46. But Isidorus the Characene, in his Description of Parthia, says, that "in the Persian sea there is an island where a great number of pearls are found; on which account there are quantities of boats made of rushes all about the island, from which men leap into the sea, and dive down twenty fathoms, and bring up two shells. And they say that when there is a long continuance of thunder-storms, and heavy falls of rain, then the pinna produces most young, and then, too, the greatest quantity of pearls is engendered, and those, too, of the finest size and quality. In the winter the pinna is accustomed to descend into chambers at the very bottom of the sea; but in summer they swim about all night with their shells open, which they close in the day-time: and as many as stick to the crags, or rocks, throw out roots, and remaining fixed there, they generate pearls. But they are supported and nourished by something which adheres to their flesh: and this also sticks to the mouth of the cockle, having talons and bringing it food: and it is something like a little crab, and is called the guardian of the pinna. And its flesh penetrates through the centre of the cockle-shell, like a root: and the pearl being generated close to it, grows through the solid portion of the shell, and keeps growing as long as it continues to adhere to the shell. But when the flesh gets under the excrescence, and cutting its way onwards, gently separates the pearl from the shell, then when the pearl is surrounded by flesh, it is no longer nourished so far as to grow at all; but the flesh makes it smoother, and more transparent, and more pure. And so, too, the pinna, which lives at the bottom, engenders the most transparent sort of pearl; and it produces them also very pure and of large size. But that which keeps near the surface, and is constantly rising, is of a smaller size and a worse colour, because it is affected by the rays of the sun. But those who hunt for pearls are in danger when they hastily put their hand into the opening of the shell, for immediately the fish closes its shell, and very often their fingers are sawn off; and sometimes they die immediately. But all those who put in their hand sideways easily draw off the shells from the rock. And Menander makes mention of Emeralds also, in his Little Boy-- There must be an emerald and a sardonyx. And the word for emerald is more correctly written μάραγδος, without a σ. For it is derived from the verb μαρμαίρω, to glisten, because it is a transparent stone.