Synthetic emerald of high gem quality has been marketed successfully. A synthetic substitute for aquamarine is also available; it is really a synthetic blue spinel. VARIETIES Emerald: Grass green Aquamarine: Blue green Morganite: Pink Heliodor: Yellow Goshenite: Colorless TOPAZ [Illustration: Three different cutting styles and colors of topaz. From top, a 235-carat colorless stone from Colorado, a 171-carat dark champagne-colored stone from Madagascar, and a 129-carat sherry-colored stone from Brazil. (Slightly less than actual size.)] Because yellow is the most popular color of topaz it has become customary to believe that all topaz is yellow. Also, there is a tendency to believe that all yellow gemstones are topaz. Neither belief is correct. Stones of yellow, sherry, blue, pink, and colorless topaz all make beautiful gems, and their characteristics are identical except for color. On the other hand, citrine (a yellow quartz), although entirely unrelated to topaz, often is disguised in the trade under the names Brazilian topaz, topaz quartz, or just topaz. Great numbers of stones described and sold as yellow topaz really are the much commoner citrine, which has few of the characteristics of fine topaz. [Illustration: A cushion-cut topaz from Brazil that weighs 1469 carats. It is an odd shade of yellow-green.] [Illustration: A 3273-carat topaz of soft blue that came from Brazil. The Smithsonian Institution had this unique gem cut by Capt. John Sinkankas of California. For several years it was the largest topaz in the collection. (Both gems are shown in actual size.)] Topaz, an aluminum fluosilicate, has a hardness of 8, a vitreous luster, and a relatively high refractive index. It is found in near-perfect crystals that range in size from very small to very large, with some giants weighing as much as several hundred pounds. Most of these crystals, especially the largest ones, are colorless, a characteristic that indicates relatively high purity of composition. Although topaz gems have little fire, they take a high polish and can be very brilliant. Great care must be taken in cutting and polishing topaz because of its ready cleavage. The desired cut and high polish can be secured by avoiding excessive heat or pressure during the operation and by planning facets so that none lies exactly parallel to the cleavage direction.