The Cut of a diamond is referred to in terms of its “proportions” which include Table, Diameter, Shown Angle, Pavilion Depth/Angle, Total Depth Percentage, and Length to Width Ratio. Proportions are simply classified as Good, Very Good, and Excellent.
An experienced master Diamond Cutter cut a rough diamond to an ultimate cut with the fewest imperfections and the least loss of carat weight. The Better the cut, the more valuable the diamond.
An Ideal Cut diamond is one that produces the maximum brilliance. In an Ideal Cut, all of the light that enters the stone refracts internally from 58 precisely placed facets and disperses through the top of the diamond, producing fire and brilliance. Only a round brilliant cut diamond can achieve the proven mathematical proportions and symmetry of an Ideal Cut.
Very Good Cut
It is close to an Ideal cut, with only slight variations in its measurements. It may achieve Ideal Cut proportions but vary in its polish or symmetry rating. Hence, a very good cut diamond still creates remarkable brilliance and luster, often reflecting back the maximum amount of light if its table and depth percentage match those of an Ideal Cut.
A good cut diamond is well proportioned and reflects back a good amount of light.
Many diamonds are “spread” in their cut to increase carat weight when cutting from the original rough. Although you may end up with a diamond that appears larger, you will sacrifice the brilliance and fire.
When cut too deep, a diamond loses light out of the bottom, leaving the center of the diamond dark in appearance.
When cut is too shallow, a diamond loses light out of the bottom, reducing brilliance and giving the stone a dark, glassy appearance.
What are the ideal proportions for a diamond?
The round shape diamond has a universally accepted set of proportions that were first discovered by a Mathematician named Tolkowsky. Today, GIA has developed proportion grading systems that are used in determining how well a diamond is cut. The following information only applies to round brilliant cut diamonds. To grade the ideal proportions for fancy shaped diamonds is more complex, but we will discuss each of the fancy shaped diamonds and provide the latest proportion information, later on this site.
GIA has divided the entire cut grading system into 4 classes. Below the explanation of each proportion is a graph showing how GIA groups together the measurements for each of the four classes. Table Percentage is calculated by measuring the size of the table and the total diameter of the diamond.
Table Size / Total Diameter = Table Percentage
Tolkowsky determined that 53% is the ideal table percentage, however GIA uses a range for grading purpose 53% to 60% because the difference between these table percentage is relatively insignificant.