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Cultured pearl farming begins when a technician inserts a small bead nucleus, along with a piece of donor mantle tissue, into a living oyster. The oyster responds to this irritant by coating it in layers of nacre, the same iridescent substance that lines its shell. Unlike natural pearls, which form by chance when a stray particle enters an oyster unassisted, cultured pearls result from deliberate human intervention, though the biological process of nacre deposition is identical in both cases. Farmed oysters are typically suspended in nets or cages in sheltered coastal waters for one to three years and must be carefully monitored for disease and water temperature. Freshwater mussels, unlike marine oysters, can produce several pearls at once because tissue can be grafted into multiple sites within a single mussel.