The brilliance of a diamond depends heavily on its cut. A diamonds cut does should not not be confused with the diamonds shape. Cut refers to the diamond’s reflective qualities. A good cut gives a diamond it’s brilliance which is the brightness the illuminates from the very heart of the diamond. The angles and finish of a diamond determine its ability to handle light, which leads to the brilliance. The light is what makes diamonds so mesmerizing.
In a poorly cut diamond, cut shallow to improve clarity factors or cut deep to maintain carat weight, the light reaches the facets and then “leaks” out from the sides or bottom rather than reflecting back to the eye. Less light reflected means less brilliance. The grades for the cut are: Ideal, Premium, Very Good, Good, Fair, and Poor.
THE "IDEAL CUT" DIAMOND PROPORTIONS
Cut grade is assigned by the GIA using the following scale: Ideal Cut - Excellent - Maximum fire and brilliance. This category applies only to round diamonds. Premium - Under normal lighting conditions, appears very similar to Excellent Cut, but for a lower price. Very Good - Reflects a majority of the light that enters the diamond, for an above average appearance. Good - A diamond graded as GOOD reflects only a moderate proportion of the light that enters it. More acceptable in diamonds of less than 1/4 carat, where differences in sparkle are more difficult to perceive. Fair & Poor - Allows most of the light entering the diamond to escape from the sides or bottom. A diamond graded as fair or poor reflects only a small proportion of the light that enters it.