Part 5

A lot of pearls which to the casual glance seem to be all quite round, will be found often on close examination to contain many, if not a majority, that are not. Upon rolling them separately, irregularities will appear which the luster and contiguity of others concealed. It will be discovered that the domes of some are slightly flattened at one part of the sphere; in others at two opposite points so as to form a double domed disk. Very many have slight protuberances above the contour of the sphere, or places in the spherical line, which though not flat, are depressed. While these minor imperfections of shape do not materially hurt the beauty of the pearl, they do decrease the value somewhat, and as they are quite common even among fine selected pearls they accentuate the rarity of the perfectly spherical.

The adventures of a pearl from the moment when the mollusk begins to cover its nucleus with nacre, until the fisher squeezes it from the folds of the creature's mantle, are many and varied. A few only escape untoward happenings. The fortunate, born where the mollusk gathers and spreads its choice secretions of mother-of-pearl, with room to grow on every side, are nursed in the lap of good fortune and uncheckered, round out layer by layer to perfection.

But some are not so fortunate. In some way cramped, they are held against the unyielding shell and grow flat on one side. These are the button pearls. Others either from an irregular rolling, or unequal action of the mollusk's mantle, become imperfectly round. Sometimes foreign particles attach themselves to a growing pearl and becoming enveloped with it in future layers, make an uneven surface.

Not infrequently two round pearls grow side by side until they touch, and together are enveloped by succeeding deposits; a twinned pearl is the result. For some reason, drop and pear-shaped pearls are seldom imperfect in shape. They may not be ideal but the form is usually good and the contour even and regular. This would imply that the simple rolling motion by the fish is more regular than the more complicated movements necessary to form a sphere.

Imperfections in the texture and luster of the skin are said to be due to the movement of the growing pearl among the zones of the mollusk's mantle supplying the varied material for the epidermis, middle shell, and lining. The difficulties confronting this theory are explained in the chapter on the "Genesis of Pearls." These imperfections consist generally of dead white chalky spots and streaks, distributed over the surface of the pearl, oftentimes so small as to escape notice except under the loup. Sometimes these imperfections take the form of rings or bands which encircle the pearl. Pearls so marked are rarely if ever round, but ovoid, capsule, or cartridge shaped, and these chalky lines always encircle the cylinder; they never cross the dome. Rings around the dome occur, but the surface over them is of equal luster. Frequently the entire outer skin is without luster. Whether this arises from lack of some element in the exudations of the mollusk from which the pearl is created, or from an imperfect crystallization of the calcium carbonate, is not known. Such skins have the usual nacreous surface wave lines and are often lustrous immediately under the outer plates of the skin.

It is possible that these chalky skins may result from the extraction of the pearl from the mollusk during a transitional stage, and that the presence of spots and streaks of that character, scattered over an otherwise lustrous surface, indicates that the secretions of the creature's mantle did not hold some essential ingredient in sufficient quantity to secure perfect crystallization and thereby cover the entire surface with transparent plates of calcium carbonate. It may be also that a lack of essential elements in the creature's exudations, causes a cessation of the mantle's action which by all signs appears necessary for the production of transparent plates of nacre.

"Peelers" are pearls of imperfect skins having indications of a better one underneath. Speculators buy these pearls at a low price and skin them. Sometimes they are rewarded by a smaller, but much more valuable pearl. Many times the under skins are no better or worse, or if better, the loss in size and weight, together with the cost of the work, make it unprofitable.

Peeling should not be attempted with cylindrical shaped pearls having chalky bands or rings around them, as such imperfections usually penetrate to the interior in pearls of that character. Cylindrical pearls are almost invariably fresh-waters. The imperfections disclosed in the under skins by peeling, are commonly irregularities of shape which have been rounded over to the improvement of the sphericity of the pearl.

It is currently reported among the pearl hunters who fish the western and southern streams, that the finding of soft pearls is not infrequent. Upon opening the mussel, they sometimes see through the mantle of the creature, an apparently fine pearl which upon being taken out proves to be a soft jelly-like substance, the form of which is usually destroyed in squeezing it out. These men do not believe that a pearl is formed in layers, but think that all pearls are originally globules of a similar soft substance, hardening later to a compact solid ball and they call them "mussel eggs."

Many pearls taken from the small thin-shelled varieties of the ocean mollusk, as for instance those of Venezuela, are devoid in part, or wholly, of the nacreous luster and instead have a china-like or waxy luster, or a dead chalky skin. A large proportion of the Abalone pearls and baroques are lustrous only in part, one section having an earthenware appearance. Many appear to be formed of interstratified layers of nacre and conchiolin. This construction is very distinct in a formation peculiar to the Abalone, consisting of two nacreous shells joined perfectly at the edges, the inside walls of both being covered with rough black conchiolin.

