A calculus or morbid concretion, formed in consequence of some external injury which the muscle or shell-fish receives that produces it, particularly from the operations of certain minute worms, which occasionally bore even quite through to the animal. The pearls are formed in the inside, on these places: hence it is easy to ascertain, by the inspection of the outside only, whether a shell is likely to contain pearls. If it be quite smooth, without cavity, perforation, or callosity, it may with certainty be pronounced to contain none; if, on the contrary, the shell be pierced or indented by worms, there will always be found either pearls, or the embryos of pearls. It is possible, by artificial perforation of the shells, to cause the formation of these substances. The process which has been chiefly recommended is to drill a small hole through the shell, and to fill it up with a piece of brass wire, rivetting this on the outside, like the head of a nail; and the part of the wire which pierces the interior shining coat of the shell will, it is said, become covered with a pearl.

As to the value of British pearls, some have been found of a size so large as to be sold for 20/. each, and upwards; and 80l. was once offered and refused for one of them.

The oriental pearl muscle, to which we are indebted for nearly all the pearls of commerce, has a flattened and somewhat circular shell, about eight inches in diameter, the part near the hinge bent or transverse, and imbricated, or covered ike slates on a house, with several coats, which are toothed on the edges. Some of the shells are, externally, of a sea-green colour, others are chestnut or reddish, with white stripes or marks, and others whitish, with green marks. These shells are found both in the American and Indian seas. The principal pearl fisheries are off the coasts of Hindoostan and Ceylon; they usually commence about the month of March, and occupy many boats, and a great number of hands: each boat has generally twenty-one men, of whom one is the captain, who acts as pilot; ten row and assist the divers; and the remainder are divers, who go down into the sea alternately, by five at a time. The largest round pearl that has been known belongs to the Great Mogul, and is about two-thirds of an inch in diameter. Pearls from the fishery of Ceylon are considered more valuable in England than those from any other part of the world. The smaller kinds are called seed or dust pearls, and are of comparatively small value, being sold by the ounce, to be converted into powder.

To make the artificial, take the blay or bleak fish, common in the Thames; scrape off the silvery scales from the belly; wash and rub these in water; then suffer this water to settle, and a sediment will be found, of an oily consistence. A little of this is to be dropped into a hollow glass bead, of a bluish tint, and shaken about so as to cover all the internal surface: after this the bead is filled up with melted white wax, to give it solidity and weight

The Roman pearls are formed of a very pure alabaster, considerable quarries of which exist near Pisa, in Tuscany. The process is as follows: - the alabaster is first sawn into slices, the thickness of the pearls required; the pearls are then formed with an instrument which bores a small hole in the centre, at the same time that the required shape is obtained. The next thing in the process is their immersion in boiling wax, to give them a rich yellow hue, and afterwards to cover them several times with the silvery substance obtained from the scales of the bleak. The singular beauty of this ornament, which perfectly resembles the real pearl, the varied patterns in which they are arranged, and their extreme cheapness, render them an object much sought after; while their solidity is such, that they may be dashed to the ground with violence without receiving the slightest injury; being thus rendered far superior to those of French manufacture, which are at once more fragile, and considerably less imitative.

The Chinese in a manner force the production of real pearl, in the animal itself. They collect the myca margarite fera, or European pearl muscle, and pierce the outsides of the shells in several parts, without completing the perforation throughout. The animal, becoming conscious of the weakness or deficiency of the shell in those particular spots, deposits over them a great quantity of its pearly calcareous matter, and thus forms so many pearly tubercles over them. The pearls thus obtained are, however, said to be generally inferior to those naturally produced. Pearls that are discoloured may be thus whitened: "Soak them first in hot water, in which some bran with a little tartar and alum have been boiled; rub them gently between the hands, which may be continued until the water grows cold, or until the object is effected, when they may be rinsed in lukewarm water, and laid on writing-paper, in a dark place, to cool." The foregoing is extracted from the scientific journals; but we have always understood that real pearls, so discoloured, are scaled by the lapidaries, that is, they take off the upper coat or lamina, which leaves them slightly diminished in size, but equally beautiful to their primitive state.