Book excerpt: A concise, systematic, yet comprehensive ``textbook'' treatment of the science. Examines origin, chemistry, crystallography, physical properties, and optical properties of gemstones, and discusses methods and instruments used to determine these properties. Highlights recent advances and important contributions; explores new methods of study and determination, crystal structures, causes of color, new gem minerals, new methods of gem synthesis, manufactured gems, chemical compounds used as substitutes, and imitations and simulations. Includes 12 full color plates, 350 halftones and line drawings, and a Gem Index that lists commonly sought characteristics along with page number references.