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Pearl farming relies on inserting a small irritant, often a bead carved from mussel shell, into an oyster along with a fragment of donor tissue that instructs the oyster to secrete nacre around the intrusion. Farmers then suspend the treated oysters in mesh cages at a carefully chosen depth, since water that is too warm encourages disease while water that is too cold slows nacre deposition to an uneconomical crawl. Over several years, farmers periodically inspect the cages, replacing dead oysters and adjusting depth as seasonal temperatures shift. A single farmed oyster does not guarantee a valuable pearl even after this lengthy process, since nacre can deposit unevenly or the oyster can reject the implant altogether, which is why commercial operations seed thousands of oysters to obtain a smaller number of gem-quality pearls.