of light widely are said to have high dispersion; gemstones not so capable of separating white light into colors are said to have low dispersion. Dopping—the act of cementing a gemstone, either rough or partly finished, to a dop-stick. Dop-stick—the wooden stick or cylindrical piece of metal to which a gemstone is cemented to facilitate handling during cutting and polishing. Dop-wax—the agent or cement used to secure a gemstone to a dop-stick. Emerald cut—a rectangular or square faceted stone with beveled corners whose surfaces are covered with several series of rectangular facets. Eocene—a division of geologic time, estimated to be the time from 50 to 40 million years ago; one of the older divisions of the Cenozoic era. Extrusive rock—igneous rock that has been extruded or forced out onto the earth’s surface. Facet—a single plane polished surface on a faceted gem. Facet head—a device used in the cutting and polishing of faceted gems; used to control the placement of facets and their relative angles (fig. 7). Facet table—the equipment used in the cutting and polishing of faceted gems and the table on which most of the equipment is mounted (fig. 7). Feldspar—a group of closely related silicate minerals including orthoclase, microcline, sanidine, plagioclase, labradorite, and others. Fire—the reflections of variously colored light from a precious opal; also the different colors of light reflected from a faceted gem owing to the dispersion of the mineral. Fracture—the texture of a freshly broken surface other than a cleavage surface, described as conchoidal, even, splintery, etc. Gem—a cut and polished gemstone. Gemology—the science dealing with the study of gemstones. Gemstone—a mineral suitable for cutting into a gem; the term gemstones is frequently used collectively to include both cut and polished stones and rough stones. Geode—a rounded or spherical rock cavity; commonly lined with crystals. Girdle—the portion of a faceted gem separating the crown from the pavilion; the girdle may or may not be polished and usually contains about 2 percent of the total depth of the gem (fig. 6). Gneiss—a coarse-grained metamorphic rock having segregations of granular and platy minerals that give it a more or less banded appearance without well-developed schistosity. Grain (pearl grain)—a unit of weight equal to 0.05 gram or 0.25 carat; not the same as the Troy grain. Granite—a granular igneous rock composed mostly of quartz, feldspar, and commonly mica and/or hornblende. Hexagonal—having six angles and six sides; a crystal system in which the crystal faces are referred to four intersecting axes;