The myth

Natural pearls are formed when a grain of sand or some other gritty
substance becomes trapped inside an oyster’s shell; the creature
covers the foreign body with secretions to prevent itself being
irritated to death.

The “truth”

If this were true, pearls – the only gems made by a living organism – would presumably be exceedingly common, given the amount of gritty grains the average bivalve must encounter in its lifetime. In fact, oysters (and the various other species which produce pearls) ingest and expel sand and the like all the time; it’s mere housekeeping, and nothing worth getting the nacre out for.