Glossary Of Gem Stones
ARTIFICIAL STONE - A gem material that is either a manmade imitation or a synthetic. (See synthetic).
ASTERISM - The optical phenomenon of seeing a rayed figure in the form of a star. This is caused by the reflection of light from minute oriented and aligned needle-like inclusions.
BAGUETTE - Rectangular style of step cut used for small gem materials.
BAROQUE - Irregular in shape, such as baroque pearls, tumble-polished stones, or freeform shaped gem materials.
BLEMISH - Any surface imperfection on the surface of a gemstone. For example, on a diamond, one would be referring to a nick, knot, scratch, abrasion, minor crack or fissure (cavity), or a poor polish.
“BLUE-WHITE” - According to the Federal Trade Commission, only diamonds with a distinct blue body color may be traded as “blue-white.” For many years the term referred to a diamond without a distinctive body color, however misuses of this term have rendered it almost meaningless.
BRILLIANCE - Total amount of white light returned to the eye from a diamond or colored stone as the result of internal and external reflections. The major factors that affect the amount of brilliancy in a gem are refractive index, proportions, polish and transparency.
BRILLIANT CUT - The most common style of diamond cutting, also used for many other gemstones, which consists of a combination of triangular and kite shaped facets. The round brilliant cut features 57 or 58 facets.
CABOCHON - An unfaceted cut stone of domed form (having a convex surface). Often called a “cab.”
CANARY - A term referring to diamonds with an intense yellow hue. The term “fancy yellow” is often used if the yellow makes the stone very distinctive.
CARAT - A unit of metric measurement used for gems. One carat (ct.) equals 100 points, 200 milligrams, or 1/5 of a gram.
CARBON SPOTS - Any black-appearing inclusions in a diamond caused by the addition of the minerals graphite or hornblende.
CHANGE OF COLOR (COLOR CHANGE) - A phenomenon of some colored stones in which the gem material appears a different color in different types of light. This is caused by selective absorption and/or transmission of the specific type of light to which the gemstone is exposed.
CLARITY ENHANCED - A gemstone that has been treated to improve its appearance by filling fissures or fractures with a transparent substance. Also called “fracture filled.”
CLARITY GRADE - One of the four value factors of a diamond. Diamonds are ranked on a scale from flawless (no inclusions visible under 10x magnification) to included (eye visible inclusions).
CLEAVAGE - The tendency of a crystalline mineral to break in certain definite directions called cleavage planes. The breakage is done by cleaving, a process where a stone is studied so that the plane may be defined and divided with a swift blow. This swift blow splits the stone into proportions quickly as opposed to sawing.
COLORED STONE - All natural gemstones except for diamonds or colored diamonds.
COLOR GRADE - One of the four value factors of a diamond. Diamonds are ranked on a scale from “D” (colorless) to “Z” (noticeable tint of color, typically yellow or brown). Diamonds with saturation greater than “Z” color are considered Fancy Colored Diamonds and are graded on a separate scale.
COLOR ORIGIN - A determination of the cause of color in fancy colored diamonds. Diamonds that are naturally colored are very rare and expensive. Two common ways of enhancing the color of diamonds are irradiation and the high pressure high temperature (HPHT) process.
CROWN - The part of any faceted gemstone above the girdle.
CULET - The small facet polished across what would otherwise be the sharp point or tip of the pavilion of a faceted stone, especially a round brilliant cut. It is cut and placed with the full intention of avoiding breakage of this tip.
CULTURED PEARL- A pearl produced by implanting a shell bead or mantle tissue into the body of a pearl bearing mollusk. Japan, China, Tahiti, Australia, the U.S., and the Persian Gulf are pearl producing regions.
DIAMOND - A mineral that crystallizes in the cubic system and is composed of carbon with a hardness of 10, a refractive index of 2.417, and a specific gravity of 3.52.
DISPERSION - The separation of white light into its component spectral colors (see fire).
