  - Amorphous—without definite molecular structure; not crystalline.
- Baroque stone—an irregularly shaped, polished stone; usually applied to tumbled stones.
- Baroque pearl—an irregularly shaped pearl.
- Brilliancy—reflecting much light; having brightness.
- Brilliant cut—a mode of arrangement of facets commonly used on round or oval stones. The standard American brilliant cut has 57 or 58 facets. Most diamonds of 5 or less carats are cut in this manner.
- Cabochon—a stone cut with a flat or convex upper surface; sometimes faceted in part. Opal, star sapphire, and agate are stones that are frequently cut in this style (fig. 2).
- Cambrian—a division of geologic time, estimated to be the time from 550 to 440 million years ago; the oldest time division of the Paleozoic era.
- Carat—a unit of weight equal to ? of a gram or 0.2 gram. One ounce avoirdupois is equal to 141.75 carats.
- Cleavage—the tendency of certain minerals to split in particular directions yielding relatively smooth plane surfaces.
- Conchiolin—an organic albuminoid substance found in pearls.
- Conchoidal—a type of fracture having curved concavities or the approximate shape of one-half of a bivalve shell. Glass has excellent conchoidal fracture.
- Cretaceous—a division of geologic time, estimated to be the time from 135 to 60 million years ago; youngest division of the Mesozoic era.
- Crown—that portion of a faceted gem above the girdle; the upper portion of a facet-cut gem (fig. 6).
- Cryptocrystalline—composed of very fine or microscopic crystals.
- Crystal—the regular polyhedral form, bounded by plane surfaces, that is assumed by a mineral under suitable conditions. Crystals have definite external symmetry and internal molecular order.
- Crystalline—possessing definite internal molecular order; not amorphous.
- Cubic—in the general shape of a cube. The isometric crystal system is often called the cubic system.
- Culet—the very bottom portion of a faceted gem; the point or line formed by the intersection of the lowest pavilion facets (fig. 6).
- Dendritic—branching or tree-like in form.
- Diaphaneity—relative transparency. The diaphaneity of a mineral is described as transparent, translucent, opaque, etc.
- Dike—a tabular rock body, usually igneous in origin, which cuts across the surrounding rock strata.
- Dispersion—a measure of the ability of gemstones to separate complex or white light into its component colors; often illustrated with a prism. Gemstones that are capable of separating colors 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; 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 i
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