Khan Academy: Gems and Their Creation

A gem is a precious or semiprecious stone that has been cut, polished, engraved, or otherwise altered to be used as a personal insignia for making sealings or as decoration. Often imported from afar, ancient gems were truly exotic, and their bright colors and luminescence added to their value. What gemstones were used in antiquity? Semiprecious gems have long been carved with decorative and figural designs. Opaque and translucent quartzes and chalcedonies, popularly known as cornelian (or carnelian), sard, jasper, onyx, and sardonyx, come in a variety of colors.