You can learn more about how diamonds are graded by reading the articles listed below.
Cut is probably the most important quality factor, and most challenging, of the four Cs to understand. The brilliance of a diamond depends heavily on its cut. More info...
Most diamonds contain some inner flaws, or inclusions, that occur during the formation process. The visibility, number and size of these inclusions determine what is called the clarity of a diamond. Diamonds that are clear create more brilliance, and thus are more highly prized, and priced. More info...
Colorless diamonds are the most desirable since they allow the most refraction of light (sparkle). Off white diamonds absorb light, inhibiting brilliance. More info...