. Bulletin - United States National Museum. Science. CATALOGUE OF GEMS AND PRECIOUS STONES. 77 PEARL. Calcium carbonate is most widely distributed in a number of forms, the varieties depending upon differences in origin, crystalliza- tion and structural condition, presence of impurities, etc. With the exception of pearl and coral, the many forms are used more for decorative purposes than for personal adornment. Pearls are concretions, or, more correctly, secretions, consisting essentially of calcium carbonate, found in the shells of certain meliusks. They are the result of an abnormal secretor
. Bulletin - United States National Museum. Science. CATALOGUE OF GEMS AND PRECIOUS STONES. 77 PEARL. Calcium carbonate is most widely distributed in a number of forms, the varieties depending upon differences in origin, crystalliza- tion and structural condition, presence of impurities, etc. With the exception of pearl and coral, the many forms are used more for decorative purposes than for personal adornment. Pearls are concretions, or, more correctly, secretions, consisting essentially of calcium carbonate, found in the shells of certain meliusks. They are the result of an abnormal secretory process caused by an irritation of the mantle of the mollusk, resulting from. Fig. 8.—Baroque pearls. the introduction into the shell of some foreign body, such as a grain of sand. Pearls possess a luster peculiar to themselves, which is called pearly or nacreous. This luster may exist on the exterior surface only of the concretion, or the outer surface may be dull and dead in luster while an inner surface may be clear and lustrous. Their specific gravity is to ; their hardness to They may be of any shape, and in some instances of considerable size. In color they range from an opaque white, through pink, yellow, purple, red, green, brown, blue, black, in all shades; in addition the}?- may be iridescent. In general, their color and luster will be that of the interior shell surface adjacent to which they are formed. The beauty and value of pearls is dependent upon their color, texture or "skin," transparency or "water," luster, and form, the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original United States National Museum; Smithsonian Institution; United States. Dept. of the Interior. Washington : Smithsonian Institution Press, [etc. ]; for sale by the Supt. of Docs. , U. S
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