Pearl (The)
.Dioscorʹidēs 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.
“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.