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Pearl is a dense, lustrous concretion, formed within the shell of certain molluscs, that is used as a gemstone and classified with precious stones.
The finest pearls (called oriental pearl) are produced by the pearl oyster. The pearl is composed of conchiolin and calcium carbonate (in the form of minuscule prisms of calcite or aragonite) that is secreted by the mantle of the mollusc and is deposited as nacre in very many thin concentric layers to surround some foreign particle (usually thought to be a grain of sand, but now believed possibly a cellular tissue that causes a resistance) that has entered the shell by nature (making a ‘true pearl’ or a ‘wild pearl’) or has been inserted by man (making a cultured pearl).
The pearl is either attached to the interior of the shell (blister pearl) or formed within the body of the mollusc (mantle pearl). The sizes vary (from seed pearl, to paragon pearl) as well as the form, from the finest, which are spherical, to the oval or egg-shaped or those that are irregularly shaped. The colours also vary, usually depending on the water where produced, from pink to various faint tints and blackish but some are artificially coloured. The finest specimens have a satin lustre.
Pearls are used as beads in a pearl necklace, suspended from brooches, earrings, and pendants, and set in finger rings, pins, brooches, etc. Probably the largest known round pearl is La Reine Perle, weighing 111 grains.
In addition to those of the pearl oyster, pearls are produced by other molluscs and are generally designated by the name of the mollusc, e.g. abalone pearl, clam pearl, conch pearl, mussel pearl.
Pearls are usually pierced through the centre for stringing or part-way for setting in earrings, pendants, studs, etc. Most drilling of oriental pearls is done in Bombay by means of a bow-drill. Pearls to be strung are drilled from each side to make a straight hole; those to be set are partially drilled and are cemented to a metal peg. Some inferior pearls are Chinese drilled.
The monetary value of a natural pearl is calculated by using the base system (once-the-weight method). Seed pearls are priced by the carat or ounce, and cultured pearls formerly by the momme.
The weight of a natural pearl is measured in grains, of a cultured pearl formerly by the momme. The size of a circular pearl is measured in millimetres. Pearls become damaged by acid (as from the skin or some hair-lotion or cosmetics), owing to the effect on the aragonite in the pearl, resulting in the pearl (especially a cultured pearl) becoming a barrel-shaped, with only two nacreous caps at the ends; by grease (as from the skin or certain cosmetics), the grease entering the drill-hole by capillary attraction from the string; and by dryness, from dry atmosphere or protracted storage, reducing the water content and causing surface cracking.
Frequent wearing is advisable, also frequent restringing by a jeweller (using a nylon string and tying a knot between individual pearls to prevent loss if the string should break) and occasional cleaning by an expert.
Among the most famous are: The Hanover pearls; Hope pearl; Mancini pearls; The Orange pearls; Peregrina, La.