E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Dioscorids and Pliny mention the belief that pearls are formed by drops of rain falling into the oystershells while open; the rain-drops thus received being hardened into pearls by some secretions of the animal.
According to Richardson, the Persians say when drops of spring-rain fall into the pearl-oyster they produce pearls.
Precious the tear as that rain from the sky
Which turns into pearls as it falls on the sea.
Pearls are believed to be the result of an abnormal secretory process caused by an irritation of the mollusk consequent on the intrusion into the shell of some foreign body, as a grain of sand, an egg of the mollusk itself, or perhaps some cercarian parasite.G. F. King: Gems, etc., chap. xii. p. 211.
Cardan says that pearls are polished by being pecked and played with by doves. (De Rerum Varietate, vii. 34.)