Peculiarities in the quality of the nacre sometimes give an appearance of uneven shape which does not exist in reality. The light falling upon such pearls produces a knobby effect, as though there were protuberances on the surface. The texture of others is such that when looked at squarely from the front they appear pyramidal in form, the rounded apex pointing toward the observer. Such pearls have a soft, waxy appearance generally.

Another common imperfection consists of pits in the surface. These may result from various causes: in many cases from the dislodgement and rolling of a pearl which has been flattened during earlier stages by pressure in one position against the shell. Freed from this hindrance to spherical growth, the later concentric layers would round over the edge of the flat spot and thereby leave a pit, or cavity, in the centre.

In other cases pressure against the pearl, or the partial inclusion of foreign substances, especially of an organic nature which decay before being entirely covered, are possible causes. The reverse of this also occurs; grains of sand or other minute particles adhering to the surface are covered by succeeding layers, thereby producing knobs, more or less observable according to the lapse of time between their inclusion and the taking of the pearl from the oyster.

If undisturbed, the fish will by the deposit of sufficient layers of nacre, fill the intervals and round the surface again. That this is done in time is shown by the occurrence of pearls having an even dome over a nucleus formed by a cluster of small round and irregular pearls enveloped together. In the process of skinning, or the removal of one or more of the layers of nacre, it is sometimes found that a depression has been filled by a thickening of the deposits in the hollow; at other times extra layers fill the space, and these flaking out with the outer skin reveal the hidden irregularity which lay beneath the round surface, thus necessitating the removal of several entire skins before a sphere is reached again. The under skins of some pearls appear to have failed to completely envelop the nucleus. The cavity resulting is then filled to an even surface and is succeeded by fully developed skins. It is, therefore, not certain that a pearl, perfect in form and skin when found, has been so at all stages of its growth. Broken pearls sometimes show not only differences of color but of thickness in the successive layers. The skins of fresh-water pearls especially are often very irregular in thickness.

Many pearls have cracks in them. These generally escape the observation of inexpert persons, as they are usually under the outer layer. The fact that they rarely extend to the surface suggests that the solidification, or drying out of the confined interior layers, may be the cause. These are considered detrimental and dangerous by dealers, so that pearls with cracks in them will not bring as high a price as they would if free from them.

As cracked pearls are liable to break, especially when pierced for stringing, it is well to avoid them, though the percentage of those which do break is small. In reality these cracks are more of an imperfection than a danger. Occasionally they are quite noticeable and are then a bad imperfection, but frequently a sharp eye or the loup only will detect them. Surface cracks however are quite perceptible. They are dangerous and are considered a serious imperfection.

There is a peculiarity of rare occurrence which, as it is a departure from the ideal, may be termed an imperfection, though some regard it of value as unique. It is a similarity under the surface of some pearls to a metal which has been hammered into small flat spots identical in appearance with the jewelry in vogue during the latter part of the 19th century made of "hammered gold." It is scarcely noticeable except under a loup, when the fine lines dividing the confused planes appear. These pearls are usually slightly pink or pinkish yellow. Sometimes these planes resemble the facets on a cut diamond, generally lozenge shape, and often grouped similar to those on the under side of a diamond.

Small holes and blisters on the surface are quite common, but ordinarily they are scarcely perceptible to the naked eye.

Many faults can be concealed by the jeweller when the pearl is mounted. Slightly buttoned pearls are set on a peg in the centre of a small shallow cup; they then appear quite round. A spot, blister, or cavity, in a round pearl can be obliterated by pegging, or hidden in the setting. Great irregularities in the sphericity are lost to the eye when the gem is set in the prongs of a ring or other piece of jewelry. Pearls shaped like a double convex lens may be made to look round, or very nearly so, by piercing them so that the flattened domes are brought in contact on the cord holding them together as a necklace.

Piercing and stringing obliterates or hides many flaws. By careful selection, the jeweller can utilize pearls having a blemish by drilling through the spot where the flaw is, and if there is another on the opposite side that also will disappear. Other imperfections near the hole are often hidden in necklaces, as they cannot be seen when the pearls are held close together on the string. It is for this reason that a string of pearls can often be bought for less than a like number of loose pearls apparently no better but which in reality are much more perfect in shape and free from flaws. Imperfections unseen in the strung pearls would be quite noticeable in the loose and undrilled.

