PEARL AND CORAL.
Two marine products bearing some resemblance to each other in origin, occurrence, mode of pro curing, artistic application, and value, are pearl and coral.
Pearl (Fe., Perle; Gee., Perle).—Many molluscs line the interior of their shells with a coating formed of alternate layers of animal membrane and carbonate of lime ; this, in some species, assumes a nacreous or pearly lustre, and forms the substance known as " mother-of-pearl.' A superabun dance of this secretion is often produced in drops or tuberosities, adhering to the interior of the shell, or lodged in the fleshy part of the occupant ; these form the " pearls " of commerce. The forma tion of mother-of-pearl is evidently a natural and unvarying process with certain species of mollusc, though little research has been made as to the conditions which favour or retard it. The produc tion of pearls, on the other hand, at least in the case of the true pearl mussel, is accounted accidental (possibly on insufficient grounds), and is generally attributed to disease or injury suffered by the occupant of the shell. This seems to be borne out in some measure by the following experiences gained in the pearl-fishery :—(l) When the shells attain great size, with a smooth, clean exterior, flee from excrescences, holes, or other blemishes, indicating full and healthy development of the animals, and particularly noticeable where their occurrence is sparse,—there pearls will be extremely scarce ; (2) where they are closely crowded, deformed and stunted in growth, studded with exores oences, and honeycombed with small perforations penetrating into the nacre,—there pearls will abound. Sometimes 100 pearls may be found in one shell, but in that case, few, probably none, will have a commercial value.
Sorts and Sources.—True pearls are formed only in bivalve shells, but handsome nacreous forma tions are obtained also from some univalves. The species whose productions are known in commerce are as follows :—(1) The true " pearl-oyster " or -mussel (Avicula margaritifcra), yielding the most valuable pearla, while the shell itself is valueless ; (2) the mother-of-pearl mussel (dreleagrina margaritifera), chiefly valuable for its mother-of-pearl, the ahells being 6-18 in. across,
while the pearls found in them also possess considerable value; (3) the conch-pearl or fountain shell (Strombus gigas); (4) the giant clam (Tridacna gigas), giving dull, opaque-white pearls ; (5) Puma squarnosa, black and red ; (6) seed-pearls from Placuna placenta [orbicularis], lead-coloured ; (7) the common oyster (Ostrea edulis); (8) the horse-mussel (liodiola vulgaris); (9) the ehank (Turbinella scolymus), pink pearl ; (10) the "green snail" (Turbo olearius [marmoratus]) ; (11) the "Turk's cap" (Turbo sarmaticus); (12) the " ear-shelle " or " aurora-shells " (Haliotis app.), including the " ormer," of the Channel Islands, K. iris, of New Zealand, H. Mida, of the Cape, H. rufescens, H. splendens, and H. Cracherodii, of Japan and other localities ; (13) the Chinese "pearl-mussel" (Anodonta herculea); and (14) the river-pearls obtained from Alamodon app., Unio app., and other river-frequenters.
River-pearls, the produce of fresh-water mussels, occur in the mountain streams of Scotland, Ireland, Lapland, Bohemia, and Canada. The marine pearl-fisheriea are confined to a portion of the Persian Gulf; the Tinnevelly Banks, on the Ceylon coast ; the E. Archipelago; the coasts of Australia, and the lagoons of many of the S. Pacific Islands ; and Central America, in both oceans.
Fresh-water pearls are almost always of greatly inferior lustre and value, yet specimens worth 3-4/. each are not unfrequently found, and individuals have been known to fetch 50-100/. The Scotch pearls have long been famous. During the years 1761-4, London is said to have received pearls to the value of 10,0001. from the rivers Tay and Isla. In the dry summer of 1862, there was an unprecedented discovery of pearls in Scotland, the average value fluctuating between 46s. and 50s., while 5/. was considered a high figure. The estimated value of the total findings in 1865 was 12,000/. Prices have since advanced considerably. The Bohemian fisheries are confined to the Horazdiowitz district of the Wotawa.