Pearls can be natural, cultured, or imitation.
Natural pearls are made by oysters and other mollusks.
Cultured pearls also are grown by mollusks, but with human intervention: an irritant introduced into the shells causes a pearl to grow.
Imitation pearls are man-made with glass, plastic, or organic materials.
Natural pearls are very rare, so most pearls used in jewelry are either cultured or imitation pearls. Cultured pearls usually are more expensive than imitation pearls. A cultured pearl’s value generally is based on its size, usually stated in millimeters, and the quality of its nacre coating, which gives it luster. Jewelers should tell you if the pearls are cultured or imitation.
Some pearls occur naturally in black, bronze, gold, purple, blue, and orange tints; others are dyed. Jewelers should tell you whether the colored pearls are naturally colored, dyed, or irradiated.