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.