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

by Wallis Richard Cattelle

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.