The irregularities of baroques cannot properly be called imperfections; nevertheless a baroque is more valuable as it is free from indentations and approaches the round in appearance, or has sides which will give it a round face when mounted. The curious forms into which nature moulds many of them are very attractive, and as they lend themselves to the imaginative skill of the jeweller, are valuable. The faults common to them are rough places uncovered by nacre and colored streaks or spots, usually yellow tending to brown. These discolorations are confined generally to the point where the baroque was attached to the shell, but not infrequently they extend far enough to leave no front which would be quite clean to the eye, when mounted.

Oriental baroques as a rule are more lustrous, more even in shape and seldom discolored. Many of them are sufficiently regular to string for necklaces, and some can be used in jewelry so that on the face they appear like round, drop, or pear-shaped pearls.

GENESIS OF PEARLS

Pearls are found in certain marine and fresh-water mollusks. The former are usually termed oysters, though zoölogists regard it in some instances as a misnomer. The sea-fish is the avicula margaritifera, a bivalve of which there are many varieties, all of similar shape and nature but differing widely in the size, weight, coloring, and quality of the shell.

Of them, the genus "meleagrina" is the largest, has the heaviest shell, and furnishes the greatest quantity of the beautiful substance known as mother-of-pearl. The other extreme is the small, frail-shelled variety taken off the coast of Venezuela, called sometimes avicula squamulosa. Similar to this is the margaritifera vulgaris, or avicula fucata, of Ceylon. The pearl oyster of the Persian Gulf though similar is somewhat larger.

Exact and uniform classification of the pearl-bearing mollusks of the sea does not exist, nor is it necessary in this connection, as the one distinctive feature which places them in the class under consideration is the possession of a nacreous lining to the shell, for no shell fish can produce a true pearl without it. The fresh-water pearl-bearing mollusk is a mussel, unio margaritifera, also found in many varieties, but all characterized alike by the nacreous lining of the shell.

These creatures, living upon the earth where water always covers it, create in the building of their habitations a material of great beauty, and sometimes produce gems which princes covet. Of the most delicate nature, they build for themselves out of the water by which they are surrounded, houses strong and enduring, fitted for their protection from the rough chances of life, yet so furnished within that they suffer no inconvenience from the rugged strength which encloses them. Few things are coarser than the exterior of these domiciles, but nothing in nature is finer or more exquisitely beautiful than the substance with which they are lined.

The avicula margaritifera is a habitant of the coral reefs and shoals about the islands and shores of the tropics; there are none living now in northern latitudes, though fossils of many species are found north of the present boundary of their habitations. An idea can be formed of the general shape and appearance of pearl-oyster shells by the neighboring illustrations of three varieties. These show the two extremes of the marine mollusk, the meleagrina of the South Sea and Australia, and the squamulosa of Venezuela.

In some of the small species, that of the Venezuelan Coast for instance, the outer shell is yellowish, with fan-like markings of dark reddish brown radiating from the boss or beak and growing darker as they near the lip. This shell is thin and frail. The nacreous lining is also thin but brilliantly iridescent and shows a series of fine lines and irregular fissure-like markings extending outward from the hinge and crossed by bands of color which curve with the outline of the lip edge of the shell.

These colors, as brilliant but more evasive than the hues of the rainbow, are not due to the presence of a pigment; they arise from a phenomenon of light and form one of the most wonderful illustrations of the ease with which our senses play tricks upon judgment and understanding. It is the striated surface and the very thin transparent plates of nacre, which cause a double interference and produce the beautiful iridescence peculiar to the lining of these shells.

"Interference," as it is called, is an optical phenomenon arising from two causes. When light falls upon a sufficiently thin transparent surface covering a denser substratum not exactly parallel with it, part of the light is at once reflected. Of that which passes through to the under surface a part also is in turn reflected through the first surface, and the confusion of rays or "interference" resulting, produces to the eye the sensation of color.

A familiar illustration is seen when a thin film of oil is spread over water. The other way in which iridescence by interference is produced in shells, may be demonstrated by drawing fine lines close together on glass with a diamond. Light falling upon them will make the surface iridescent. Melted wax dropped upon this striated surface would, upon removal, show a like iridescence, reproduced with the impression of the fine lines. The outer markings of the large Australian shell are similar to the small Venezuelan. The mother-of-pearl interior is not so iridescent.

Pearls and the shells in which they grow are composed almost entirely of calcium carbonate or lime. A small percentage of organic matter and water are the other ingredients.

