Part 6

The effect is similar to that made by a pack of cards set diagonally so as to spread the edges sufficiently to show the merest trifle of the faces of the cards between the edges. The arrangement of these plates, not only produces a series of fine lines curving about the umbo, but, as the edges are slightly irregular, another series of fine lines cross the others at right angles, radiating from the umbo. This doubly striated surface, by interference, produces an iridescence more full of color than the mother-of-pearl of any but the thin-shelled varieties.

Though similar in construction, these plates differ from those of the epidermis. In some respects they suggest a transitional stage between the outer and inner shell. A plate, as it separates from the series and which appears as one line in the striated surface of plate edges, is in reality a number of very thin plates, or waves, so welded together that they cannot easily be separated. In this and the presence of fine surface lines marking the wave edges, they resemble the nacreous plates.

The composite plate is opaque, but when split so that light can penetrate there appears on the face, markings similar to the unexposed portions of the Venezuelan epidermis plates only the hexagonal faces are very much smaller and less distinct. So also the edge of the composite plate appears as series of prisms crossing it from face to face on the plate, in sets which show plainly, lines marking the juncture of the individual plates or waves. Although the individual plates or waves, can only be separated with great difficulty, together, as composite plates, they can be flaked off from the shell very easily, and they crumble and break into fragments under slight pressure. The component plates or waves are very thin, and appear under the microscope as white and translucent planes marked by outlines of the prism faces.

The inner series of these plates as they near the nacreous lining become harder and more compact, and incline more and more to a horizontal position, so that at the point where they abut upon the nacre it is not easy to distinguish them from the nacreous plates. At the thinner end of the shell, about the edges, the plates are all of this nature. They grow more friable and chalky as they incline to the perpendicular, where the series are more numerous and are situated at the thicker part of the shell about the umbo.

Adjoining the inner edges of the middle shell plates is the nacreous lining. In this the calcium carbonate takes the same form as the mineral aragonite and is identical with it. As a mass however, the specific gravity is somewhat less, owing to the inclusion of organic matter with the mineral in the shell. This material is harder, finer, more compact, and contains less organic matter than that of which the middle and outer shell is composed.

The lining is constructed of thin waves of transparent calcium carbonate set in animal tissue of great tenuity. This is the mother-of-pearl, and the gem differs from it only in its more or less rounded and independent formation. The plates of which the lining is composed lie almost parallel to the plates of the epidermis. They are bent a little toward the interior at the inner surface of the shell, but the general sectional structure of a shell, cutting from the umbo to the lip, is fairly represented by that stem of the letter X which extends from the right upper to the left lower, the diagonal line representing the middle shell; the horizontal lines at the extremities show the general trend of the epidermis and the nacreous lining. The diagonal trend downward is from the epidermis toward the boss-end of the shell.

The nacreous plates, or mother-of-pearl, unlike those of the middle shell of the nigger-head, cannot be easily separated. On cutting them across the grain they appear as distinct and separate strata and show dividing lines, yet the mass is compact to a great degree. Upon being broken, these strata separate only at the edges, so that the entire set usually breaks diagonally, showing a small strip of the surface of each plate along the broken edge and forming a series of ragged edge steps.

These plates or strata are composed of a great many very thin waves following one upon the other, and thereby producing series of fine, irregular lines upon the surface which, though trending generally in straight lines, curve and twist about as do the edges of water waves, when they run up on the sands of the sea-shore. It is the lapping of these thin transparent waves, and the minute undulations of the layer edges reflecting through the transparent plates, which produce the soft luster peculiar to the linings of the shells and the surface of pearls, and which is known as "pearly."

The wave edges do not usually produce iridescence, but if the waves are very thin and transparent the undulating lines of many under waves following close upon each other appear on the surface, under the microscope, as dark lines when the light is passed through the skin, or silvery lines if the light be thrown upon it from above; to the naked eye this becomes the tempered brilliancy of the pearl's orient. Under the microscope these waves appear to be constructed of minute hexagonal plates or prisms set in animal membrane.

A set of waves forming a plate, when broken at right angles to the trend of the wave, shows under the microscope a rough irregular edge, and the small plates of which they are composed sometimes appear separated individually from the mass though more often they are dislodged in clusters or strips. Broken with the trend of the wave edges, the plate breaks diagonally in steps with undulating edges, which correspond in appearance with the successive underlying waves as they are seen through the surface under the microscope.

