Pearls have sometimes been found where the outer layer, or skin, as it is technically called, has been discolored or otherwise injured, and when this top skin has been carefully removed the result was a somewhat smaller but perfect pearl. This, however, is a very delicate operation, and at the pearling grounds is only resorted to by men of experience. The composition of the pearl is carbonate of lime, with a small proportion of organic matter, and the specific gravity 2.5 to 2.7. The pearl is affected by acids, and is easily calcined on exposure to heat. In color, the pure white, slightly transparent, is the most highly prized; while in India and China the bright yellow colors are sought after. Decided colors, however, such as black, pink, and golden-yellow bring a high price, and, in fact, black pearls, if perfect in color and shape, are at present more valuable than any other kind. The beauty and value of a pearl depend on form, quality of texture or skin, color, transparency or water, and lustre or orient. In form, the perfectly round shape comes first in value, then a finely formed drop or pear shape, and lastly the oval or egg shape. Pearls that are flat on one side and rounded on the other are called boutons or button pearls. These are frequently found attached to the shell, and are cut out and the bottom part smoothed and polished. It is easy, however, to detect this class of pearls by the lack of pearly lustre on the side that was attached to the shell. When a pearl is rough and odd-shaped it is called a baroque, and some extremely fantastic shapes are found, especially in fresh-water oysters. The texture or skin of a fine pearl should be perfectly smooth and free from all spots, indentations, wrinkles, or scratches. Pure white is the desirable color for a gem pearl, but many others that are slightly tinted with blue, pink, or yellow will pass for gems if they are otherwise perfect. The transparency or “water” of a pearl, while not existing in fact, is still one of the requisites of a fine pearl; there must be an appearance of transparency, which adds to the beauty of the gem. To describe the lustre or orient of the pearl, the author can only use the term pearly, as there is no other substance that approaches the brilliancy and color of a pearl, excepting, of course, mother-of-pearl—the nacre in the pearl-oyster. Without orient or lustre, the pearl of finest form and color has but little value. Lustre is to the pearl what brilliancy is to the diamond; when the orient is absent there is no life, no beauty. Pearls are principally supplied by two groups of pearl-oysters or mussels: the marine or meleagrina margaritifera, a round-cornered square shell with very thick sides, measuring six to eight inches in length. The color of this shell is mostly blackish-green, but it is also sometimes yellowish; the edges of the inner part of the shell are black, but the rest of the interior is the beautiful mother-of-pearl.