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. [Illustration: _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.