DURABILITY - A combination of hardness, toughness and stability that describes a specific gemstone’s ability to resist wear.
EMERALD CUT - A style of cutting a gemstone in which the outline is a rectangular shape with cut corners and the shape of the facets are rectangular and trapezoid.
ENAMEL - A vitreous glaze. In jewelry, it is usually fused glass that is heated to a base–most commonly to the surface of metal, glass or pottery.
FACET - One of the small, flat surfaces that is polished on a gemstone to increase brilliancy and beauty. A round brilliant cut gemstone has a total of 57 or 58 facets.
FACETED GIRDLE - A girdle of a stone that is lapped and polished for maximum surface reflection and has many flat planes around the circumference of the stone.
FEATHER - A fracture or break in a gemstone that is typically white and feathery in appearance.
FINISH - The term referring to the quality of fashioning a gemstone including the polish and symmetry.
FIRE - Flashes of spectral colors seen in gemstones as a result of dispersion.
FLUORESCENCE - The process of a material emitting visible light when it is subject to ultraviolet light. For example, diamonds under ultraviolet light will often emit a visible light of a blue hue.
FOUR C’S - A phrase used to describe a diamond’s value characteristics, all starting with the letter “C”: cut, color, clarity, and carat weight.
FRACTURE - A break within a gemstone.
FRESHWATER PEARL - A pearl that forms in a freshwater living mollusk.
FULL CUT DIAMOND - A description of a brilliant cut, round stone with 57-58 facets.
GIRDLE - The narrow band around the widest part of a polished or faceted gemstone. It divides the crown and pavilion facets.
HARDNESS - A gem’s ability to resist scratching.
HUE - The primary impression of color such as red, green or blue.
INCLUSION - Internal characteristics present in gemstones. Common diamond inclusions include feathers, crystals, fractures, graining, pinpoints, and cavities.
IRIDESCENCE - Prismatic colors inside or on the surface of a material caused by light interference from thin layers of differing refractive indices. These layers may be thin films of liquid, gas or solid. Pearls are best known for exhibiting iridescence.
LUSTER - The quality of reflected and refracted light from the surface of a gemstone or pearl.
MAKE - Trade term referring to the proportions, symmetry, and polish of a gemstone.
MARQUISE - A fashioning style for gemstones with the girdle outline in an elliptical shape with pointed ends.
MELEE - A term used primarily to describe small, round faceted diamonds or colored stones of approximately .18 carat or less.
MOHS SCALE - A loose scale of hardness, used for field collecting, which allows for identification of specimens. Devised by Friedrich Mohs, a German mineralogist, in the 19th century. The comparative scale of hardness is as follows:
1) talc
2) gypsum
3) calcite
4) fluorite
5) apatite
6) moonstone
7) quartz
8) topaz and beryl
9) corundum
10) diamond
MOUNTING - Trade term for that portion of a piece of jewelry in which a gem or other object is set.
NACRE - The iridescent substance secreted by a mollusk during the formation of a pearl that consists of layers of aragonite and calcite crystals.
NATURAL - A trade term for a portion of the original surface of a rough diamond left by the cutter when polishing and faceting a diamond. Naturals are usually found near the girdle of a diamond and are represented in green on a plotting diagram.
NATURAL PEARL - A pearl that originates naturally in a mollusk, as distinguished from cultured or imitation pearls.
NICK - A minor chip on the surface of a diamond, usually found near or on the girdle of the stone.
OPAQUE - A term referring to the transparency of a gemstone; opaque materials do not transmit light.
ORIENT - The soft iridescent glow on or just below the surface of a pearl or cultured pearl. It is caused by the interference and diffraction of light from the minute crystals comprising the pearl’s nacre.
OVAL CUT - A fashioning style for gemstones with the girdle outline in an elliptical or oval shape.
PAVE - A style of setting small stones as close together as possible in two or more rows.
PAVILION - That portion of a faceted diamond or other gem material that lies below the girdle.
