Diamond cut grading...
A well cut or faceted diamond, regardless of its shape, scintillates with fire and light offering the greatest brilliance and value...
While nature determines a diamond's clarity, carat weight and colour, the hand of a master craftsman is necessary to release its fire, sparkle and beauty. When a diamond is cut to good proportions, light will reflect from one mirror-like facet to another and disperse through the top of the stone, resulting in a display of brilliance and fire. So it follows that a better cut diamond does a better job of dazzling us with its beauty.
Here's the secret to how a diamond sparkles.
Many gemmologists consider Cut the most important diamond characteristic because even if a diamond has perfect colour and clarity, a diamond with a poor cut will have dulled brilliance. The width and depth have the greatest effect on how light travels within the diamond, and how it exits in the form of brilliance.
The diamonds proportions, specifically the depth compared to the diameter, and the diameter of the table compared to the diameter of the diamond, determine how well light will reflect.
Excellent / Ideal cut: Represents roughly the top 3% of diamond quality based on cut. Reflects nearly all light that enters the diamond. An exquisite and rare cut.
Very good cut: Represents roughly the top 15% of diamond quality based on cut. Reflects nearly as much light as the ideal cut.
Represents roughly the top 25% of diamond quality based on cut. Reflects most light that enters.
Represents roughly the top 35% of diamond quality based on cut. Still a quality diamond, but a fair cut will not be as brilliant as a good cut.
This includes all diamonds that do not meet the proportion standards of a fair cut. These diamonds are deep and narrow or shallow and wide and tend to lose most of the light out the sides and bottom.
A round brilliant cut has 58 facets. Since the quality of the cut is directly responsible for the stone's beauty, the precision with which the facets are arranged is of prime importance - they determine the amount of light reflected to the eye, called brilliance.