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HOW TO ESTIMATE A DIAMOND'S PROPORTIONS
Posted by MICHAEL Hart on Apr 06, 2017
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:
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)
Posted by MICHAEL Hart on Apr 06, 2017
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%
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)
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