The very highest position among all valuables belongs to the pearl. It is principally the Indian Ocean that sends them to us. Across many a sea, and over many a lengthened tract of land, scorched by the ardent rays of a burning sun, must the pearl seeker pass, amid those monsters so frightful and so huge which we have already described. The places most productive of pearls are the islands of Taprobana and Stoïdis, and Perimula, a promontory of India. But those most highly valued are found in the vicinity of Arabia, in the Persian Gulf, which forms a part of the Red Sea. The origin and production of the shell-fish is not very different from that of the shell of the oyster. When the genial season of the year exercises its influence on the animal, it is said that, yawning, as it were, it opens its shell, and so receives a kind of dew, by means of which it becomes permeated; and at length small, hard bunches form in its shell, in the shape of pearls, which vary according to the quality of the dew. If this has been in a perfectly pure state when it flowed into the shell, then the pearl produced is white and brilliant, but if it was turbid, the pearl is of a clouded color also; if the sky should happen to have been lowering when it was generated, the pearl will be of a pallid color; from all which it is quite evident that the quality of the pearl depends much more upon a calm state of the heavens than of the sea, and contracts a cloudy hue, or a limpid appearance, according to the degree of serenity of the sky in the morning.[142] If, again, the fish is satiated in a reasonable time, then the pearl produced increases rapidly in size. If it should happen to lighten at the time, the animal shuts its shell, and the pearl is diminished in size in proportion to the fast that the animal has to endure: but if, in addition to this, it should thunder as well, then it becomes alarmed, and closing the shell in an instant, produces what is known as a physema, or pearl-bubble, filled with air, and bearing a resemblance to a pearl, but in appearance only, as it is quite empty, and devoid of body. Those which are produced in a perfectly healthy state consist of numerous layers. It is wonderful, however, that they should be influenced thus pleasurably, by the state of the heavens, seeing that by the action of the sun the pearls are turned of a red color, and lose all their whiteness, just like the human body. Those which keep their whiteness the best are the pelagiæ, or main-sea pearls, which lie at too great a depth to be reached by the sun’s rays. Those pearls which have one surface flat and the other spherical, opposite to the plane side, are for that reason called tympania, or tambour-pearls. I have seen pearls still adhering to the shell; for which reason the shells were used as boxes for unguents.