True pearls are divided primarily into two classes, "oriental," and "fresh-water." By true pearls those creations are meant which consist of concentric layers of nacre or mother-of-pearl, as distinguished from similar formations by mollusks out of material that is not pearly.

In the early days pearls brought from the Orient were therefore called "Oriental" pearls. For the same reason the fine mellow luster which characterized and made them superior to others came to be known as the "orient" of the pearl. These pearls were taken from oysters found on the coasts of Ceylon, Arabia, and the Red Sea. Later, when the same kind of oysters containing similar pearls were found in other seas, they were also classified with them, until the term "oriental" is now applied usually to all true pearls taken from salt water mollusks, to distinguish them from those found in the fresh water mussels and other products of ocean shell-fish which, though similar in construction and composition, are not nacreous. Occasionally, however, the term is still applied specifically to pearls from the Indian Seas, though their "orient" or luster is not always finer than that of like pearls found in many other localities.

Pearl oysters are varieties of the Avicula Margaritifera, of which the Meleagrina Margaritifera is the most prolific of mother-of-pearl and pearls combined, and, the Indian excepted, yields the finest pearls. All pearl oysters do not produce sufficient mother-of-pearl to make their shells valuable, nor do they all contain pearls. The name therefore applies to all oysters whose secretions are productive, in some degree, of mother-of-pearl and therefore under favorable conditions of pearls also.

"Fresh-water" or "sweet-water" pearls are, as the name signifies, those found in the mollusks of inland waters. This mollusk is a mussel. The name "mussel" in Anglo-Saxon signifies something which retires on being touched. It is known as "Unio" of which there are many pearl-bearing varieties.

In both the sea oyster and the fresh-water mussel, other nacreous formations occur of irregular shape called "baroque" pearls. The orientals approach more nearly to the globular and hemispherical form of true pearls, having frequently the lumpy rotundity of a snowball and sometimes sections which are smooth and round. The fresh-water baroques are usually very irregular, often fantastically so. Many resemble the incisor teeth of man or distorted grains of corn. Slender pieces similar to the wing of a bird and therefore called "wing" pearls, or "hinge" pearls because they are found near the hinge of the shell, are common. Some are shaped like a flat spike nail. Unlike oriental baroques, the surface of a large proportion of the fresh-waters is grooved or indented and some show a beautiful iridescence. Large button baroques of fine luster and iridescent, especially when they have a decided tinge of pink, have come to be known of late as "rose" pearls. Another variety of pink baroques having a fairly regular shape with a lustrous and finely irregular pimply surface are known as "strawberry" pearls. These terms are applied indiscriminately to the two varieties however.

Another nacreous formation found in the mother-of-pearl oyster shells is the "blister." It is produced by the raising of the nacreous deposits above the level of the shell to cover some intruder of considerable size. This results in a growth similar in shape to a blister on the flesh, hence the name. It is cut out of the shell and used in various ways as a set for jewelry, or to imitate the bodies of insects or small animals. Others with a slightly higher dome and rounded oval shape, regular in form, are called "turtlebacks."

Some of these hollow shells of pearl have been found to cover small fish, lizards, etc. The writer saw one which appeared to be a large button-pearl. On lifting, it proved to be a shell of several thicknesses of nacre covering a small shell-fish about a half-inch in diameter. The imprisoned mollusk was shrunken and crumbling so that the nacreous covering could be lifted from over it, a hollow dome of pearl. Mud blisters are common in some waters and depreciate the quality of the shell and are otherwise useless. A typical mud-blister appears in the shell illustrated herewith.

The Abalone pearl occurs usually as a baroque or blister but occasionally it is found solid and spherical. Although it is not classed among true pearls, a few globular pieces found are entitled to a place among them because they are sometimes identical in construction and have a similar pearly luster, it is however very liable to crack and break and can seldom be pierced with safety.

The shell-fish from which it takes the name is the Haliotis, called here the Abalone. It is known under many names—ear-shell, Venus's ear, etc. In the English Channel Islands it is the ormer, and on the adjacent coast of France where it is very abundant the name for it is similar— "ormier." The Aelonians called it the "Ear of Venus." The shell is ear-shaped, flattened, slightly spiral and has a series of round holes near the edge curving with the last whorl toward the boss. As it grows, the oldest of these are successively filled up and the last remaining open, serves as the anal channel. The exterior is very rough and unsightly, but the mother-of-pearl interior is one of the most exquisite pieces of color work painted by the hand of nature and to this is added an enlivening iridescence most fascinating. Like it, the pearl formations are deeply tinted. Brownish reds, peacock greens, and dark grays are the prevailing colors. They are seldom of even color or luster, many of them having but one lustrous point where a pearly glaze seems to have been incorporated with the earthenware like surface.

