What exactly is diamond color? While most people think of the classic, transparent diamond, which happens to be the most rare and valuable, the colors of diamonds actually range from transparent all the way to a brownish color. There are different saturations of diamond color, and tones.
The Gemological Institute of America, known as the GIA for short, is the most commonly used system for evaluating diamond color. They've created something known as the “GIA Color Grade Scale”, and it has placed a grading system on the many different colors and shades of diamonds, classifying into 22 letter grades.
How Diamonds are Graded
The Gemological Institute of America has certified diamond color grades as “master color comparison” diamonds. When a diamond is being graded, it is compared against these master color comparison grades, to find the shades that match it most closely. The diamond that is being graded must be thoroughly cleaned and all oil must be removed from the surface before being examined, since even small amounts of dirt or oils on the surface of the diamond can alter the color.
A line of master color comparison diamonds or shade chart is set up, with each grade placed about one inch apart, ranging from the lightest and highest graded D shade, to the darkest, graded Z diamonds. The diamond color that is being graded is moved from the left side of the comparison units to the right, until it matches one of the shades.
Diamond grading should take place in a colorless environment, using a balanced fluorescent light source that has a filtered, cool white sun like ray. The room where the grading takes place is dark, all except for the special grading light, in order to obtain the most accurate grading of the diamond color 
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