Pearl
The pearl is found in a shell, and it is called unio (union), because it is always found alone. The wise say that the oyster shells are open at certain times, and they receive the dew of heaven; the morning dews become white and clear pearls, while the evening dews are obscure. The young shells produce clearer pearls than the old ones do. The more dew the shells receive, the larger is the pearl, but no one is ever more than half an ounce in weight. If there is thunder when the dew is received, then the pearls perish. They grow in India and in Great Britain.