Although none of the substances considered in this chapter come within the strict definition of a stone, since they are directly the result of living agency, yet pearl at least cannot be denied the title of a gem. Both pearl and coral contain calcium carbonate in one or other of its crystallized forms, and both are gathered from the sea; but otherwise they have nothing in common. Amber is of vegetable origin, and is a very different substance. PEARL From that unrecorded day when some scantily clothed savage seeking for succulent food opened an oyster and found to his astonishment within its shell a delicate silvery pellet that shimmered in the light of a tropical sun, down to the present day, without intermission, pearl has held a place all its own in the rank of jewels. Though it be lacking in durability, its beauty cannot be disputed, and large examples, perfect in form and lustre, are sufficiently rare to tax the deepest purse. 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