The oldest known fossil pearls date from 230-210 million years ago, although mollusks have undoubtedly been producing pearls since they first appeared about 530 million years ago. Always rare, fossil pearls are almost always associated with marine bivalves, although ancient freshwater mollusks also produced pearls. During a pearl's fossilization, the aragonite (the mineral that makes up most of the pearl) is replaced by calcite or another mineral, but in cross-section the fossils show the same concentric layering as in modern pearls. Occasionally the original aragonite is preserved with its nacreous luster.
These fossil pearls have retained their nacreous luster, possibly because the fine marine mud in which they were buried--now turned to rock--prevented the nacre from dissolving during fossilization.