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EGL Asia Diamond Grading Lexicon Glossary

By Paul Glen,
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“Carat” is one of the four Cs according to which a diamond is graded, and one of the components that help a diamond buyer to determine the quality of the gemstone he or she is about to purchase. It also is the easiest to understand and measure, inasmuch as it refers to weight. Furthermore, because the density of diamonds is essentially constant, as the caratage of the diamond increases, so does its relative size. The term “carat” is the measurement used to designate the weight of gemstones, including diamonds. It originates from the Greek word "karaton," which was the carob seed that was used to balance scales in ancient times. A carat is equal to 200 milligrams. Carats are subdivided into points and there are 100 points in one carat.

Clarity is one of the four Cs according to which a diamond is graded, and one of the components that help a diamond buyer to determine the quality of the gemstone he or she is about to purchase. It essentially refers to the purity of the diamonds, or to the degree to which it is free of blemishes and imperfections. On principal, as the clarity of the diamond is improved, its value is enhanced. The clarity of a polished diamond is affected by both external blemishes and internal imperfections or inclusions, which were created in nature when the diamond was formed. Internal inclusions include piqués, or dark spots, gas bubbles or lines, and even tiny crystals. Clouds are clumps of microscopic inclusions that lower the transparency of the diamond crystal. Sometimes cracks in the diamond crystal, or fractures or feathers affect clarity, passing through the interior and sometimes reaching the surface. Most occurred during the crystallization of the gemstone, but on occasion they resulted during the cutting and polishing process, or some cane about as a result of wear and tear after being worn in jewelry.

Color is one of the four Cs according to which a diamond is graded, and one of the components that help a diamond buyer to determine the quality of the gemstone he or she is about to purchase. To first-time diamond buyers, it often comes as a surprise that diamonds come in a wide range of colors, ranging from colorless to yellow, but even blue, green, orange and sometimes red. Nonetheless, the majority of diamonds fall within a color range that runs from colorless (sometimes called white) to near- colorless and then to lightly yellowish or brownish. Color is evaluated by gemologists who compare stones to a standard master set. Today there are electronic systems which can assign grades, but members of the trade still consider a human evaluation to be more accurate. The slight variations that occur in a diamond’s color form the basis of a generally accepted color grading system that uses the letters of the alphabet, in which D is the highest grade, and it is assigned to a completely colorless or white diamond – a rare phenomenon. Near-colorless diamond are graded with the letters E and F. The most common color grades encountered by diamond consumers run from the color grade G through the color grade M. Diamonds that display a slight yellowish or brownish hue receive the color grades K, L or M. The color grades N, O, P, Q and R represent stones with a progressively light yellowish tint, while the grades S down to Z represent diamond that show an increasingly yellowish or brownish hue. When diamonds are of a truly vivid color, for instance a lively “canary” yellow or a distinctive pink, they are defined as fancy colored diamonds. Here, the alphabetical color scale does not apply and they generally are described according to their hue, and the intensity of the color. Fancy colored stones are valued very highly—even more so than colorless diamonds.

The section of a diamond above the girdle plane

Cut is one of the four Cs according to which a diamond is graded, and one of the components that help a diamond buyer to determine the quality of the gemstone he or she is about to purchase. The shape of the diamond is also sometimes referred to as the cut. In such instances, the term “cut” is not a criterion for placing a value on a diamond. The quality of the cut, or the “make,” as diamond dealers often say, is crucial. It is this that decides how the light entering the stone will react, and consequently how brilliant the stone will appear. A poorly cut diamond will actually loose light and appear dull. In the modern age, diamond cutters do their utmost to fashion each stone according to an exact mathematical formula, in order to achieve a finished product that is most pleasing to the eye. The most famous formula for a round brilliant stone was introduced in 1919 by a Belgian mathematician, Marcel Tolkowsky. Experts agree that cut is one of the most difficult qualities to judge. Currently most gem labs assign descriptions of “poor,” “fair,” “good” or “excellent” in their diamond grading reports.

This is the grade given to a polished diamond where no external blemishes or internal inclusions are visible using a standard 10X-magnification loupe. Experienced jewelers seldom see such stones, and some even argue that flawless diamonds simply do not exist.

Fluorescence is the effect viewed when a diamond glows under ultra-violet light (UV). Some 50 percent of polished diamonds display fluorescence, most of them in a blue hue, but sometimes other colors, especially yellow, can be observed. When diamond displays relatively strong fluorescence, it can cause a diamond to look somewhat “hazy” and therefore, less transparent.

