Estimating Diamond Proportions (Part 2) – GemSelect AU
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Estimating Diamond Proportions (Part 2)

Posted by MICHAEL Hart on

In recent years, the cut of a diamond has become an important component that consumers consider. There are set ranges of proportions that are favourable and diamonds outside these ranges will receive a lower cut grade. This post is Part 2 of how to estimate a diamonds proportions. It reviews how to estimate the pavilion, cullet and finish of a diamond.



The Pavilion
The Pavilion is an important part of the diamond. It breaks white light into spectral colours and creates the fire we see in diamonds. There are two ways that diamond professionals describe the pavilion: Pavilion Depth Percentage and Pavilion Angle.

Pavilion Depth Percentage

To estimate the pavilion depth Percentage, look at the diamond face up and look for a circular image around the cutlet - this is the reflection of the table. A table reflection that reaches halfway from the culet to the table is 44.5%. Pavilion Depth percentages greater than 50% make a diamond look dark. Pavilion Depth Percentages less than 38% give the diamond a grey circle called a fish-eye

Pavilion Angle Generally, the greater the pavilion Depth Percentage, the steeper the Pavilion Angle will be. A good Pavilion Angle is between 37.7 and 44 degrees. Angles outside of this range with receive lower cut grades


Girdle, Lower Half Facets and Culet

 Lower Half Length percentage

The Lower Half Facets start at the Girdle and extend towards the culet. Think of the distance between the girdle and the Culet as 100%. If the lower half facet reaches 1/2 way to the Culet the length would be 50%, 3/4 of the way would be 75% and so on.

Girdle Thickness Percentage -The Girdle Thickness Percentage is the thickness of the Girdle expressed as a percentage. It can be calculated by subtracting the Crown Height percentage and Pavilion Depth Percentage from the Total Depth Percentage. A high Girdle Thickness Percentage or thick girdle can lower a Diamonds cut Grade. 


Culet size- the cutlet is the point at the bottom of a diamond's Pavilion. It counts as a facet and is an important part of the diamond. Generally, the larger a culet is, the lower the cut grade will be. The main purpose of the culet is to prevent abrasions before a stone is set. To estimate culet size, hold the diamond face up and look through the table to see where the Pavilion ends meet. A culet can be described as pointed, small, medium, large etc.

Total Depth Percentage
Total Depth Percentage is calculated by adding the Crown height Percentage, Girdle thickness Percentage and Pavilion Depth Percentage. A Depth Percentage of less than 55% usually indicates a thin crown. Over 65% means a steep crown angle or a thick girdle. 

Finish
A diamonds finish is summed up by its polish and symmetry. Polish and Symmetry are each graded separately. Most stones have good to very good polish and symmetry. Cutters could give every stone an excellent polish however, the time that this takes is usually only reserved for high-value stones

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