Among the synthetic gems produced commercially are rubies, sapphires, emeralds, and spinels. Diamonds of gem quality have also been manufactured. Color changes are produced in diamonds by exposing them to radioactive bombardment. Doublets are made by combining a crown, or upper part, which is a thin slice of either the true stone or some inferior but hard gem, with a lower part of the true stone, a substitute stone, colored glass, or colored paste. Triplets generally consist of a layer of paste between two genuine stones of poor color.
Imitation pearls are glass or plastic beads coated with a preparation made from fish scales.
The Columbia Encyclopedia, Sixth Edition.