Usually the pearls when round and lustrous are not constructed as compactly as those of the bivalves. The texture of the skins vary in quality and the frequent presence of intermediary strata of black conchiolin which shrink, makes them liable to crack and break. The blisters run very even in these two qualities of color and luster and though seldom quite as brilliant as the nacre of the shell, are very beautiful and often curiously formed. These blister-baroques are like two blisters joined at the edges, and are liable to separate there. The interior consists chiefly of black conchiolin, rough and somewhat shiny.

The "Conch" pearl, found in the Conch (Strombus gigas) of the West Indies, also is not a true pearl. The shell is used largely for ornamental purposes, especially for the cutting of cameos, and also in porcelain works. It is a large shell, sometimes weighing four or five pounds. Formerly great quantities were exported to England from the Bahamas; in one year as many as three hundred thousand. Conch pearls are devoid of nacreous luster, the surface having an appearance like china. They are slightly transparent and show under the surface a series of delicate wavy markings.

The silky sheen of these lines causes them to appear lighter than the body color of the pearl, and they seem to branch toward the surface, changing kaleidoscopically as the pearl is turned. Almost without exception the shape is ovoid, or a flattened ovoid, though some are distorted. In color they range from very pale to deep pink and coral red, the ends being usually much lighter than the body and often white. In the deeper tints they are more uniform in color, and as they are apt to be less lustrous and transparent as the shade deepens to red they show less plainly the distinguishing wavy lines, and may be easily mistaken for pieces of coral cut to the shape and polished. They are very delicate and therefore easily fractured or cracked. As the natives usually obtain the pearls by cooking the fish, for which they have a great liking, a large proportion of the few which come into the market are cracked. It is claimed also that the color fades with time. They are sometimes called "Nassau" pearls.

Pearls similar in appearance to the Conch, except that the wavy lines are absent and the skin rarely as brilliant, are taken with true pearls from the small varieties of the Avicula, especially about the coast of Venezuela. Some are white as chalk, many are tinted in various shades of gray, yellow and brownish reds. They have the shining appearance of china in different degrees, but no nacreous luster. The skins of many of these are peculiarly constructed, they show modified characteristics of various parts of the shell. The surface wave lines are present to some extent, together with curious malformations of prisms and conchiolin.

The hexagonal faces look as though they had been doubled up upon themselves together with a layer of conchiolin, the latter appearing as thick black V or U shaped marks in the faces of the distorted hexagons. Heretofore these have been considered valueless, but it is possible that with the increasing vogue of pearls and the growing desire for oddities, they will be utilized in the cheaper forms of jewelry.

Creations similar in construction to pearls are found occasionally in the common oyster and clam. Though entirely devoid of the pearly texture and luster, some of them are very perfect in shape and smoothness of skin. Whether taken from the oyster or clam they are usually called "clam pearls." The color of the oyster pearl is generally a light drab. The clam pearls are mostly purplish red or blue, often dark enough to appear black. Those taken from the oyster are generally round; those from the clam are more frequently ovoid. Occasionally one or both ends of the oval are lighter in color, as the Conch pearl is, changing there to a dark red or purple. When the color is very dark and the skin uncommonly good, they have been sold for black pearls by unscrupulous dealers. They are accounted of little value, though exceptionally large pieces will sometimes sell for as much as one hundred to a hundred and fifty dollars. Similar to these, pearly formations characterized by a glazed, or glassy, or shiny surface, are found in many molluscan varieties, bivalves and univalves, but none of these are true pearls.

Pearls similar to the pink Conch are found in the shank or chank of Ceylon (Turbinella scolymus). This is the sacred shell of the Hindus and the national emblem of Travancore in the Madras presidency, India. Vishnu carries a chank called "Devadatta" in his hand. It is said his first incarnation was for the purpose of destroying ShankhÁsura (the giant chank shell), and thereby regaining the Vedas, which had been stolen and taken to ocean deeps.