PEAR SHAPED CUT - A fashioning style for gemstones with the girdle outline in the shape of a tear drop or pear.
PERFECT - The Federal Trade Commission considers it an unfair trade practice to use the word “perfect” or any other word, expression or representation of similar derivative, as a description of any diamond with inclusions that are visible to the naked eye or up to 10X power magnification. This term has been so flagrantly misused in the sale of diamonds that many jewelers avoid its use entirely.
PLOTTING DIAGRAM - A schematic diagram included on the EGL USA Diamond Certificate that illustrates the facets (printed in black) of a diamond and the approximate location and type of internal and external characteristics (printed in red and green).
POINT - A measurement in the weight of a diamond equal to 1/100 of a carat. Thus, 0.50 carats is equal to 50 points.
POLISH - The smoothness of the surface of a fashioned gem in which optical reflection is maximized and shows no visible wheel or burn marks.
POLISHED GIRDLE - A girdle that has been lapped or polished to yield a uniform, highly reflective surface.
REFLECTION - The return of light that strikes the surface of stone.
REFRACTION - The change in direction of a ray of light as it enters a gemstone.
ROUGH - Any uncut or unpolished gem material.
SCINTILLATION - Mirror-like reflections from the facets of a gemstone as it turns in the light.
SIMULATED STONES - Any substance fashioned to imitate the appearance of a gemstone.
SINGLE CUT - A brilliant cut (round) with 17 or 18 facets: 8 bezel, 8 pavilions, a table and a culet facet.
SOLITAIRE - A term referring to a ring containing a single diamond or other gem.
STAR FACETS - The eight triangular facets that surround the table facet of a round, brilliant-cut diamond.
SYMMETRY - A term referring to the quality of the fashioning of a gemstone. The balance of the outline shape and the precision of the facet arrangement are considered in the symmetry grade.
SYNTHETIC - A manmade gem material that has essentially the same physical, optical and chemical properties as that of its natural counterpart.
TABLE - The horizontal, top flat facet on the crown of a faceted gemstone. The table percentage is a comparison of the size of the table facet to the width of the stone at the girdle.
TONE - An attribute of color that determines its lightness or darkness of shading.
TOUGHNESS - The ability of a mineral or gemstone to resist breakage (fracturing). Toughness is not the same as the hardness of a stone.
TRANSPARENCY - The ability of a substance to transmit light.
TREATED GEMS - A general term used to described gemstones that have been artificially modified to improve their color or clarity. Techniques include laser drilling, fracture filling (clarity enhancement), high pressure high temperature (HPHT) annealing, irradiation, and surface coloration. Gemological laboratories have the equipment and experience to detect the more difficult treatments.
TRILLION - A triangular shaped and faceted gemstone. More specifically, it is a gemstone that has an arched shaped triangle cut with 25 facets on the crown and 19 facets on the pavilion.
TWIN CRYSTALS - Two or more mineral crystals that have grown together in a symmetrical nature, usually nonparallel. Sometimes called twinned crystals.
TROY OUNCE - A weight unit for precious metals predominately used in England and the U.S. Twenty pennyweights (dwt) equals one troy ounce, and twelve troy ounces equals one troy pound.
ULTRASONIC CLEANER - A machine commonly used by jewelers to clean jewelry effectively. The ultrasonic vibrations free most types of dirt, grime, and oils from the items. Some ultrasonic cleaners come with a heating unit. When using this type of cleaner, extreme care is recommended because certain gems cannot withstand the heat and vibration. Consult your professional jeweler.
ULTRAVIOLET LIGHT - Light that is invisible to the naked eye because it consists of wavelengths shorter than those of visible light.
VERMEIL - Silver overlaid with gold (gold-plated silver).
WHITE GOLD - An alloy of yellow gold that has silver, zinc, or platinum as a whitening agent.
YELLOW GOLD - The natural color of pure gold as well as the color of karat gold when alloyed with certain metals.