‘Hearts & Arrows’ diamonds are precisely cut round brilliants that, when examined using a special viewer, display eight hearts though the pavilion from below and eight arrows through the crown from above. As top-quality cut stones, they typically fetch a premium in the market.

With I1 the best of the three, followed by I2 and I3, these grades are assigned to diamonds where the inclusions can be seen easily with the naked eye.

In an ideally-proportioned diamond, all of the light entering the diamond from the top will bounce within the diamond and be reflected back through the top, giving the stone maximum brilliance and fire. If the stone is too shallow or too deep, some light will escape through the bottom part of the diamond, giving the appearance of shadows when viewed from the top.

Measurement is the diameter of the diamond by the depth of the diamond, expressed in millimetres.

Natural diamonds are diamonds that have been found in nature, and have not been subject to unnatural treatments that may affect their appearance, such as High Pressure, High Temperature (HPHT) treatments and irradiation, which could affect color, and fracture filling and drilling which could affect clarity. In and of themselves, these treatments are not illegal, but they should be disclosed to the buyer before a transaction is completed, because they may have a bearing on price. Diamonds that are created in the laboratory or factory are alternatively known as synthetic, man-made, laboratory grown or factory grown.

The section of the polished gemstone below the girdle. The pavilion depth percentage is the distance from the girdle plane to the culet, expressed as a percentage of the girdle diameter of round diamonds or as a percentage of the width of the polished stone in a fancy shaped diamond.

The overall quality of the work done in producing a polished stone. Polish influences how well light is able to pass through a diamond and is very important to a diamond's brilliance.

Essentially “proportion” refers to the polished stone’s proportions, or the size of each of its various facets—flat, polished planes—and the angles at which they lie relative to each other.

Shape refers to the way that the stone is cut. Common shapes are round, marquise, oval, pear-shaped, heart-shaped, princess (square), and radiant (also square).

With SI1 being the better of the three, these diamond's inclusions can be detected with difficulty with the naked eye, but are quite obvious under the magnification of a 10X loupe. When of good color and make, such stones are an excellent choice for an expensive designer jewelry piece.

Symmetry refers to the exactness of the shape and arrangement of facets. This includes naturals, misshapen facets and extra facets, off center culets and tables, wavy and out of round girdles, misalignment of crown and pavilion facets

The table is the large, central plane or facet on the crown of a polished diamond, through which most of the light enters the stone, and out of which most of it is reflected. The relative size of the table is different for every cut.

Diamonds can be scientifically classified into 4 types, known as type 1a, 1b, 2a, and 2b. Diamonds are made of carbon, and are extremely pure, but in almost all diamonds there are tiny proportions of other elements, interspersed within the carbon as part of their crystal structure. Type 1 diamonds contain nitrogen. If the nitrogen atoms are clustered together within the carbon lattice, then the diamond is said to be a Type 1a diamond. About 98% of all diamonds are type 1a. Because these diamonds absorb blue light, they can have a pale yellow or brown color. In Type 1b diamonds, the nitrogen atoms are evenly spread out throughout the carbon lattice. These diamonds absorb green light as well as blue light, and have a darker color than type 1a diamonds. Depending on the precise concentration and spread of the nitrogen atoms, these diamonds can appear deep yellow ("canary"), orange, brown or greenish. Less then 0.1% of diamonds belong to Type 1b. Type 2a diamonds contain no, or minuscule amounts of non-carbon elements and are usually colorless. However, an imperfect carbon lattice will make the diamond absorb some light, which will give it a yellow, brown or even pink or red color. 1-2% of diamonds belong to Type 2a. Type 2b contain no nitrogen, but they do contain boron, which absorbs red, orange and yellow light. These diamonds therefore usually appear to be blue, although they can also be grey or nearly colorless. All naturally blue diamonds belong to Type 2b, which makes up 0.1% of all diamonds

With VS1 being the better of the two, such diamonds appear clean when seen with the naked eye, but the inclusions are relatively easily spotted with a 10X loupe. More readily available in the market, these stones are still in the high-end market range.

With VVS1 being the better of the two, such grades are assigned to diamonds in which imperfections and inclusions are very difficult to spot, even under the magnification of a 10X loupe. Such diamonds are rare and most definitely are high-end market stones.


Published - July 2009

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