The substance composing the pearl is identical with the iridescent lining—mother-o’-pearl or nacre, as it is termed—of the shell. Tortured by the intrusion of some living thing, a boring parasite, a worm, or a small fish, or of a grain of sand or other inorganic substance, and without means to free itself, the mollusc perforce neutralizes the irritant matter by converting it into an object of beauty that eventually finds its way into some jewellery cabinet. Built up in a haphazard manner and not confined by the inexorable laws of intermolecular action, a pearl may assume any and every variety of shape from the regular to the fantastic. It may be truly spherical, egg- or pear-shaped—pear-drops or pear-eyes, as they are termed—or it may be quite irregular—the so-called baroque or barrok pearls. The first is the most prized, but a well-shaped drop-pearl is in great demand for pendants or ear-rings. The colour is ordinarily white, or faintly tinged yellowish or bluish, and somewhat rarely, salmon-pink, reddish, or blackish grey. Perfect black pearls are valuable, but not as costly as the finest of the white. Though not transparent, pearl is to a varying extent translucent, and its characteristic lustre—‘orient’ in the language of jewellery—is due to the same kind of interaction of light reflected from different layers that has been remarked upon in the case of opal and certain other stones. The translucency varies in degree, and some jewellers speak of the ‘water’ of pearls just as in the case of diamonds. If a pearl be sliced across the middle and the section be examined under the microscope, it will be seen that the structure consists of concentric shells and resembles that of an onion. These shells are alternately composed of calcium carbonate in its crystallized form, aragonite, and of a horny organic matter known as conchiolin, and they evidently represent the result of intermittent growth. Because of their composite character, pearls have a specific gravity ranging from 2·65 to 2·69–2·84–2·89 in the case of pink pearls—which is appreciably less than that of aragonite, 2·94: the hardness is about the same, namely, 3½ to 4 on Mohs’s scale. That the arrangement of the mineral layers is approximately parallel is evinced by the distinctness of the shadow-edges shown on examination with the refractometer. Pearls require very careful handling, both because they are comparatively soft and therefore apt to be scratched, and because they are chemically affected by acids, and even by the perspiration from the skin. Acids attack only the calcium carbonate, not the organic matter; the well-known story therefore of Cleopatra dissolving a valuable pearl in vinegar, which is moreover, too weak an acid to effect the solution quickly, must not be accepted too literally. Pearls are not cut like stones, and therefore as soon as the precious bloom has once gone, nothing can be done to revive it. Attempts are sometimes made in the case of valuable pearls to remove the dull skin and lay bare another iridescent layer underneath, but the operation is exceedingly delicate. Even with the best of care pearls must in process of time perish owing to the decay of the organic constituent. Pearls that have been discovered in ancient tombs crumbled to dust at a touch, and those formerly in ancient rings have vanished or only remain as a brown powder, while the garnets or other stones set with them are little the worse for the centuries that have passed by.
The largest known pearl was at one time in the famous collection belonging to the banker, Henry Philip Hope. Cylindrical in form, with a slight swelling at one end, it measures 50 mm. (2 inches) in length, and 115 mm. (4½ inches) in circumference about the thicker, and 83 mm. (3¼ inches) about the thinner end, and weighs 454 carats. About three-quarters of it is white in colour with a fine ‘orient,’ and the remainder is bronze in tint. It is valued at upwards of £12,000. A large pearl, 300 carats in weight, is in the imperial crown of the Emperor of Austria, and another, pear-shaped, is in the possession of the Shah of Persia. A beautiful white India pearl, a perfect sphere in shape, and 28 carats in weight, is in the Museum of Zosima in Moscow; it is known as ‘La Pellegrina.’ The ‘Great Southern Cross,’ which consists of nine large pearls naturally joined together in the shape of a cross, was discovered in an oyster fished up in 1886 off the beds of Western Australia. The collection of jewels in the famous Green Vaults at Dresden contains a number of pearls of curious shapes.
Large pearls are sold separately, while the small pearls known as ‘seed’ pearls come into the market bored and strung on silk in ‘bunches.’ The unit of weight is the pearl grain, which is a quarter of a carat, and the rate of price depends on the square of the weight in grains. The rate per unit or base varies from 6d. to 50s. according to the shape and quality of the pearl. Spherical pearls command the best prices, next the pearl-drops, and lastly the buttons; but whatever the shape, it is imperative that the pearl have ‘orient,’ without which it is valueless. The cheaper grades of pearls are sold by the carat.
PLATE XXX
NATIVES DRILLING PEARLS
For use in necklaces and pendants pearls are bored with a steel drill, and threaded with silk, an easy operation on account of their softness. They harmonize well with diamonds. Small pearls are often set as a frame to large coloured stones, to which they form an admirable foil. Pearls set in rings or anywhere where the upper half alone would show are generally sawn in halves; ‘button’ pearls find an extensive use in modern rings.
Any mollusc, whether of the bi-valve or the uni-valve type, which possesses a nacreous shell, has the power of producing pearls, but only two, the pearl-oyster, Meleagrina margaritifera, and the pearl-mussel, Unio margarifer, repay the cost of systematic fishing. The outside of the shell is formed of the horny matter called conchiolin; while the inside is composed of two coats, of which the outer consists of alternate layers of conchiolin and calcium carbonate in its crystallized form, calcite, and the inner of the same organic matter, but with calcium carbonate in its other crystallized form, aragonite. The latter coat forms the nacreous lining known as mother-o’-pearl, which is identical in consistency with pearl, but somewhat more transparent. The iridescence of mother-o’-pearl is due not only to the fact that it is composed of a succession of thin translucent layers, but also to the fact that these layers overlap like slates on a house, and form a series of fine parallel lines on the surface; diffraction therefore as well as interference of light takes place, and a similar diffraction phenomenon is displayed even by a cast of the inside of the shell. The animal has the property of secreting calcium carbonate, which it absorbs from the sea-water, in both its crystallized conditions as well as conchiolin. At the outer rim it secretes conchiolin, further in calcite, and at the very inside aragonite. The shape and appearance of a pearl therefore depend on the position in which the intruding substance is situated within the shell. The most perfect pearl has been in intermittent motion in the interior of the mollusc, and has received successive coats according to the position in which it happened to be. A parasite that bores into the shell is walled up at the point of entrance, and a wart- or blister-pearl results. The thinner the successive coats the finer the lustre. Pearls have even been discovered embedded in the animal itself. The number of pearls found in a shell depends on the number of times the living host was compelled to seal up some irritant object, and may vary from one up to the eighty-seven which are said to have been found in an Indian oyster. That an oyster thus distinguished has not led a happy existence is testified by the distorted shape of its shell, a clue that guides the pearl-fishers in their search. Moreover, pearl-oysters never have thick nacreous shells, and on the other hand molluscs with fine mother-o’-pearl seldom contain pearls.
Beautiful white and silvery pearls are found in a small oyster that lives at a depth of 6 to 13 fathoms (11–24 m.) in the Gulf of Manaar, off the coast of Ceylon. About seven-eighths, however, of the pearls that come into the market are obtained from a larger oyster which has its home on the Arabian coast of the Persian Gulf. These famous fisheries have been known since very early times. The pearls found here are more yellowish than those from Ceylon, but are nevertheless of excellent quality. The pearl fisheries off the north-west coast of Western Australia and off Venezuela are also not unimportant, and fine black pearls have been supplied by molluscs from the Gulf of Mexico.
PLATE XXXI