Although distinct dividing lines between the plates appear when a sectional cut is made across the grain, there is no indication of a division between the waves which make up the plates, and there is no apparent difference in the structure or compactness at the junction of the plates though a clean division can only be made there. It would appear, therefore, that the plates mark intervals in the process of construction and that the animal tissue is somewhat thicker between the plates than between the waves of which they are composed, where the formative process has been continuous.

In all parts of the shell, the calcium carbonate takes the hexagonal form: in the nacre, as thin waves composed of hexagonal faces, and in the middle shell and epidermis, as plates of hexagonal particles grouped as hexagonal prisms whose terminations form the front and back of a plate. All the parts show a similar plan of construction, _i.e. _, separable plates composed of thinner plates more compacted together, and these in turn of infinitesimal hexagons of calcium carbonate; full plates, component plates, and particles, all alike surrounded by animal tissue.

The shell is built up of secretions from the water in which the oyster lives, made by the mantle, a membraneous covering of the fish. The function of this mantle, in part, is to obtain from the water the elements required and exude it at different parts of its folds in the various forms required for the several parts of the shell. The necessary lime exists in the surrounding water and is supplied sometimes by the calcareous beds upon which the oysters grow, and in other cases by surrounding vegetation.

In all mother-of-pearl oysters and the fresh-water mussel unio, the lining is usually quite thick, but in some pearl-bearing species having small, frail shells, it is, though beautiful, too thin to be of use. In the meleagrina, this nacreous lining lies in the interior of the shell like a congealed pearl wave, the smooth even rim following the curve of the shell about an inch to an inch and a half within the jagged edge of the epidermis, as shown in the Manilla shell illustrated herewith, in which the lip, usually trimmed off for commercial purposes, is preserved. The lining of the meleagrina is not as iridescent as that of the thin shell varieties.

Thus the shell is being constantly enlarged at the edge, by a deposit of the exudations of the mantle; conchiolin for the epidermis outside, lime for the prisms and inner layers of transparent plates, until the shell has attained its full growth in size, after which some varieties continue to lay on nacre only.

The linings of some have a black rim, extending from the hinge on one side, around the edge to the hinge on the other side. Viewed from the edge this dark band appears to be a sixteenth to half an inch wide (widest at the lip), fading out as it becomes lost under the thicker white nacre of the interior, but turn the shell up and look at it squarely from the front and it is black only around the extreme edge where it joins the epidermis. This kind of shell is found in the Pacific about the islands of Polynesia and is called the black shell. In others the nacre is white to the edge. The iridescence of the white shell generally shows more play of color than that of the black. The white shell is usually somewhat flatter and broader than the black, and the epidermis is light yellowish-brown. This variety is found in great abundance on the northern and western coasts of Australia. The yellow, greenish and grayish shells (these colors refer to the edge of the lining), are similar in every way, but inferior, the yellow being the best of the three.

The shell lining of a common form of the unio, or fresh-water mussel pictured at page 146, like that of the meleagrina, shows little iridescence except at the edges outside the pallial lines, where the nacre is comparatively thin, and at the striated surface of the scar or bed of the adductor muscle. In quality of color and luster it is inferior to the nacre of the sea fish, the white being more chalky in appearance and the luster less pearly. The material of which the shell is composed and its construction are however almost identical with that of the salt-water mollusk. In fact all shells are made of the same ingredients and are constructed on the same general principles by the animals inhabiting them.

This description of pearl shells has been given here because a knowledge of the shell enables one to understand the formation and characteristics of a true pearl, and the differences which exist between the gem and other similar formations formed in pearl and other oysters, mussels, and univalves. Many such formations are found, having the elements and constructed like one or both of the outer parts of the shell, and some, in part like the lining, but these are not true pearls; the gem has neither the material nor construction of the middle and outer shell. Except that the pearl, because of its form, is rarely iridescent even to a slight degree, whereas the nacreous lining of some pearl-bearing shells is brilliantly so, the pearl and the nacre of the shell in which it grows, are essentially the same. Pearls are more or less spherical and independent formations, made by the fish on the same plan and from the same secretions with which it lines the shell, misdirected by abnormal conditions. Those constructed like any other part of the shell are not true pearls.

