Glossary of Gemological Terms

Abraded culet When the small culet facet becomes chipped or scratched by another diamond, or resulting from being carried loose in a diamond paper with other diamonds.
Adamantine Diamond like. The appearance of a material's surface in reflected light, as determined by the quantity and quality of light reflected. The luster of most cut and polished diamonds is described as adamantine luster.
Alluvial deposits Sometimes environmental forces break down the kimberlite at the surface of the pipe, and diamonds get washed out of the pipe into riverbeds. These riverbeds (or ancient riverbeds) are referred to as alluvial or secondary deposits.
Amsterdam In Holland , a traditional diamond cutting center that has declined markedly in recent years, although it was very important at one time.
Antwerp In Belgium , the most important diamond cutting center in the world and a major distribution center for polished goods.
Bahia A gem producing state in Brazil , diamonds were discovered here as early as 1755. Bahia is also an important source of carbonado.
Bezel facets The eight large, four sided facets on the crown of a round, brilliant-cut diamond, the upper points of which join the table and the lower points, the girdle.
Black diamond When a diamond is dark gray, a very dark hue, or truly black, it is referred to in the trade as a black diamond. Such a stone may be opaque to nearly semi-transparent.
Blemishes Imperfections in a diamond that are confined to the surface of the stone.
Blocking The process of placing the first 18 (main) facets on a diamond brilliant (17, if there is no culet) while fashioning a diamond.
Boron One of the many impurities in diamond which is now believed to cause the semi-conductor properties and the blue color in Type IIb diamonds.
Bort (also Boart) The lowest quality of diamond so badly flawed and imperfectly crystallized that it is suitable only for crushing into abrasive powders for a multitude of industrial purposes.
Brilliance Brilliance is the quality and quantity of light returned from reflections within the gemstone to the eye.
Brillianteering The placing and polishing of the 40 remaining facets on a brilliant-cut diamond after the main bezel and pavilion facets have been placed and polished by the blocker.
Bruting The step in the fashioning process of a diamond in which the stone is given its circular shape.
Canary diamond A trade name for an intensely colored yellow diamond. The yellow may be very slightly greenish or slightly orangy, but it must be deep enough to be an asset. Such a diamond is called a fancy.
Cape A broad range of diamond color grades for stones that show a distinct yellow tint face-up.
Cape spectrum Diamonds with a significant depth of yellow body color of natural origin frequently show a characteristic absorption spectrum when examined with the spectroscope. Since yellow stones are known as capes, the absorption lines are often called Cape lines of Cape spectrum.
Carat A unit of weight for diamonds and other gems.
Carbon A non metallic chemical element C, occurring native in the crystalline form as diamond, graphite and lonsdalite.
Carbonado Black industrial diamond, a massive, impure, slightly cellular aggregate of minute diamond crystals, forming a mass with a granular to compact structure. It is the toughest form of industrial diamond.
Carob seeds The origin of the 'carat weight' stems from the seed of carob tree. The dried dark-colored seeds are very uniform in weight and were first used by ancient pearl merchants as units of weight.
Central Selling Organization (CSO) De Beers created the Central Selling Organization (CSO) to market their rough diamond production through their office in London , to ensure that the world's diamond markets receive the rough diamond required to meet their needs.
Certificate A written report describing a diamond as to its weight, proportions, color grade and clarity grade.
CIBJO Acronym for the French equivalent of the International confederation of Jewelry, Silverware, Diamonds, Pearls and Stones. CIBJO is a European organization which has established rules of nomenclature and their application for gem materials.
Clarity Clarity refers to a diamond's purity.
Cleavage Cleavage refers to the tendency of a single crystal to break or split along smooth flat planes parallel to atomic planes.
Cleaving A process occasionally used in fashioning diamonds. The splitting of a stone along a cleavage plane, or grain, into two or more portions, to produce pieces of a size or shape that will produce fashioned stones more economically or of better qualities.
Color center A structural defect in the atomic lattice of a mineral that absorbs light.
Color grade The relative position of a diamond's body color on a colorless-to-yellow scale.
Crown That portion of any faceted gemstone above the girdle.
Crown angle The angle measured between the table and the crown facets and is read as the reciprocal angle.
Crown height The distance from the girdle to the table.
Crown height The vertical distance between the girdle plane and the table.
Crystalline Gems with orderly atomic structures are termed crystalline.
Crystal The term crystal refers to a crystalline substance that displays outward signs of its internal symmetry - it shows flat surfaces ('faces').
Crystal form The geometric form or shape of a crystal, e.g., a common diamond form is an octahedron.
