Reading a Diamond Certificate for a Round Shape:
Even for a well-educated consumer, much of the information on a diamond grading report can look like gibberish. In the interest of helping you find the diamond that is best suited to your expectations, we offer the explanations and opinions below.
Do note that there are always exceptions to the rule, but this should serve as a concise guide in most situations.
1. Evaluating the Cut Dimensions of a Diamond
If you know how to read them correctly, the dimensions are the key to how much brilliance and fire your diamond will display. Every report lists the table percentage and total depth of a diamond. One popular formula among diamond experts is the 60/60 rule. If both the table percentage and total depth of a diamond equals near 60% of the width, you probably have a well cut diamond on your hands.
The 60% table makes the diamond appear larger when viewed from the top down and the depth is sufficient to create a good return of light. Do note that numbers and percentages only provide a rough estimation of performance. There is not really enough information here to properly evaluate the brilliance of a diamond.
GIA reports (which are considered to be very reliable), and some others do not provide additional specifications on diamond dimensions. They do, however, list some important information in a text format.
If a diamond has a table% value of under 50% or over 65%, the brilliance and beauty of a stone will be negatively affected. The smaller table will result in a deep cut that makes the diamond appear dark in the center, while the larger table will result in a shallow cut which makes the diamond appear milky and dull.
In a grading report that lists all of the angles of a diamond cut, the most crucial factor is the pavilion depth. The optimum pavilion depth for the most beautiful and brilliant diamond is 43.1%. A depth near this will create a dramatic shower of brilliance, scintillation and fire to the eye of the viewer.
This falls within the AGS (0) or Ideal Cut standard, and these diamonds command a premium price in the marketplace. If you find an Excellent Cut near this best pavilion depth, you probably have a good bargain on your hands.
The three important dimensions that determine brilliance are the Pavilion Depth, the Table % Diameter and Total Depth % Diameter. AGS has established strict tolerances for diamond cut grades.
The three examples below illustrate the brightest diamond (0) Ideal, a (2) Very Good diamond and a (6) Fair diamond, which would be dull and lifeless in comparison.
AGS (0) Ideal Tolerances
Pavilion Depth: 42.2%-43.8%
Table Diameter: 52.4%-57.5%
Total Depth Diameter: 59.8%-62.4%
According to traditional guidelines, this diamond will exhibit the best return of brilliance, fire and scintillation.
AGS (2) Very Good Tolerances
Pavilion Depth: 41.7%-42.1% or 44.4%-44.8%
Table Diameter: 59.6%-61.5%
Total Depth % Diameter: 55%-63.9%
Some experts feel that a 60% table diameter combined with a 60% depth provides better value for the cost, an excellent return of light and a larger looking diamond. However, the optimal pavilion depth is still 43.1%
AGS (6) Fair Tolerances
Pavilion Depth: 45.9%-46.3%
Table % Diameter: 67.6%-69.5%
Total Depth % Diameter: below 53%
This kind of proportions will result in a dull and lifeless diamond. The table should not exceed 63% and the pavilion depth will leak a substantial amount of light.