GENESIS OF PEARLS
TUAMOTU PEARL-SHELL
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.
AUSTRALIAN PEARL-SHELL
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.
VENEZUELAN PEARL-SHELL, WITH PEARL ATTACHED
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.
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.
MANILA PEARL-SHELL WITH THE LIP CONSERVED
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.
MISSISSIPPI NIGGER-HEAD PEARL MUSSEL
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.
Many of these dead pearls are formed throughout of this material. Others, perfectly spherical, are simply successive layers of prism groups like the conchiolin plates of the shell. Upon cutting these through the centre the skins are shown by the concentric rings marking their divisions and the prismatic formation appears as glistening lines radiating from the nucleus to the surface. Under the microscope these layers, which are thicker than the nacreous skins of true pearls, appear identical with the epidermis plates, except that they are concentric instead of flat, and are free from the coarse, rough, conchiolin deposit which forms the extreme outer coating of the shells. This deposit is also found, however, in some pearl formations, as many of the abalone baroques, especially when they are somewhat flat in shape, are like two pearl blisters joined, with the shell-building process reversed, the rough, black conchiolin being inside, and the nacre outside. Undoubtedly pearls containing hidden qualities which made them once gems are thrown away as valueless, while others found just as nature had covered their earlier coarseness with a coat of beauty, are worn and excite much admiration for their skin-deep beauty.
Though the successive skins of a pearl do not usually vary much in color, except in abalone pearls, it does happen occasionally, for the removal of dark yellow skins sometimes discloses another of better color—a good pink for instance. From the sectional appearance of pearls it seems probable, that in the majority of cases the color of yellow pearls would be improved by the removal of the outer waves of the outer skin.
Changes in shape sometimes occur during the growth of the pearl, the tendency being always toward the rounding of the surface. If the nucleus is fast to the shell, a dome is built over and around it. If the nucleus permits, the nacre is deposited not only over but under its edges to the point of contact with the shell, so that a button pearl connected with the shell at the centre only, results. Two pearls held against the shell and growing side by side are separately enveloped until they touch each other, after which they are included in single deposits of nacre and the depression between their domes becomes less distinct with each successive coating. Similarly, a cluster of small pearls lying together often forms the nucleus of a large rounded baroque or button pearl. Examination of such formations shows, that up to a certain period the pearls have a separate existence and growth. They then become joined in an irregular mass of twinned pearls, and finally, if allowed to remain in the oyster long enough, all individuality is lost in the tendency to round over. The same thing occurs when grains of sand or other intrusions become attached to a growing pearl. They are quite prominent when first included in the nacreous deposit and can be easily detached from the under pearl by breaking through the layer which binds them on; but they are soon obliterated by succeeding deposits. This filling-in process is sometimes accomplished by additional layers in the depression, sometimes by thicker layers. It happens occasionally, when skinning a round pearl, that one of these fillings is uncovered and flakes out, leaving the pearl irregular, as it was in a former stage of its growth.
Although pearls naturally grow spherically, many free pearls are more or less buttoned, that is, have a flat place from which the pearl rises like a dome, high or low. This happens when the pearl is held during growth by the fish against the shell with a part of its body intervening. According to circumstances, the pearl varies in form from slightly button, to a low dome, rising from a plane at its greatest diameter. Should a pearl of this description become dislodged, the rounding action of the mollusk would begin at once to obliterate the plane.
If undisturbed, the process would result eventually in changing the button to a round or nearly round pearl, but should the pearl be taken from the fish before the metamorphosis is completed, a depression, or pit, would mar its contour. When borers intrude through the shell, the presentation is at once covered with nacre, and successive deposits are built up around it resulting in the nacreous wart known as a baroque. The rounding action of the mollusk is clearly shown in these excrescences, as the borer is not simply covered and levelled with the shell, but the slight elevation above the level of the lining receives a continuity of concentric deposits which finally raise it very considerably above the surface and separate it in construction from the lining to which it is attached. The shell herewith reproduced illustrates the result. Borers pierced it at the thick part of the hinge, and burrowing down, entered the interior at the point where the baroque is shown. In rare instances, pearls attached to the shell do escape the concentric deposition, for they have been found buried under even layers of nacre, when the mother-of-pearl was cut up in the process of manufacture.
