How is the clarity of a diamond determined?

Gem-stone diamond clarity is documented by a certified gemologist -- with an 'important diamond', potentially more than one gemologist -- according to a scale.

From Wikipedia:

"Diamond clarity is a quality of diamonds relating to the existence and visual appearance of internal characteristics of a diamond called inclusions, and surface defects called blemishes. ... Inclusions may be crystals of a foreign material or another diamond crystal, or structural imperfections such as tiny cracks that can appear whitish or cloudy. The number, size, color, relative location, orientation, and visibility of inclusions can all affect the relative clarity of a diamond. A clarity grade is assigned based on the overall appearance of the stone under ten times magnification."

You can read more, below.