The normal instinctive action of the mollusk is self-protective and adaptive. By the secretive action of its mantle it gathers from the water in which it lives, material to build a shell with a rough and rugged exterior for its enemies, and adapted to resist the chemical activities by which it is surrounded, and a perfectly smooth lining suitable as an interposition for its own delicate organism.

Barring accidents, the building functions of the animal are employed only in the extension of the shell to meet the needs of its own growth and protection. But should a particle of secretion intended for the shell, harden within the folds of the oyster's mantle, or some parasite or other intruder present itself within the nacre-forming sphere, the instinctive action which lines the rougher part of the shell is also directed toward the foreigner, and it is at once covered with a like deposit. This is the birth of a pearl, and it grows layer by layer as long as it remains within the scope of the nacre building instinct. These layers, or skins as they are called, are seldom iridescent. Occasionally a pearl of that character is found, but it is generally from a fresh-water mussel, and the nacreous plates are of unusual tenuity.

Although the pearl like the lining of the mollusk's shell is composed of carbonate of lime in series of thin waves lapping each other, each series constituting a plate or separable layer, there is a distinct difference in construction.

Whereas the lining is a series of horizontal layers, the pearl is made up of concentric layers, each addition enveloping those preceding it. These skins however are not always absolutely distinct and separate. Instead of being like a succession of globular skins, each completely covered by its successor, the growth is often spiral and the construction is as if the nucleus had been rolled one, two, or three complete revolutions in a continuous plate of nacre, and the spiral envelope then finally merged into another plate and the process repeated. That which to a casual glance, therefore, appears to be six rings of nacre in a sectional cut, is in reality, several spirals of two or three turns each.

It is also noticeable that whereas the wave edges, with all their eccentricities, trend generally in one direction in the shell nacre, in the pearl, the lines twist and curl with a concentric tendency, as though the waves had been laid on by turning or rolling the pearl in the material of which it is composed.

A white pearl on being cut in half shows a number of faint dark rings one within the other, from the surface to the nucleus in the centre; usually these rings occur at almost regular intervals. Upon close examination under the microscope, it will be seen that the inner part of these intervals is white, and that the color gradually changes to a yellowish tint which deepens until it culminates in that which appears as a dark line against the succeeding outer formation, the material of which is also white in the beginning. Although this change of color is very slight, a section between two rings will often show three distinct bands; the inner white, the centre one faintly yellow and the outer one of a deeper tint. In some cases the dark concentric rings succeed each other very closely, in which case no abrupt changes of color between them are noticeable. The material occupying the space between the rings is the sectional appearance of the skin of pearl. Upon applying a weak acid to the surface of an entire section of a pearl, it effervesces, and the inner colorless parts of the bands are at once attacked. After several hours the white inner part of the skins will show depressions where the calcium carbonate has been dissolved, and the outer parts of the skins will be marked by coarse black rings of undissolved animal tissue, similar in appearance to the epidermis of the shell. Now as these skins are made up of many very thin waves of calcium carbonate lapping each other and set in animal tissue, it would appear, therefore, that in the beginning these waves of transparent calcium carbonate are set in animal tissue of extreme tenuity and that the proportion of animal tissue increases with the growth of the skin until it reaches a stage provocative of a new skin, which begins with purer layers of the smoother crystallized mineral like its predecessor, and identical with the nacre of the shell. If this be so, it would account for the various tints of color and degrees of luster in white pearls and for the fact that the outer skins of very lustrous pearls are usually very thin also. Similar conditions exist in colored pearls, though the presence of a pigment makes them less noticeable. The skins of the haliotis pearl, which separate easily, usually show remarkable luster on the inner surface.

Sometimes the nucleus is surrounded by a confused mass without apparent concentric markings, as though it had been enveloped in nacre which had solidified while stationary, or the first deposit shows the concentric skin arrangement at one segment of the circle only; followed by layers which appear in the depressions of the mass and are continued until they finally include the whole pearl. These layers are usually very thin, and the partial or segmentary layer formation is quite common in the early stages of the pearl's growth. At that period the concentric lines are also irregular, and in many cases where the curve is true, they extend about one quarter of the circumference only, another concentric skin being lapped on the ends, as though the globular skin had been formed in sections.