VENEZUELAN PEARL-SHELL, SHOWING BAROQUE
From the appearance of the striæ when they are divided lengthwise, pear-shaped pearls appear to have been spherical at one time. During a stage in the growth, the forming layer has curved away from the centre at one section of the sphere to a point. Succeeding layers, following the innovation, are deposited around the extension until it becomes sufficiently elongated to give the pearl the obovoid form.
Many pearls are shaped like a capsule. The ends of most are rounded up to a full dome; some have somewhat flatter ends; many are long and cylindrical like an ordinary capsule; others are short and appear in shape like two high button pearls joined at their bases; while some resemble a cartridge, one end being almost flat and the other a somewhat pointed dome. It is noticeable that such pearls have a chalky line around the middle, and sometimes there is a lustrous band between two. These chalky lines are found, on peeling such a pearl, to extend through all the interior layers. Similarly, a high button joined at its entire circumference to the shell, if the junction is abrupt, has an intersecting chalky line, marking the juncture of the two, between the luster of the pearl and the shell lining. If the base of the pearl and the shell form a curve there is no chalky line of demarcation.
This suggests that whenever the animal is unable to envelop the thing upon which the mantle deposits its secretions completely or is not in touch with every part of it, there is at the extremity of its action, an unnacreous deposit, corresponding to the deposit of conchiolin or calcite, at the extreme edge of the shell which precedes the nacreous layers following within and slightly back of it. As the luster of the pearl arises from the transparency of the calcium carbonate modified by the undulating lines formed by the edges of the wave-plates, it may be that the lapping action of the mantle is necessary for the regular formation and crystallization of these plates, and that at points beyond the reach of this action, the depositions of the mantle are therefore not pearly.
Much is necessarily conjectural as to the modus operandi by which the shell and the pearl are formed but the invariable tendency toward sphericity suggests that the nucleus of a pearl, when free within the mollusk's mantle, is not only enveloped in its exudations, but is either kept constantly moving with a rolling motion or lapped on all sides by the membrane which exudes upon it the nacreous material.
The instances cited of the short capsule shaped pearl and the high button joined to the shell, which seem to escape the nacreous deposit at the basis of the domes, favor the lapping or licking method of depositing the nacreous solution and this action by the mollusk would result in a constant rolling or turning motion imparted to the object if it were free within the creature's body. The licking and rolling action of the mollusk, modified by the conceivable influences of position in the shell, would account for the spherical form with all the various modifications in which the pearl is found.
To account for the variation of quality which undoubtedly exists in the successive skins of some pearls, and the imperfections in the nacre of the same skin, the theory has been advanced that the secretions for the lining, the shell proper, and the epidermis, are exuded by different parts of the mantle; the pearl traverses during growth these different bands and its skins are modified by the secretions, as they come within the various zones of influence. But there are several facts which seem to oppose the theory.
In the first place all these parts of the mantle which supply the material for the epidermis, the middle shell, and the lining, are enclosed within the shell and in touch with the lining yet each receives the exudations of that part of the mantle which supplies the material suitable for it, the mantle invariably pushing the coarser excretions outwardly to the shell's exterior. Again, whatever the quality of the skin of the pearl may be, it is never of conchiolin like the outer epidermis and though sometimes similar to the plates, of which the conchiolin is the exposed fringe, it always contains sufficient nacre to render the surface smooth. The fact that the skins of a pearl do sometimes correspond with the different parts of the shell, and that the same skin on the surface is occasionally partly nacreous and unnacreous, in connection with the variation of quality which exists in the internal composition of the skin, favors an idea that the mixed and variable quantity of nacre in the skins may be caused by the abnormal position of the mantle wrapped about the growing pearl which would thereby come more or less under the influence of the calcite and conchiolin zones distorted from their normal extension and action.
