Pearl, a peculiar product of certain marine and fresh-water molluscs. Most shell-bearing molluscs are provided with a secretion with which they line their shells, and give to the otherwise harsh granular material of which the shell is formed a beautifully smooth surface, which prevents any unpleasant friction upon the tender body of the animal. This secretion is laid in extremely thin semi-transparent films, which, in consequence of such an arrangement, have generally a beautiful Iridescence (q.v.), and form in some species a sufficient thickness to be cut into useful and ornamental articles. The material itself in its hardened condition is called nacre by zoologists, and by dealers mother of pearl. Besides the pearl lining of the shells, detached and generally spherical or rounded portions of the nacre are often found on opening the shells, and there is great reason to suppose that these are the result of accidental causes, such as the intrusion of a grain of sand, or the frustule of one of those minute siliceous vegetables known as diatoms, or a minute parasite, or even one of the ova of the pearl-oyster itself, which, by irritating the tender body of the animal, obliges it in self-defence to cover the cause of offence, which it has no power to remove. Around this foreign body thin layers of nacre are deposited one after another, like the successive layers of an onion, until the object is completely encysted, and a pearl is formed. The pearl is formed of concentric layers of carbonate of lime of extreme tenuity, but of the same general character as those composing the shell.
The most famous pearls are those of the East; the ancients obtained theirs mainly from Ceylon (Taprobane) and the Persian Gulf, whence many of the best pearls still come. Other pearl-fisheries are in the Sulu Archipelago, off New Guinea, off some parts of the Australian coast, and amongst some of the Polynesian islands. In the Persian Gulf the most important pearl-yielding mollusc is the Avicula (Meleagrina) fucata. It is specially fished for pearls, and yields them in greatest number and of the finest kinds. It has a much smaller shell than the species which has been long known as the pearl-oyster, Avicula (Meleagrina) margaritifera. This last and another species (A. macroptera) are also extensively fished for in the gulf; but, though pearls are found in both, they are much more valuable for their shells than for the pearls they sometimes contain. Like the A. margaritifera, the A. macroptera has also a large shell. All three species are chiefly fished about the Island of Bahrein on the west side of the gulf. The centre of the trade is the port of Lingah, but all the pearls that come hence are called Bombay pearls. A steamer with a diver and diving-dress on board was sent out by an English firm of merchants in 1884 to try to obtain shells and pearls in the deeper water on the east side of the gulf, but the experiment did not succeed.
Of the Ceylon pearl-fishery, which, like the Indian ones on the Madras side of the Strait of Manaar, is under government supervision, and is only allowed at irregular intervals, some account is given at CEYLON, Vol. III. p. 78. The method of fishing may be thus described. The season of the fishery, when permitted, lasts from four to six weeks. For each diver there is provided a diving-stone, weighing about 40 lb., which is fastened to the end of a rope long enough to reach the bottom, and having a loop made for the man's foot. To each boat there is usually allotted a crew of thirteen men and ten divers, five of whom are descending whilst the others are resting. This work is done very rapidly; for, notwithstanding the stories of divers who can remain below for four minutes (see DIVING), the best divers cannot, as a rule, remain longer than eighty seconds below, and few are able to exceed sixty. The greatest depth they descend is thirteen fathoms, and the usual depth about nine fathoms. When the diver gives the signal by pulling the rope he is quickly hauled up with his net and its contents. Probably the rarity of accidents from sharks, usually so abundant in tropical seas, is to be attributed to the bustle and to the excitement of the waters during the fishery frightening away those dreaded creatures. The divers are sometimes paid fixed wages, others agree for one-fourth of the produce. When a boat-load of oysters has been obtained it returns to shore, and the cargo, sometimes amounting to 20,000 or 30,000, is landed and piled on the shore to die and putrefy, in order that the pearls may be easily found. In Ceylon, in 1889, in twenty-two days fifty divers brought up 11 million oysters, which sold for 24s. per 1000 shells, the government receiving £10,000 and the divers £3200. When the animals in the shells are sufficiently decomposed the washing commences, and great care is taken to watch for the loose pearls, which are always by far the most valuable; the shells are then examined, and if any attached pearls are seen they are handed over to the clippers, who, with pinchers or hammer, skilfully remove them. Such pearls are only used for setting; whilst the loose ones, being usually quite round, are drilled and strung, and can be used for necklaces, &c.
The pearls vary much in size; those as large as a pea, and of good colour and form, are the best, except unusually large specimens, which rarely occur, the most extraordinary one known being the pearl in Mr Beresford Hope's collection at South Kensington, which measures two inches in length, and four in circumference, and weighs 1800 grains. The smaller ones are sorted into sizes, the very smallest being called seed-pearls. A considerable quantity of these last are sent to China, where they are said to be calcined and used in Chinese pharmacy. Amongst the Romans the pearl was a great favourite, and enormous prices were paid for fine ones. The single pearl which Cleopatra is said to have dissolved and swallowed was valued at £80,729; and one of the same value was cut into two pieces for earrings for the statue of Venus in the Pantheon at Rome.
