If undisturbed, the fish will by the deposit of sufficient layers of nacre, fill the intervals and round the surface again. That this is done in time is shown by the occurrence of pearls having an even dome over a nucleus formed by a cluster of small round and irregular pearls enveloped together. In the process of skinning, or the removal of one or more of the layers of nacre, it is sometimes found that a depression has been filled by a thickening of the deposits in the hollow; at other times extra layers fill the space, and these flaking out with the outer skin reveal the hidden irregularity which lay beneath the round surface, thus necessitating the removal of several entire skins before a sphere is reached again. The under skins of some pearls appear to have failed to completely envelop the nucleus. The cavity resulting is then filled to an even surface and is succeeded by fully developed skins. It is, therefore, not certain that a pearl, perfect in form and skin when found, has been so at all stages of its growth. Broken pearls sometimes show not only differences of color but of thickness in the successive layers. The skins of fresh-water pearls especially are often very irregular in thickness. Many pearls have cracks in them. These generally escape the observation of inexpert persons, as they are usually under the outer layer. The fact that they rarely extend to the surface suggests that the solidification, or drying out of the confined interior layers, may be the cause. These are considered detrimental and dangerous by dealers, so that pearls with cracks in them will not bring as high a price as they would if free from them. As cracked pearls are liable to break, especially when pierced for stringing, it is well to avoid them, though the percentage of those which do break is small. In reality these cracks are more of an imperfection than a danger. Occasionally they are quite noticeable and are then a bad imperfection, but frequently a sharp eye or the loup only will detect them. Surface cracks however are quite perceptible. They are dangerous and are considered a serious imperfection. There is a peculiarity of rare occurrence which, as it is a departure from the ideal, may be termed an imperfection, though some regard it of value as unique. It is a similarity under the surface of some pearls to a metal which has been hammered into small flat spots identical in appearance with the jewelry in vogue during the latter part of the 19th century made of "hammered gold." It is scarcely noticeable except under a loup, when the fine lines dividing the confused planes appear. These pearls are usually slightly pink or pinkish yellow. Sometimes these planes resemble the facets on a cut diamond, generally lozenge shape, and often grouped similar to those on the under side of a diamond. Small holes and blisters on the surface are quite common, but ordinarily they are scarcely perceptible to the naked eye.