Imitation pearl

Imitation pearls are man-made objects (often beads) that are designed to resemble real pearls. A variety of methods are used to create imitation pearls from starting materials that include glass, plastic, and actual mollusc shell. Some beads are coated with a pearlescent substance in order to imitate the natural iridescence of nacre or mother of pearl. Varieties of imitation pearls include:

The pearlescent substance used to coat various cores may contain essence d'orient, isinglass, fish scales, oyster scales, mother-of-pearl powder, along with binders or dyes.

Coral is sometimes used to imitate pearls from the pink conch, and hematite is sometimes used to imitate black pearls.

...
Wikipedia