Enamel work and gemstones

In enamel work, powdered glass coloured with metal oxides diluted with water and adhesive is applied to certain parts of the piece of jewelry that have been cut lower or surrounded with a raised rim made of gold, silver, or copper. The object is then heated until the glass melts and adheres to the metal. As the enamel gradually cools, it crystallizes and, when smoothed, takes on greater lustre and colour. The enamel applied to jewelry can be opaque or translucent. By letting light through, transparent enamel catches reflections from the metal to which it is applied and makes visible any engraving done on the metal. Enamel is also distinguished according to the way it is applied, as in cloisonné, champlevé, basse-taille, painted, and plique-à-jour.

Enameling preceded the polychromy created by precious stones. In the beginning, in Egypt, Greece, and the Sāsānian period in Iran, unpolished enameled parts of jewelry were often used to imitate lapis lazuli or malachite.

To a limited extent, jewelry also was decorated with the niello technique (from the Latin nigellus, an adjective derived from niger, meaning black). This consists of cutting grooves in gold or silver with a graver and then filling these with a powder made of red copper, silver, lead, sulfur, and borax. When heated, the powder melts and fills the grooves, adhering to the metal. After the piece has cooled, the surface is smoothed and polished, and the design shows up in black.

Gems
In addition to gold, silver, and platinum, the precious materials most widely used in jewelry are gems—any precious or semiprecious stone. By definition this group also includes some animal and vegetable products with precious characteristics, such as amber, pearls, and coral. Conventionally, the following are classified as precious stones: diamonds, rubies (corundum), emeralds (beryl), and sapphires (corundum). To these, however, can be added chrysoberyl, topaz, and zircon because of their hardness and their refraction and transparency index.