Soude emeralds are crystal cemented over green glass. Foilbacks have been made for nearly 4,000 years, using a variety of techniques. One kind of foilback involves placing a backing of foil or a metallic coating on a stone to give it a more brilliant color. Colored substances on foilbacks add color to clear stones or deepen the color of pale stones. Cat's-eyes and star effects are created with etched backings.
Rhinestones are one example of a popular foilback.
Cultured pearls fall into a special category. Mollusks have shells and most of them live underwater. The purpose of nacre is to make the inside of the shell smooth so that it doesn't irritate the soft body of the mollusk. A pearl is simply an abnormal growth of nacre. Natural pearls are formed when something, such as a grain of sand, gets inside its shell and irritates the mollusk.