Glossary:

Understanding the Language of Gemology

Adularescence: The white or silver-blue haze displayed by top-quality moonstones and certain other gems.

Akoya oysters: Mollusks of choice in the cultivation of saltwater pearls.

Alluvial deposits: Gem deposits found in water after they have been separated from the mother rock.

Appraisal: An evaluation of the actual replacement value of a piece of jewelry. This practice is often done by a licensed gemologist.

Asterism: The star effect displayed by certain gems with properly intersecting inclusions. Gems of this variety are always cut en cabochon (see Cabochon cut).

Baroque: Odd-shaped pearls formed during cultivation.

Brilliant cut: Round-shaped stone consisting of a minimum of fifty-eight facets.

Cabochon cut: The art of rounding a gem without facets into the shape of a highly-polished dome.

Cameo: The art of carving a shell or similar matter above its background.

Carat: Used to denote the weight of gems.

Chatoyancy: The ability of certain gems to display a “cat’s-eye” effect, due to the precise occurrence of narrow inclusions. These gems are always cut en cabochon.

Cleavage: Breakpoint or weakness of a gem, connected to its atomic structure. Ironically, gems defined as having perfect cleavage are the ones most likely to break when being cut or faceted.

Color: Important property used in the evaluation of a gem, particularly the diamond. Interestingly enough, the quality of a gem may be measured based on either the presence or the absence of color.

Commercial jewelry cleaner: A solution that usually incorporates water and a mild detergent, although many often contain small amounts of ammonia. These solutions are formulated to maintain a proper pH balance (between too much acidity and too much alkalinity).

Conchiolin: Dark substance secreted by a mollusk for protection during cultivation.

Crown: Denotes the top part of the gemstone.

Culet: The lowest part of a gem.

Cultivation: The process of forming a pearl by inserting tissue from a sacrifice mollusk into another.

Cut: The method of faceting a gem.

Dichroism: The ability of certain gems to display a second color when viewed from a different angle.

Dispersion: The splitting of light as it enters a gemstone. Also known as the stone’s “fire.”

Doublet: A stone made of two components, generally held together with a clear or colored adhesive.

Emerald cut: See Step cut.

Enhancement of gems: The process by which the appearance of a gem is improved. An excellent example of this is heat enhancement, which most often is used to clarify a gem or improve its color.

Facet: The cut and polished portion of a gemstone.

Faceting: The art of cutting a stone or precious metal to enhance its appearance.

Fire: The rainbow of colors resulting from light rays as they enter a stone.

Fluorescence: The ability of certain gems to glow when heated in low-light situations.

Foiling: The practice of inserting a colored or silver foil behind a gem in a closed-end setting, to enhance its appeal. Most often used in the production of rhinestones.

Full cut: A round-shaped, brilliant-cut gem of small proportions. Although this is most often used to describe accent diamonds, the term also may apply to colored gems.

Gemstone: A mineral or combination of minerals displaying a high degree of beauty, rarity, durability, and desirability.

Geode: A hollow rock cavity, usually containing some form of one or more gems. Two excellent examples of gems often found in geodes are amethyst and peridot.

GIA: The Gemological Institute of America, which most experts consider the established authority on gemology. Among many other things, the GIA offers in-home study courses that cover an extensive range of subjects.

Girdle: The widest point (in circumference) of a gemstone.

Gram: Unit of measurement of weight (with just over twenty-eight grams equaling one ounce), often expressed in milligrams (at one thousand milligrams to a gram); in jewelry, grams are used in expressing the weight of precious metals such as gold and silver.

Handmade: A legal term used to describe an item made without the use of machine tools.

Heat treatment: The application of heat to a gem for the purpose of improving its color or clarity. Many gems are treated in this manner.

Illusion setting: Any setting that is not as it appears. Most often, illusion settings combine one or more small diamonds with rhodium to enhance the diamonds, or to give the impression of a higher carat weight.

Inclusions: Foreign matter within a gem or mineral, which often displays different colors from the rest of the stone, or other effects.

Intaglio: The art of carving a shell or similar matter beneath its background. An intaglio is the opposite of a cameo.

Iridescence: The play of colors seen within a gemstone as a result of inclusions interfering with light entering the stone.

Karat: Unit of measurement that indicates the quantity (or parts) of fine gold in a piece of jewelry. This karatage is always based on pure 24-karat gold. For example, 18-karat gold contains eighteen parts fine gold and six parts other metal alloys.

Lapidary: Gifted craftsperson who cuts and polishes gems to their finished state.

Lava: Molten rock that forms above the surface of the Earth as a result of volcanic activity.

Loupe: A compact magnifying glass used to examine stones and settings. The most common jewelers loupe magnifies objects ten times.

Luster: The outward appearance of a gem or an organic material. Luster is of particular importance in determining the quality of a pearl.

Mabe: Literally translated, mabe means “half,” and this term is used to denote the large half pearl or shell often seen in jewelry. Also known as blister pearls.

Magma: Rock that forms below the surface of the Earth as a result of volcanic activity.

Mineral: An inorganic element of the Earth of consistent atomic structure and chemical composition.

