How do pearls form in oysters?

When a small irritant, such as a parasite or fragment of shell, becomes trapped inside an oyster or other mollusc, the animal protects itself by covering the intruder. It secretes layers of nacre, also known as mother-of-pearl, the same material that lines the inside of its shell. Over months and years, thousands of thin layers build up around the irritant, gradually forming a pearl. The many layers scatter light, giving pearls their soft, shimmering glow. Cultured pearls are made by deliberately inserting a tiny bead so the mollusc coats it, which is how most pearls are produced today.