Because they consist of the morbid secretions of the fish, situated either in the body, or lying loose between, it and shell; or, lastly, fixed to the latter by a kind of neck. It is said they do not appear until the animal has reached its fourth year. There is nothing peculiar in their chemical composition, being merely carbonate of lime.
The Romans gave almost incredible prices for pearls. Their finest pearls were from the Gulf of Persia, and the Indian Ocean; though it is matter of history that Caesar was induced to invade Britain from some exaggerated accounts he had heard of the pearls of our coasts, or rather of our rivers; but, if these were his objects, the mercenary conqueror was disappointed, for they were found to he of had colour and inferior size, nor have they since improved.
Because he received that elevation for a discovery of causing the fresh-water mussels of Sweden to produce pearls at his pleasure. It is conjectured that he accomplished this by drilling small holes through the shells; but his method is not certainly known.