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HCA - Defining the Beauty and Desirability of Round Brilliant Diamonds

Girdle Thickness Adjustment

The GIA Brilliance study found the impact of girdle thickness on diamond beauty was minimal. I concur. Ray path analysis shows that polished girdles function just like any other facet. A diamond set with an exposed girdle can gather light and refract it back to a viewer or the light can leak out the pavilion. However thickening girdles result in reduced light return., so the penalty score shown in the table is been applied by HCA. Girdles can look like inclusions, especially if the girdle is bruted, this is covered under the fisheye section.
 
Diamonds with thick, very thick and extremely thick girdles spread poorly, and this is also taken into account in the advanced HCA. However a thicker girdle remains the most effective way for a cutter to maintain yield with a minimal sacrifice in beauty.
 
We have also calculated a penalty for diamonds with extremely thin and very thin girdles because of an increased risk of damage.
 
Diamonds with a low total angle for crown and pavilion combined (shallow stones) are at the greatest risk of breakage. An additional penalty factor has been added where the addition of crown and pavilion angle is less than 72 degrees. This penalty increases as the combined angle reduces, unless the girdle becomes progressively thicker.
 
If a diamond’s girdle has an extreme variation in thickness, it can receive a double penalty, one for very thin and one for very thick. In this case an average score is calculated. For instance: 0% to 9% (with say an average of 5%) 0.5 + 0.8 = 1.3 divided by 2 = 0.65 penalty score.
 
Please note that HCA does not ‘see’ the diamond and it is common for girdle variations to occur in only one very small area, or across 90% of the girdle. We can only work with the data provided and we can not replace expert visual assessment.

Table 5.The ‘beauty’ deduction for girdle thickness for overly thin or overly thick girdles. An additional spread penalty is calculated on the advanced HCA for thicker girdles.

Girdle	0%	1%	2%	3%	4%	5%	6%	7%	8%	9%	10%
Score	0.5	0	0	0	0	0	0.2	0.4	0.6	0.8	1.0

 

Thin girdle % score	Automatic warning
0.0 - 0.5	Warning – The girdle of this diamond may chip if worn in an exposed ring setting. Warn jewelers to handle with care.
0.5 - 1.0	Warning – The girdle of this diamond is likely to chip if worn in an exposed ring setting. Warn jewelers to handle with care.
1.0 - 1.5	Warning – The girdle of this diamond is very likely to chip if worn in an exposed ring setting. Warn jewelers to handle with care.
1.5 - 2.0	Warning – The girdle of this diamond is extremely likely to chip if worn in an exposed ring setting. Warn jewelers to handle with care.
> 2.0	Warning - Reject this diamond because of an extreme risk of chipping.

 

Thick girdle % score	Automatic warning
0.0 - 0.5	You are paying for a little extra weight because of the girdle thickness.
0.5 - 1.0	This diamond’s thick girdle means it has a smaller spread and may also have slightly less brilliance.
> 1	This diamond’s extremely thick girdle reduces both its spread and its brilliance.

Mr. Jasper Paulsen has found a relationship between increasing girdle thickness and the other proportions by adding a ‘girdle’ to Tolkowsky’s original calculations in ‘Diamond Design’ 1919.
 
His excellent web version and review are at http://www.folds.net/diamond/index.html
 

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