The above information was taken off GIA's website.  For more information on diamond grading:

www.gia.edu

Carat

Diamond carat weight is the measurement of how much a diamond weighs. A metric "carat" is defined as 200 milligrams.

Each carat can be subdivided into 100 'points.' This allows very precise measurements to the hundredth decimal place. A jeweler may describe the weight of a diamond below one carat by its 'points' alone. For instance, the jeweler may refer to a diamond that weighs 0.25 carats as a 'twenty-five pointer.' Diamond weights greater than one carat are expressed in carats and decimals. A 1.08 carat stone would be described as 'one point oh eight carats.'

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Cut
Diamonds are renowned for their ability to transmit light and sparkle so intensely. We often think of a diamond's cut as shape (round, emerald, pear), but a diamond's cut grade is really about how well a diamond's facets interact with light.

Precise artistry and workmanship are required to fashion a stone so its proportions, symmetry, and polish deliver the magnificent return of light only possible in a diamond.  The five grades are Excellent, Very Good, Good, Fair, and Poor.

Clarity

The GIA Diamond Clarity Scale has 6 categories, some of which are divided, for a total of 11 specific grades. Flawless (FL) No inclusions and no blemishes visible under 10x magnificationInternally Flawless (IF)No inclusions visible under 10x magnification Very, Very Slightly Included (VVS1 and VVS2) Inclusions so slight they are difficult for a skilled grader to see under 10x magnification Very Slightly Included (VS1 and VS2) Inclusions are observed with effort under 10x magnification, but can be characterized as minor Slightly Included (SI1 and SI2) Inclusions are noticeable under 10x magnification Included (I1, I2, and I3) Inclusions are obvious under 10x magnification which may affect transparency and brilliance. 

Color

The diamond color evaluation of most gem-quality diamonds is based on the absence of color. A chemically pure and structurally perfect diamond has no hue, like a drop of pure water, and consequently, a higher value.


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