The Pearl, its story, its charm, and its value

by Wallis Richard Cattelle

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.