Gems sometimes cultured in a lab (6)

I believe the answer is:

pearls

(Other definitions for pearls that I've seen before include "Lustrous structures in bivalves" , "They're found in the shells of oysters" , "Oyster gems" , "Jewels from bi-valves" , "They're found in 9dn" .)

I've seen this clue in the USA Today.

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