It has also been suggested that the oyster deposits the nacreous layer in a fluid state and then rests until the deposit hardens, when the process is repeated. To a certain extent this may be true though apparently it could not be a yearly process as pearls found in the small varieties of the avicula which mature in four to six years and die out in seven years, often contain a greater number of layers than the years of the mollusk's life, and no pearl is ever found with a soft exterior, though it seems possible that pearls with a dead white chalky exterior are taken from the oyster at a period when the crystallization of the outer skin has not been perfected, or that they have escaped some action, chemical or of the animal, necessary for the formation of the lustrous waves of nacre. Mr. Ludwig Stross, who has had much experience at the pearl fisheries, says that he has frequently found pearls of fair size in shells of the Lingah type which could not be over twelve to fifteen months old. Some of these pearls weighed fully three grains. As there are many apparent skins in a pearl of that size, the divisions could not mark either years, seasons, or breeding periods. In some experiments made by Mr. Stross, he found that borings made to the interior of a living mollusk's shell were closed by a film of hard nacre in two days.
The known facts about a pearl are these. It is composed of about ninety-two per cent. carbonate of lime, about six per cent. organic matter and a little over two per cent. water in combination almost identical with the lining of the shell in which it grows and similar to the mineral aragonite. In construction it is usually a series of layers, which can sometimes be peeled off entirely, each one successively enveloping its predecessors apparently as an independent structure though itself composed of a number of thin lapping waves. Upon cutting through these layers the divisions appear as a series of rings and the intervals, though composed of many thin waves, appear compact. It grows spherically or with such modifications as the exigencies of position in the shell would reasonably account for. These facts seem to justify the hypothesis that a foreign substance upon entering the shell of a pearl oyster is at once enveloped or washed in the creature's exudations; that the organic matter of the secretions forms a filmy envelope in which the mineral contained in them is precipitated or crystallizes in wave-like layers of crystals of great tenuity, and that as these layers harden the process is repeated, and that during the process the creature either revolves the object, or about it, as it is free, or fastened to the shell. It is also possible that changes in the organic matter interwoven with the calcium carbonate may produce some chemical action resulting in the crystallization of the lime, and the crystallization in turn be provocative of another deposit, each process in turn being almost simultaneous and that the process is continued until a paucity of mineral in the exudations induces a rest for recuperation, after which the process is repeated, the result being a succession of composite skins as we find them. Whatever the cause, it is evident in all parts of the shell and in the pearl that continuity of construction is periodically arrested to be resumed upon exactly the same plan, except that the material used in the succeeding layer of the pearl may be formed occasionally like another of the shell sections though usually it is like the preceding one.
Marked differences in the same skin occur more frequently in the pearl formations of univalves. The skins of the abalone pearl especially, are frequently nacreous in part only.
Pearl oysters are found in immense numbers on banks having a calcareous foundation. They are extraordinarily prolific, the spat of one oyster being estimated at upwards of several hundred thousands to millions, so that were it not for the natural enemies of their young and the liability of being swept away and scattered by storms before they have anchored, the banks would be over-crowded with the myriads produced. Some idea of the numbers may be gained from the fact that during the fishing season the Ceylon divers raise about one million each day.
The oysters are seldom found in water with a temperature below 75 degrees and they seem to thrive best in warm sheltered bays and inlets, especially when the banks are situated far from the equator. They attach themselves to the beds by a bunch of tough threads which pass out through an aperture in the shells, near the hinge, and fasten on the rocks and stones; consequently the oysters do not lie flat, as might be supposed, but maintain an upright position, hinge down, lip end up, and the shell slightly open for the passage of the food-laden water, as the fresh-water mussels do. These threads are called the beard or byssus, and are composed of material similar to the epidermis of the shell.
The abalone, which is a univalve, holds on to the rocks by the foot, a flat muscular appendage used for locomotion and also as an anchor on the principle of the leather toy known to boys as a sucker.
