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The Four C's
Because diamonds are so valuable, its essential to have a universal grading system for comparing their quality. In the 1940s and 50s, The Gemmological Institute of America (GIA) developed the 4Cs and the GIA International Diamond Grading System to objectively compare and evaluate diamonds.
The carat weight measures the mass of a diamond. One carat is defined as 200 milligrams. The point unit equal to one one-hundredth of a carat (0.01 carat, or 2mg) is commonly used for diamonds of less than one carat. The price per carat increases with carat weight, since larger diamonds are both rarer and more desirable for use as gemstones.
The price per carat does not increase linearly with increasing size. As an example, a 0.95 carats (190 mg) diamond may have a significantly lower price per carat than a comparable 1.05 carats (210 mg) diamond, because of differences in demand.
Colourless diamonds, which are graded as D, are the finest quality and have the highest value as far as colour is concerned. A diamond with slight traces of colour (which can only be observed by an expert diamond valuer or a grading laboratory) is the next best choice. However, when studded into a piece of jewellery, these very light coloured diamonds do not show any obvious colour to the naked eye of the layman. These are graded as E or F colour diamonds. Diamonds which show very little traces of colour are graded as G or H colour diamonds. Slightly coloured diamonds are graded as I or J or K. Diamond can also be found in other more solid and identifiable colours than the very sought after colourless diamonds. These coloured diamonds, such pink diamonds, are however extremely rare and priceless.
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Clarity is a measure of internal defects of a diamond called inclusions. Inclusions could be crystals of a foreign material or another diamond crystal or structural imperfections such as tiny cracks that can appear off-white or cloudy. The number, size, colour, relative location, orientation, and visibility of inclusions all affect the relative clarity of a diamond. The Gemmological Institute of America (GIA) and other organizations have developed systems to grade clarity, that take into consideration the inclusions that are visible to a trained professional when a diamond is viewed under 10x magnification. Diamonds become increasingly rare when considering a higher clarity grading. Diamond clarity can be measured on a scale ranging from flawless to imperfect.
FL = highest clarity grading
I3 = considered lowest clarity grading
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Visit the Wikipedia page on Diamond Clarity for different rating scales.
Diamond cutting is the art and science of creating a gem-quality diamond out of mined rough. The cut of a diamond describes the manner in which a diamond has been shaped and polished from its virgin form as a rough stone to its final proportions as a gem. The cut of a diamond describes the quality of workmanship and the angles to which a diamond is cut. Often diamond cut is confused with shape.
There are mathematical guidelines for the angles and length ratios at which the diamond is supposed to be cut in order to reflect the maximum amount of light. Round brilliant diamonds, the most common, are guided by these specific guidelines, though fancy cut stones are not able to be as accurately guided by mathematical specifics.