Mohs’ hardness scale: Numerical scale developed in the nineteenth century by Friedrich Mohs that assigns a rating to a gem according to its ability to resist scratching (with hardest rated 10 and softest, 1).

Mother-of-Pearl: The actual substance that lines the inside of the oyster or other mollusk.

Nacre: The substance secreted by the oyster or other mollusk that surrounds the darker conchiolin and subsequently forms the outer layers of the pearl.

Natural: Gem formed without the assistance of humans.

Opalescence: A variety of iridescence that is most often light blue in color.

Organic gem: Matter that is not technically a gemstone, but rather is derived from animal or plant life. In jewelry, this group consists of amber, coral, ivory, jet, pearl, and tortoiseshell.

Origination: In this book, origination refers to the country or countries where a particular gem or mineral can be found.

Overlay: Generic term used to describe any variety of costume jewelry that involves a base metal.

Oxidation: The breakdown of a metal over time as a result of exposure to oxygen and other natural elements.

Pavé: By definition, in order for a setting to be classified as pavé (pronounced “pah-vay”), one prong must touch three or more stones. If not, it is classified as a cluster. The pavé setting is most often used with diamonds and cubic zirconia.

Pavilion: The lower portion of the gemstone. The pavilion begins just below the girdle.

Pearl essence: A liquid coating that adds a pearl-like luster to simulated pearls. This coating is actually derived from the scales of a certain type of herring.

Phenomena: Special properties exhibited by certain gemstones. An excellent example of a phenomenon is the color-change property of the alexandrite.

Pleochroism: The ability of certain gems to display two or more colors when viewed from different angles.

Points: Units of measurement used to express the carat weight of a gem or simulant. One carat is equal to one hundred points, a half carat (.50) is equal to fifty points, a quarter carat (.25) equal to twenty-five points, and so on.

Precious gem: At one time, only four gems were considered to be precious: the diamond, emerald, ruby, and sapphire. Changing conditions in the colored-gem market have pretty much made this term a thing of the past.

Precious metals: The industry defines gold, silver, platinum, and palladium as precious metals. Unlike gemstones, the term precious is still widely accepted when used to delineate metals.

Primary deposit: When a gem (or mineral) is found resident in its original rock.

Refraction: In gems, the bending of light as it enters the stone and slows down.

Refractive index: Developed by Willebrord Snell in the seventeenth century. This process usually incorporates a refractometer, which measures the speed and angle of light as it enters a gemstone. The refractive index is often the key to gem identification.

Rose cut: An age-old art of faceting a stone to look like an opening rose. This type of cut is usually seen in groups of six. This method of cutting is sometimes used in the creation of marcasite jewelry.

Rough: In gemology, this refers to the raw, natural state in which gems are found.

Rutiles: Needle-like inclusions within a stone, which can produce such gem phenomena as an asterism (or star) and a cat’s-eye, depending on the direction of the rutiles.

Scarab: The art of carving the intricate detail of the sacred scarab beetle into stone. The ancient Egyptians considered the scarab a symbol of the soul.

Secondary deposit: A deposit of gems that has been worn away from its original site, usually by weather. An alluvial deposit is one example of a secondary deposit.

Semiprecious gem: At one time, this category was reserved for all gems other than diamonds, emeralds, rubies, and sapphires. Like the category of precious gems, this term is seldom used today.

Shaving: Cutting a stone generously across the table, while allowing for very little depth. Shaving makes a stone appear to be of higher carat weight than it is.

Sheen: Another name for iridescence.

Simulant: A human-created gem having the same look, but not the same physical properties, as its natural counterpart. An excellent example of a simulant is the cubic zirconia, which has a diamond-like appearance yet none of its properties.

Single cut: Stones consisting of seventeen facets or less.

Soudé: Assembled gems of multiple layers, held in place by an adhesive. A newer term sometimes used interchangeably with triplet.

Species: A gem with distinct characteristics that are well defined.

Specific gravity: In gemology, this refers to a process that determines the weight (or density) of a gem when compared to that of water. The result is a ratio expressed as a single number.

Steam cleaning: Used primarily by jewelers. This aggressive cleaning process combines steam with pressure. Steam cleaning is not suitable for all gemstones.

Step cut: A manner of cutting a gem with rectangular facets along its perimeter.

Swiss cut: Stones consisting of thirty-three facets.

Synthetic gemstone: A man-made stone that, unlike a simulant, has the same chemical composition and crystal structure as its natural counterpart.

Table: The flat top part of a gemstone.

Table facet: Central facet on the table (or crown) of a gem or simulant.

Trichroism: Those gems that display precisely three different colors when viewed from different angles.

Triplet: A man-made creation of three parts that normally includes a clear protective top layer fused together with a thinly sliced gem and a clear or colored adhesive.

Ultrasonic cleaners: Ultrasonic cleaners convert high-frequency sound waves into electrical energy, which causes a solution to bubble. This type of cleaning method is quite useful when intricate pieces of jewelry are involved. Delicate gems and organics such as amber, coral, jet, and pearls should not be cleaned in this manner.

Veins: In gemology, this usually is a term loosely used to describe long thin lines that occur on the surface of the gem. The black lines normally seen in turquoise are an excellent example of veins.