued Strombus of the Bahamas. Ceylon pearls are seldom of any other color than white, and Sharks Bays are almost invariably quite yellow or straw-colored, while those of Venezuela are commonly yellowish tinged. But from other localities, pearls simulate every tint of the rainbow, as well as white and black. The most common, as well as the most desirable ordinarily, is white, or rather, silvery or moonlight glint,—“la gran Margherita,” as Dante calls it; but yellow, pink, and black are numerous. They may also be piebald—a portion white and the rest pink or brown or black. Some years ago there was on the market a large bean-shaped pearl of great luster, one half of which was white and the other quite black, the dividing-line being sharply defined in the plane of the greatest circumference. The pearls from Mexico, the South Sea islands, and the American rivers are especially noted for their great variety of coloration, covering every known tint and shade, and requiring such a master as ThÉophile Gautier to do justice to them.

Many theories have been advanced to explain the coloration of pearls. When the old idea of dew formation prevailed, it was considered that white pearls were formed in fair weather, and the dark ones when the weather was cloudy. It was further considered that the color was influenced by the depth of the water in which they grew: that in deep water they were white, but where it was so shallow that the sunlight easily penetrated, the pearls were more likely to be dark in color. Tavernier curiously explained that the black pearls of Panama and Mexico owed their color to the black mud in which the pearl-oysters of those localities lived, and that Persian Gulf pearls were more inclined to yellow than those of Ceylon, owing to the greater putrefaction of the flesh before they were removed therefrom.^[77] Two centuries ago the color of a pearl was attributed to that of the central nucleus, and it was concluded that if the nucleus was dark, the pearl would be of a similar hue.^[78] This theory has also been upset, for pearls are found white on the exterior and quite dark within, and also with these conditions reversed.

The color of a pearl is determined by that of the conchiolin, as appears from its remaining unchanged after decalcification. While generally it is the same as that of the mother-of-pearl at the corresponding point of the shell in which it is formed, there are many exceptions to this, and the reasons for the varying tints and colors are probably to be found in the changes in position of the pearl, the ingredients of the water, the health of the mollusk, accidents of various kinds, etc. These factors will be referred to later in discussing the pearls from different mollusks and regions; but in general it is no more easy to explain the colors of pearls than it is to say why one rose is white and another is yellow.

Medieval writers had much to say regarding unripe or immature pearls, likening them to eggs in the body of a hen, which follow a uniform rate of growth; and this idea is not entirely absent even in contemporaneous writings. However, it is an interesting fact that the humble mollusks, like the five wise virgins with prepared lamps, keep their gems perfect in beauty and luster at all times. It matters not whether the pearl be removed when it is only the size of a pinhead or not until it reaches that of a marble, it is at all times a complete, a ripe, a perfect pearl, and the largest surpasses the smallest only in the characteristics and properties which are incidental to size. Imparting perfection and completion every day, every moment, the mollusk utilizes the added time simply in enlarging its beautiful work.

Although art has made wonderful progress in that direction, the pearl, like truth, is not easily imitated. There is as much difference between the ubiquitous imitations and the perfect gem as there is between a chromolithograph and a silvery Corot, or between the effects of cosmetics and the freshness of youth. While to the unskilled, or under superficial inspection, the false has some of the properties of the genuine, it is only necessary to place them side by side to make the difference apparent. However clever the imitation may be in color, in form, and in density, it always lacks in richness, in sweetness, and in blended iridescence.