As pearls are accidental and the result of a misdirection of normal processes, a general knowledge of those processes is necessary to an insight into the nature and genesis of the pearl, and as pearl shells and the pearls in them are constructed on the same general plan, a knowledge of the former will assist to a better understanding of the gem and its eccentricities. The mother-of-pearl shell is built up of a series of calcium carbonate plates or prisms set in organic matter. In the material of the inner shell, the calcium carbonate greatly preponderates; on the outside of the shell, the organic matter is largely in excess. In the building of its shell, the animal deposits the finest material and does the best and most compact work where the house is in touch with itself, the walls becoming coarser in construction and quality as they approach the outer surface.

In the inside of the shell, the calcium carbonate plates are very fine and transparent, and the animal membrane in which they are set is of extreme tenuity. In the middle shell these plates become more chalky and less compact; in the exterior shell they are set in a thicker binding of organic matter and terminate outside in rough, horny fringes, completely covering the shell.

In a general way therefore, the animal deposits the best of its secretions about itself and pushes out to the outer extremities, the coarser elements which are fitted to preserve the finer parts of the shell, as the finer parts of the shell are fitted to protect the delicate organism which they enclose. The building of the shell is done by a membraneous covering of the fish which entirely envelops the body and is attached to the shell a short distance from the inner edge, leaving a rim of membrane free around the fish and the edges of the two valves. This membrane is called the mantle. It extracts lime from the water, and at different parts exudes modified solutions of it mixed with animal tissue, suitable for the construction of the various parts of the shell.

The exterior of the shell or epidermis consists of conchiolin, an organic compound. It is a horny-looking substance, and in the large salt-water shells and in most of the fresh-water mussels, the nigger-head of the Mississippi Valley especially, it appears to the eye as a series of extensions, sometimes terminating in ridges, which curve about the umbo and spread to the edge of the shell, each extension coming from under the one preceding. In some varieties it is attached as an excrescence to the prismatic formation immediately under it, and may be easily detached in thin flakes: a rusty black in some, brownish-yellow in all on the inner surface and in some on the outside. The substance is generally opaque, but contains spots of which some are translucent, resembling horn or amber, while others are more transparent, similar in formation to the inner parts of the shell.

In most of the marine and fresh-water varieties, unlike the nigger-head, the conchiolin exterior does not easily flake off. In these the outer shell is composed of wave-like plate extensions, superimposed one upon the other recedingly from the lip to the umbo as in the others, but without the ridges, the plates being flat and the edges more irregular. These extensions are formed of a number of horizontal composite plates, which penetrate the shell to the mother-of-pearl.

Not only may they be separated into thinner horizontal plates, but they divide vertically into prisms. Under the microscope the edge of a composite plate appears as a number of prisms placed side by side lengthwise across the plate edge, but showing dark, intersecting lines through the series where they divide as plates.

These prisms appear on the face of the plates as translucent hexagons, separated by dark lines like a tessellated floor, and under a powerful microscope are seen to be composed of similar smaller particles, also joined together by a binder of tissue. The exposed parts of the epidermis plates, forming the outer skin of the shell, are more dense than the unexposed portions; the hexagonal dividing lines are thick and blurred, and the faces are almost opaque, whereas in the unexposed parts, the faces are translucent and the hexagonal markings are clear and fine.

Though constructed in the same way throughout, these plates appear to follow the general plan of shell construction, the preponderance of calcium carbonate in the interior parts gradually changing to an excess of organic matter as they become exposed to form the outer part of the shell. The outer shell is in some varieties of a brownish-yellow with radiating fan-like markings of a deeper tint or red; in others, dark gray and brown to almost black. Immediately under the surface, the plates become lighter in color, and finally almost white as they approach the nacreous interior.

In all varieties the outer plates lie almost parallel with the extension of the shell, so that, lapping each other as they do, the outer contour of the shell is raised by a series of low steps from the edge to the umbo. These plates appear to have been superimposed one upon the other. On the contrary, they are added on the under side. Starting from the umbo, which is the oldest part, the shell is enlarged by the addition of a succession of plates from beneath, each series extending a little beyond its predecessor, the rough conchiolin fringe at their extremities forming the outer covering of the shell. Following the growth of the epidermis, the shell and the lining are also extended and built up, so that the entire shell is constantly pushed to dimensions necessary for the proper and commodious housing of its growing tenant.

Under the thin coat of epidermis on the Unio nigger-head, is a stratum of prism plates similar to the outer plates of the Venezuelan oyster. The prism faces are however smaller and the organic intersections are thicker and darker. Immediately under and abutting, is another series of plates which penetrate the shell almost horizontally at the lip end, to the lining; diagonally at the thick part of the shell near the umbo to another series of the same kind. Here, owing to their diagonal set, upon peeling off the epidermis and the epidermis plates, the edges appear as a series of fine lines curving about and spreading out from the umbo. The plates set outward, away from the umbo, from the lower or inner edge.