As before stated, it often happens that the skin division lines are spiral, as though the nucleus had been rolled one way in the nacreous material. In all cases the first deposits of a skin, that is the first of the nacreous waves of which a skin is composed, appear to be most transparent and lustrous. The component waves of nacre then gradually become more impregnated with animal tissue until they apparently reach a stage which induces either a rest on the part of the fish, to gather nacreous material, or a new deposit of less impure nacre, to protect itself from the increasing impurity of the pearl's skin.

The skins undoubtedly mark certain stages in the formation of the pearl, though the skin and the nacreous waves of which it is composed are often confounded. In the skinning of pearls an entire skin is seldom peeled off. The surface is scraped, a number of the component waves being taken off, until the luster is improved and it is then supposed that the entire outer skin has been removed. A close examination however, will show, by breakages in the surface of the waves, that the under skin with its peculiar and systematic arrangement of surface wave edges, has not been reached.

A sectional view as seen in a half pearl would lead one to infer that a free pearl in the beginning lies stationary in the oyster; is turned or partially rolled as it grows larger; and finally, on attaining about a one grain size, is kept in constant motion with a concentric rolling in the nacreous exudations of the mantle which are deposited upon it.

The nuclei of pearls were long thought to be grains of sand, but late and careful research has shown that in the majority of cases they are minute parasitic or domiciliary worms.

Professor Herdman and James Hornell, after three consecutive inspections of the oyster banks in the Gulf of Manaar in 1902-3, stated in a paper contributed to the British Association for the advancement of science, that after examining many hundreds of oysters and decalcifying a large number of pearls, they had come to the conclusion, that grains of sand and other inorganic particles formed the nuclei of pearls only under exceptional circumstances, as for instance, when the shell was injured by the breaking of the ears, which would enable sand to get into the interior.

Pearls, or pearly excrescences on the interior of the shell, were due to the intrusion of leucodore, clione and other borers. Pearls found in the mussels, especially at the levator and pallial insertions, were formed around calcospherules, minute calcareous concretions produced in the tissues. But most of the fine pearls found free in the body of the Ceylon oyster, contained the remains of platyhelminthian parasites. These observations agree with the opinions formed, after careful study, by several eminent conchologists.

The action of the mollusk results differently as the object to be covered is free within the folds of the creature's mantle or, rising above the surface of the nacreous lining, presses upon it. If free, the intruder is enveloped by the animal's exudations and the deposits become concentric instead of level, or nearly so, as in the construction of the shell. It is said that the foreign substance acts as an irritant, causing the fish to exude its secretions abnormally in order to protect itself, and thereby creating a diseased condition; but from the fact that the process continues after the intruder has been enveloped and rendered as non-irritant as the natural lining of the shell, it would appear that the introduction of a foreign element simply draws upon it the normal impulse of the fish to cover with nacre anything with which it comes in contact, and that the method of doing it is similar to the instinctive rolling action of the tongue when some insoluble globule is put in the mouth, for not only do free pearls grow spherically, but a nucleus fast to the shell is not covered simply but it grows to a pearl, round and domelike, as nearly spherical as its juncture with the shell will permit.

Not only is the composition of a pearl identical with the lining of the shell where it is formed, but in a general way its appearance and characteristics are the same, except that free pearls are sometimes colored when the nacre of the shell is white.

Button pearls, warts and baroques, grown fast to the shell, are usually like the surrounding nacre in every respect.

Salt-water pearls are characterized by the soft velvety luster of the oriental mother-of-pearl, and fresh-waters, like the lining of the unio, have a somewhat thinner looking and more chalky texture.

Abalone pearls have the irregular surface and coloring of the haliotis. Conch pearls resemble the delicate pink china-like lining of the shell, and clam pearls have the glazed earthenware appearance of the inside of a clam shell. The one material difference between a pearl and the lining of the shell in which it grows is, that in the one case the fish deposits the nacre over an even surface, and in the other wraps it around a central point with delicate precision in successive filmy layers.

Dissection shows that a pearl during growth is liable to many mishaps. As with the human creature, a promising youth may end in a wretched maturity. It is also possible that an ugly period may be redeemed by later happenings, and the thing that was worthless in its early existence, be found in its age worthy of a place among the great gems. Pearls found with a dull, chalky exterior sometimes have lustrous skins beneath. Sometimes a bony-looking formation will be found, on breaking it, to have a variety of skins in the interior, some of which are very lustrous, others white and chalky, like the middle shell of the mollusk.