Coal-black pearls occur in one of the pinnas, the interior of which is sooty, shot with iridescent purple, and since the pearl, whether produced by oyster, mussel, pinna, or window-shell, is generally more brilliant than the containing shell, that of the black pinna, with the high lights of its environment concentrated, may be a gem of surpassing novelty and beauty. But the habitual product of this pinna is small, dull, mud-tinted or brown, and of no value whatever. Another of the genera grows “seed” of excellent lustre, corresponding with the azure brightness of the shell.
The chief source of orient pearls on the coast of North Queensland is the gold-lip mother-of-pearl PINCTADA MAXIMA, while the black lip PINCTADA MARGARITIFERA occasionally yields fine and flawless specimens of a silvery lustre. One which is still lovingly remembered was of pale blue and wonderfully lighted. The commonest of the giant clams TRIDACNA GIGAS sometimes betrays evidence of past internal trouble by the presence of a concretion of porcelain whiteness and of porcellaneous texture, but such are not to be described as pearls and to be prized as rarities only.
That some huge molluscs produced pearls before man, with his faculty for admiration, came on the scene is proved by their existence as fossils in chalk. Hemispherical specimens have been found on the inner surface of a shell which has no living representative--viz., the Inoceramus (some of which attained a length of two feet)--and spherical ones of the same prismatical structure occur detached in the chalk. It were curious to let the imagination run over the fact that the hosts of these uncommended gems died ages before the advent of man. The best of modern prizes may be puny in comparison with those which caused distress to the giant molluscs of the age when the Ichthyosaurus, Plesiosaurus, and Pterodactylus were the aristocrats of the animal world. Such gems have gone for ever, and even during this age of insatiable and adventurous search man does not secure a tithe of the ocean's tribute, for, since a pearl is a source of discomfort to its host, the unceasing effort of the animal is towards expulsion. The greatest and possibly the most magnificent are cast out as rubbish on the ocean floor, or are retained within the valves when the animal dies of old age.