The finest pearls are found within the mantle of the mollusc, close to the lips of the shell, or in the soft part of the oyster near the hinge of the shell; the worst pearls are those found within the close, coarse fibres of the adductor muscle. At intervals they are found loose in the shell outside the body of the oyster, and may when large get washed out of the shell and thus be lost. Lastly, pearls are often found embedded more or less deeply in the shell, having in some cases escaped from the soft tissues. It is notable that the adherent pearls occur almost invariably in the flat or lower valve; occasionally, it is true, they are found embedded in the rounded or upper valve, but in such cases it is observed that the shell has been found lying at the bottom in the reverse position. The pearls found embedded in or under the 'muscular impression' are always small, irregular, and worthless, similar to those found embedded in the adductor muscle itself. Pearls are found in infinite variety of form, and the consecutive layers vary in brightness, colour, and perfection. The most highly prized pearls are quite spherical, and it is evident from their shape that these must have been formed free in the mantle or in the soft tissues of the mollusc, and not cemented to the shell. Some pearls show defects caused apparently by the contact of new foreign substances, organic or inorganic, such as a grit or film of weed; and in some cases it requires a number of layers to completely hide these defects. Thus every new layer secreted changes the value of the pearl. When a pearl that has been cut from the shell presents a hemispherical surface, it is sometimes called a 'perle bouton;' such a pearl is flat on one side, and rounded or convex on the other. If a solid pearl has an irregular shape, having grown over a rough object, it is known as a baroque pearl. Sometimes warty pearls are hollow, and pass under the name of 'coq de perle.'
Pearl-oysters frequently renovate their shells, and are in the habit of burying such intruders as they cannot otherwise dispose of. Stones, mud, small shells, wood, and especially layers of weed are found thus embedded in shells forming unnatural excrescences on the surface. These protuberances are gradually removed by the oyster secreting thinner layers of nacre on the top of them than on the base until the surface becomes again level. Slowly, but steadily, the exterior surface of the shell decays and disappears, until the foreign substance, of whatever nature it may be, comes within the reach of advancing dissolution, and thus the oyster literally passes a stone or other intruder through its shell. In the Natural History Museum, South Kensington, and in the Museum of Practical Geology in Jermyn Street there are specimens which clearly illustrate the processes of refining the shell and of burying foreign substances. They are flat shells (the lower valves), with a number of the figures of Buddha lying embedded at equal distances apart, on the upper portion of the shell near the lips, but not so deeply buried as to be hidden. These are produced artificially in China; the little figures are slipped carefully below the mantle of the oyster, and the process of deposition covers them with nacre.
The value of a pearl depends upon its size, shape, colour, brightness, and freedom from defects. The most valuable pearls at the present time are those which are perfectly round; the button-shaped ranks next, and then comes the drop or pear-shaped pearl. Perfectly round pearls over 25 grains in weight are extremely scarce, and secure high prices. They are greatly sought after to form the centre of necklaces, and large pearls of this character are safe and very profitable investments. The varying tints and colours of pearls are less difficult to understand than some of their eccentricities of growth. The changing condition of the sea, both as regards purity and temperature, the health of the oyster, accidents, such as the discharge of the inky fluid of the cuttle-fish in the neighbourhood of the oyster, all will probably affect the colours of the successive growth-periods of the pearl. Pearls, when of extraordinary beauty, size, and brilliancy, will sell for sums which appear extravagant.
The chemical composition of the pearl is carbonate of lime associated with a small proportion of organic matter. It is easily affected by acids and fetid gases, and may be calcined by exposure to heat. Its sp. gr. is 2.5 to 2.7. Three varieties of pearls being examined, British, Australian, and Sinhalese, the qualitative analyses showed that they all had an identical composition, and that they consisted solely of water, organic matter, and calcium carbonate. There was a total absence of magnesia, and of all the other mineral ingredients of sea-water, from which the inorganic part of pearls must of course be obtained.
The most important marine pearl-fishery on the American continent is that of Lower California, the central point being at La Paz. The largest and finest black pearls which come into the market are the specialty of these fisheries. In Australian waters pearls are fished on the coast of West Australia and of Queensland, and in Torres Strait.
River-pearls are produced by fresh-water mussels in Scotland, Wales, Ireland, various parts of Russia, Germany, Canada, and the United States. British pearls are spoken of by Tacitus and Pliny; and in the end of the 17th century the Scottish pearl-industry was of some importance. The Spey, Tay, South Esk, Doon, Dee, Don, Ythan, and Forth have all yielded pearls; and a temporary revival of the industry took place in 1860 and succeeding years. In the United States the chief river-pearl fishery is in the Little Miami, in Ohio; but China is the great headquarters of the trade in river-pearls.
False pearls are made by blowing very thin beads or bulbs of glass, and pouring into them a mixture of liquid ammonia and the white matter from the scales of the bleak, and sometimes of the roach and dace. The scales of the lower part of the fish are very carefully washed and put to soak in water, when the pearly film falls off and forms a sediment at the bottom of the vessel, which is removed and placed in liquid ammonia for future use. This pearl mixture, when of the best quality, is very costly, being as much as £4 or £5 per ounce. For use it is diluted with ammonia, and injected into the glass beads, so as to thinly coat them inside; afterwards the better kinds have melted white wax or mucilage of gum-arabic poured in, which renders them much more durable. In this way are produced imitations of the finest oriental pearls, such as only the practised eye can detect. The art of giving the irregular forms of large pearls to the glass bulbs increases the resemblance; and the glassy appearance caused by the exterior glass coating is removed by exposing it for a short period to the action of the vapour of hydrofluoric acid. Roman pearls are lighter, as they have the coating of pearly matter on the outside.
See Streeter's Pearls and Pearling Life (1886), and G. F. Kunz's Gems and Precious Stones of North America (New York, 1890).