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How to estimate a diamond's proportions

A Diamond’s proportions and their relationship with each other have a big effect on the Diamond’s appearance and the Brilliance and Fire it displays. Features such as a thick girdle or a deep pavilion can lower the cut grade. This post reviews how industry professionals estimate the table, Crown and Girdle of a Round Brilliant and why these proportions are so important.

To properly assess a diamond, we need to better understand a diamonds proportions and their relationship with each other. The formulas below apply to the most popular diamond cut, the standard Round Brilliant.

Average Girdle Diameter – This is one of the most important measurements that is used to calculate many other Diamond proportions. Average girdle diameter = smallest girdle diameter + largest girdle diameter / 2

Total Depth Percentage – Total Depth Percentage is used to determine why a diamond is overweight or underweight. To calculate, Measure the distance from the table to the culet and divide this figure by Average Girdle Diameter. A diamond with Total Depth Percentage under 55.0 is underweight and over 65.0 is overweight

Table percentage – Table percentage is important because some people prefer large tables and some people prefer small. This figure is stated as a percentage of the diamond’s average girdle diameter. A diamond with average girdle diameter of 10mm with 50% table percentage would have a table that is 5mm across. The table percentage can also be estimated using the following methods:

Flash method – The Flash method is done by looking at the diamond face-up and rocking it back and forth. If the flash is small, the table percentage is 60%. A medium flash = 60-64% and a large flash = 65% or greater. For this method to be accurate, the grader must inspect allot of diamonds to gage what a small and large flash is.
Ratio method – The Ratio method involves looking at the diamond face-up and comparing two sections to determine the ratio. The two sections are the Girdle to the edge of the table and table to the culet.
Bowing method – The Bowing method involved examining the star facets to see how far they extend around the table. No Bow = 60% a slight bow= 53% and a noticeable bow = 67%
Star length percentage - Star Facets are the facets around the table that point towards the girdle. They effect the brightness and fire of a diamond. The Star Length percentage is estimated by holding the diamond face up with tweezers and estimating how far each star facet extends towards the girdle. Consider the distance between the table facet and the girdle as 100%. The average percentage for the 8 Star Facets is calculated to come up with the figure

Crown Angle – The Crown Angle can be estimated in the profile or face-up position. The easiest way for beginners is the profile method. Hold the diamond table-to-culet and look at it from the side under 10X magnification. Half a right angle is 45 degrees and one third is 30 degrees. The grader should use these angles as reference when doing the estimates.

Crown Height Percentage - The Crown Height percentage can be calculated once the table percentage and crown angle are determined. Use these two figures and refer to a Crown Height percentage chart. (Attached)

Girdle Thickness – If the girdle is too thick it can cause grey a shadow to appear in the diamond. A thick girdle also adds weight without enhancing the appearance. Girdle thickness is estimated by inspecting the circumference of the stone and using a descriptive term o describe the girdle. Typical terms include Extremely Thin (ETN), Very thin (VTN) Thin (THN), Medium (MED) and Slightly Thick (STK)