The pearl oyster (_Meleagrina margaritifera_) is the most valuable and interesting of all the nacre (mother‐of‐pearl) bearing shells. The shell is nearly round, and greenish in colour on the outside; it furnishes at once the finest pearls, under favourable circumstances, and the nacre so useful in many industrial arts. Fine pearl and nacre have, in short, the same origin. The nacre invests the whole interior of the shell, being the same secretion which, in the pearl, has assumed the globular form; in one state it is deposited as nacre on the walls of the bivalve, in the other as a pearl in the fleshy interior of the animal. Between nacre and pearls, therefore, there is only the difference of the form of the deposition. The finest pearls—“solidified drops of dew,” as the Orientals poetically term them—are secretions of nacrous material spread over foreign bodies which have accidentally got beneath the mantle of the mollusc. The animal, if irritated by the intrusion of only a grain of sand, and being unable to remove it, covers it with a natural secretion, and the pearl gradually grows in size. Almost invariably some foreign body is found in their centre, if broken, which has served as a nucleus to this concretion, the body being, perhaps, a sterile egg of the mollusc, the egg of a fish, or a grain of sand, round which has been deposited in concentric layers the beautiful and much prized gem. The Chinese and other Eastern nations turn this fact in the natural history of this bivalve to practical use in making pearls and cameos. By introducing into the mantle of the mollusc, or into the interior of its body, a round grain of sand, glass, or metal, they induce a deposit which in time yields a pearl, in the one case free, and in the other adhering to the shell. Pearls are sometimes produced in whole chaplets by the insertion of grains of quartz connected by a string into the mantle of a species of _Meleagrina_; in other cases, a dozen enamelled figures of Buddha seated have been produced by inserting small plates of embossed metal in the valves of the same species. The pearls are very naturally small at first, but increase by the annual layers deposited on the original nucleus, their brilliancy and shade of colour varying with that of the nacre from which they are produced. Sometimes they are diaphanous, silky, lustrous, and more or less iridescent; occasionally they turn out dull, obscure, and even smoky.