lens or dike shaped. Perthitic—a plaid-patterned structure resulting from intermixture of soda- and potash-rich feldspars. Phantom crystal—a crystal outline seen within another crystal, mostly due to entrapping of inclusions during the crystal’s growth. Pleochroism—the property of transmitting different colors of light in different crystallographic directions. Point—a unit of weight equal to ¹/₁₀₀ (0.01) carat. Porous—containing pores or void spaces. Precambrian—a division of geologic time, estimated to be all of geologic time prior to 550 million years ago; the time before the Paleozoic era. Preform—a gemstone that has been ground to a rough outline of the finished shape of a gem. Rhyolite—a fine-grained extrusive or shallow intrusive igneous rock of approximately the same composition as granite. Rough—uncut, not worked by a lapidary, not cut and polished. Schist—a metamorphic rock that contains an abundance of oriented platy minerals that enable the rock to be split with relative ease parallel to the flat surfaces of the platy minerals. Silicified—replaced by or containing a large amount of quartz or silica. Skill facet—a term often used for the pavilion girdle facets of the standard American brilliant cut (fig. 6). Specific gravity—the weight in air divided by the loss of weight in water at a given temperature, or the weight of an object in air divided by the weight of an equal volume of water; also called relative density; the most commonly used standard temperature for this measurement is 4° C. or 39.2° F. Star facet—one of the eight facets surrounding the table facet of a standard American brilliant cut (fig. 6). Step cut—a mode of faceting in which the surface of the gem is covered by a series of square or rectangular facets; stones thusly cut are usually square, rectangular, or irregular with straight sides in outline. Streak—the color of a mineral when finely powdered; usually determined by rubbing the mineral against a piece of unglazed porcelain. Symmetry—the number, location, and balanced arrangement of crystal faces in reference to the crystallographic axes or other crystallographic planes or directions. Synthetic gem—a gemstone manufactured by man that has approximately the same chemical composition and properties as a natural gemstone. Table facet—the large horizontal facet found on the crown of many gems, often called simply the table (fig. 6). Tenacity—the resistance of minerals to breakage, described by such terms as malleable, ductile, sectile, and brittle (p. 6). Termination—the end of a crystal that is completely enclosed by crystal faces, the crystal end that is not attached to the matrix.