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.