Cutting shapes Shape refers to the basic girdle outline of a stone.
De Beers Consolidated Mines, Ltd Privately owned company formed in 1888 in South Africa, controlling the major production and distribution of rough diamonds.
Depth percentage The depth percentage is calculated by measuring the depth of the diamond (the distance from the table to the culet) in millimetres, and dividing that number by the average girdle diameter, which is also measured in millimetres.
Diamond Trading Company (DTC) DTC is the marketing arm of the De Beers group of companies. DTC sorts, values and sells the majority of the world’s rough diamonds.
Diamond appraisal A monetary evaluation of a diamond or diamond jewelry, usually for insurance or estate purposes. An appraisal is only an opinion on the identity, quality, and value of a diamond at a given point in time.
Diamond Bourse A wholesale diamond exchange dealing mainly in polished stones. The Diamond Exchange with the purpose of protecting trade’s integrity and interests with its own infrastructure, constitution and by-laws and strict requirements for membership.
Diamond sieve A round, perforated metal plate for rapid grading of loose, fashioned diamonds for size. A number of such plates are usually used, each having perforations of a different size.
Dispersion Dispersion is the breaking up of white light into its six component colors of red, orange, yellow, green, blue, and violet.
Dop Any device that is used to hold a diamond during the sawing or faceting process. A cutter's dop is usually made of bronze or copper with either a cup-like or flat surface to which the diamond is fixed with "cutter's cement".
Doublet A composite assembled stone usually of two parts, crown and pavilion, cemented or fused together. Diamond doublet has a diamond top and pavilion of synthetic colorless sapphire or spinel, quartz, glass etc.
Durability Durability depends on a combination of many factors, particularly hardness and toughness.
Electric conductivity (of diamond) The electrical resistance of diamond is very high: therefore, it is essentially a non conductor.
Extinction Should the pavilion be too deep, light passes out the side and extinction results, also referred to as black center.
Eye clean A term that implies that no internal flaws are visible to the unaided eye of a qualified diamond clarity-grader.
Fancy colored diamonds All other natural colors in diamond are considered fancies regardless of hue and tone.
Four C’s Color. Clarity, Cut and Carat weight.
Gemstone fashioning The cutting and polishing of a gemstone from the rough to a finished stone.
Girdle diameter External circular diameter of a diamond. Girdle separates the crown to the pavilion of a diamond.
Grain The atoms of a diamond are arranged in planes. This is termed the "grain" of the diamond, and is similar to the grain in a piece of wood.
Grainer Stones with weights near multiples of 0.25 cts, or one grain, are referred to as grainers, qualified by the appropriate numerical designation, e.g.: four-grainer for a one-carat stone.
Hardness Hardness is the resistance to scratching of a smooth surface.
HPHT High pressure - high temperature. A treatment process developed to improve color in a diamond using high pressure high temperature technique.
Hue The pure color defined by position on the color spectrum; what is generally meant by ‘color’ in lay terms.
Ideal cut The term ideal cut, used predominantly for round brilliant stones refer to the set of proportions that yield the highest degree of brilliance. These were calculated by Marcel Tolkowsky who published his findings in 1919. The objective in faceting a gemstone is to achieve beauty through brilliance, scintillation and dispersion. A faceted stone is designed to be a light trap - to collect as much light as possible - and throw it back to our eyes.
Inclusion Any visible internal foreign object, or any crystal or grain of the same material as the host, or any break in a diamond or other gemstone can be called an inclusion.
Inclusions Imperfections in a diamond that are found inside the stone.
Inert No detectable reaction, such as fluorescence, to stimulus when exposed to radiation from x-rays, ultraviolet or cathode rays.
Kimberlite The Kimberly Process ensures that conflict or illicit unofficial rough diamonds will be excluded from the legitimate distribution channels.
Loupe Any small magnifying glass mounted for use in the hand as a hand loupe. A loupe usually contains a system of lenses (corrected) to the minimum specifications for a magnification of 10x power.
Macle A term used in the diamond trade for a flat, triangular rough diamond, which is a twinned crystal.
Make A trade term that refers to the proportions and finish of a diamond; for example, good make refers to a stone that is well proportioned, symmetrical and well polished.
Master diamonds Fashioned diamonds of known color grades that are used as comparison stones when grading other diamonds for body color.
Melee From the French, meaning a confused mass, a skirmish, an affray, etc. 1) In the trade, the term is used collectively to describe smaller brilliant-cut diamonds. 2) Usually, all small gemstones used in embellishing mountings, settings, or larger gems are called melee.