three of these axes are equal, lie in the same plane, and intersect at angles of 60 degrees; the fourth axis is perpendicular to the other three. Igneous rock—rock formed by solidification from a hot melt. Index of refraction—a measure of the relative ability of a gemstone to “bend” incident light rays; sine of the angle of incidence of a light ray divided by the sine of the angle of refraction. Intrusive rock—rock that has been pushed (usually in a molten state) among pre-existing rock strata, commonly along faults or fissures. Intrusive rocks do not reach the earth’s surface but are commonly exposed at the surface by later erosion. Isometric—a crystal system in which the crystal faces are referred to three equal intersecting axes at right angles to each other. Lap—a disc-shaped piece of metal or other material which is impregnated with diamond dust, or some other cutting or polishing agent, that is revolved while the gemstone is worked against it. Lap plate—a metal plate to which a cutting or polishing lap is attached, usually by means of a threaded bolt and wing nut. The lap plate is attached to the shaft which is turned by the motor under the facet table. Lapidary—one who practices the lapidary arts; a gem cutter. Limestone—a sedimentary rock composed mostly of calcium carbonate. Luster—the appearance of the freshly broken or unweathered surface of a mineral in reflected light (p. 5). Main facet—as applied to the standard American brilliant cut, one of the first eight facets cut on either the crown or pavilion of a gem (fig. 6). Matrix—the material in which a specific mineral is embedded; also the rock to which one end of a crystal is attached. Metamorphic rock—rock that has been changed from its original state by heat, pressure, chemical action, or some combination of these factors. Millimeter—¹/₁₀ centimeter; approximately ¹/₂₅ inch. Mineralogy—the science concerned with the study of minerals, including their occurrence, composition, forms, properties, and structure. Monoclinic—a crystal system in which the crystal faces are described in relation to three intersecting unequal axes, two of which are at right angles and the third inclined. Oligocene—a division of geologic time, estimated to be the time from 40 to 28 million years ago; part of the Cenozoic era. Opaque—does not transmit light. Orbicular—containing orbs or spherical or eye-like markings or structures. Orthorhombic—a crystal system in which crystal faces are referred to three unequal intersecting axes at right angles. Pavilion—the portion of a faceted gem below the girdle (fig. 6). Pegmatite—a body of coarse-grained intrusive igneous rock, commonly