Artificial and imitation gems are of various kinds. Synthetic stones are made in the laboratory of the same chemical elements as natural stones. 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. Synthetic stones may sometimes be detected by the presence of air bubbles, which, when numerous, cause a cloudy appearance; by having curved rather than straight striae; and by their unnatural color. 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. Paste (glass) gems usually contain lead and are consequently very soft; they soon lose their brilliance and color. Imitation pearls are glass or plastic beads coated with a preparation made from fish scales. A cultured pearl is made by inserting a small bead inside the oyster; the irritation causes the oyster to deposit pearly material upon the bead, leading to the formation of a pearl.