The clarity of a diamond is simply a measurement of the absence of any internal flaws. There are guild lines for each of the clarity levels, however in the end it is a subjective science because the diamond grader’s judgment determines the final clarity grade. The grading scale most commonly used was developed by the GIA.
It is important not to associate the “brilliance” of a diamond with its clarity, they are two distinct features. The brilliance of a diamond is strictly a function of the quality of the cut, the clarity is strictly a measure of the presence/absence of flaws. The only time the clarity can possibly affect the brilliance is if the inclusions are so large they actually dull the diamond. This would be in the I2 and I3 category (see chart below).
No two diamonds are alike… each one is truly unique. The clarity grading is done under a magnification of 10X (Ten times normal 20/20 vision). A jeweler’s loupe or a microscope are the two instruments used by gemologists to grade the diamonds. There are many different types of inclusions.
Below is a listing of clarity categories and their descriptions.