Although pearls of value are found only in shells containing mother-of-pearl, a small proportion only of the mother-of-pearl shells contains pearls, and many varieties in which pearls are found do not yield enough nacre to make the shells valuable. The size of the meleagrina in some seas is remarkable. That at page 127, photographed from a Tuamotu shell, measures 8-7/8 inches by 6-7/8 inches and weighs twenty-eight ounces troy.
It is of the black-edge variety, contains a large quantity of fine quality mother-of-pearl, and has a beautiful small pearl attached to the lining near the center of the shell. Though large, it is not full grown. It is probably twelve to fourteen years old and would continue to lay on mother-of-pearl and so grow thicker and heavier until sixteen to eighteen years of age, when the oyster would reach maturity. The Australian white shell at page 129 is a young shell—that is, it has not attained the full thickness and weight of a mature shell. The shells at pages 131 and 161 are from the coast of Venezuela; they measure 2-1/4 by 2-1/4 inches and weigh seven pennyweights each.
The common form of the pearl-bearing fresh-water mussel unio (nigger-head) is illustrated at page 146. This shell measures 3-3/4 by 2-3/4 inches and weighs 3-1/2 ounces. It is from the Middle West of the United States. In construction it resembles the meleagrina, the epidermis being dark, though not as rough as that of the oyster, and the lining white, showing slight iridescence around the lip-edge and to a greater degree on the adductor muscle scar. The mother-of-pearl under the epidermis at the thick or hinge end is quite iridescent, and the lines which make the color play are plainly discernible under the loup.
The largest and finest pearls, also the greatest number, are found usually in distorted shells. This has given rise to the idea that they are a symptom of disease in the fish, but having in mind the functions of the three zones of the creature's mantle by which they supply separately material for the epidermis, middle shell and lining, one may conceive that if, by some extraordinary cause, the secretions of one of these is largely withdrawn from the natural channel, the losing part of the shell would warp the normal growth of the others to its own dwarfage.
When the nacre grows to a pearl, contrary to the intent of nature, instead of a lining for the shell endeavoring to keep pace with the growing oyster, the full-growing exterior is distorted in accommodating itself to the undersized lining. In view of the fact that an oyster sometimes contains a large number of pearls (one shell in New Caledonia contained 256) the diversion of nacre sufficient to cover them, or to produce one large pearl, might reasonably be expected to result in a considerable distortion of the shell. It may also be that the displacement of the mantle, caused by the wrapping of itself about the growing pearl, interferes with the even deposit of shell material about the edges of the shell and so distorts it.
Because deformed shells are more fruitful of pearls some have advocated the practice of throwing perfectly-formed shells back into the sea unopened, but, inasmuch as the mother-of-pearl of the shells often exceeds in value the pearls found in them, this is not likely to happen. Few fisheries could be made to pay if they were fished for the pearls alone. In many of them the shells yield 90 per cent. of the total value and are in fact the sole incentive for the investment of the necessary capital.
Luckily for the world's supply of pearls, however, the disturbers of the mollusk which cause these gems by their intrusions appear to be more abundant in waters where the shell is valueless, the banks about Ceylon especially being infested with the cestodes which are commonly the nuclei of Indian pearls. It is interesting also to learn that Mr. James Hornell (inspector of the pearl banks) finds these worms in another stage in the file-fish, which frequents the banks to prey upon the oysters, and confidently expects to find them in the adult stage in the shark, which in turn devours the file-fish.
It is the opinion of Jameson of London and others, that the parasite which causes the formation of pearls in the mussels of Europe is frequently the larva of distomum somaterœ, from the eider-duck and scoter, and that the larva first inhabits Tapes, or the cockle, before getting into the mussel.
Generally the nuclei appear to be the bodies or eggs of minute parasites—distoma, filaria, bucephalus, etc., and they vary in different localities according to the animal life of the neighborhood. In the still parts of the river Elster, where water-mites (Limnochares anodontœ) were abundant, Kuchenmeister found that the mollusks contained more pearls.