Natural pearls
Natural pearls are produced without human intervention, generally with a core that comes from the environment, such as gravel. Although pearls have been used as decorations for thousands of years, natural pearls are extremely rare and account for only a small percentage of the total number of pearls traded.

Cultured pearls
Pearls formed by manual intervention. To grow cultured pearls, the farmer implants individual mantle membranes (usually for freshwater aquaculture) or together with a mother-of-pearl bead (all marine cultures) into the mollusks. As a consequence, the mollusks secrete a pearly nacre to wrap the irritant, which is consistent with the formation of natural pearls.
http://cwordsworth.com/june-birthstone-alexandrite-pearl-moonstone/
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