Girdle thickness is somewhat of a judgement call. If a girdle is Medium to Thick, then this could mean 1/3 of it is Medium, 1/3 is Slightly Thick, and the remaining 1/3 is Thick. The term “Medium to Thick” may have several meanings. It may be that 85% of the diamond’s girdle is medium, whereas the remaining 15% is thick. In such a case, the entire adjustment percentage cannot be added into the formula. A stone with a more consistently thick girdle will require you to add a larger adjustment percentage to the formula.

Thin: less than 0.15 millimeters
Medium: between 0.15 and 0.20 millimeters
Slightly Thick: between 0.20 and 0.23 millimeters
Thick: between 0.23 and 0.33 millimeters
Very Thick: between 0.33 and 0.40 millimeters
Extremely Thick: grater than 0.40 millimeters

These measurments might be difficult to take for the thickness, but it will give you an idea of what to look for. The measurments are consistent regardless of the stone size. Courtesy of Adamas Gemological Laboratory.

Girdle Thickness Range=Girdle Thickness Average Girdle Thickness Adjustment
“-” represents slightly thinner
“+”represents slightly thicker
3.1mm-4.1mm 4.15mm-6.90mm 7.00mm+
Extremely Thin 0.97 0.96 0.95
Extremely Thin to Thin = Thin – 0.98 0.97 0.965
Extremely Thin to Medium = Thin 0.985 0.98 0.975
Extremely Thin to Slightly Thick = Medium – 0.99 0.985 0.98
Extremely Thin to Thick = Slightly Thick – 1.025 1.015 1.005
Extremely Thin to Very Thick = Thick 1.04 1.03 1.02
Extremely Thin to Extremely Thick = Thick + 1.045 1.035 1.025
Thin 0.985 0.98 0.975
Thin to Medium = Medium – 0.99 0.985 0.98
Thin to Slightly Thick = Medium NA NA NA
Thin to Thick = Slightly Thick 1.03 1.02 1.01
Thin to Very Thick = Thick – 1.035 1.025 1.015
Thin to Extremely Thick = Very Thick – 1.07 1.05 1.035
Medium NA NA NA
Medium to Slightly Thick = Medium + 1.015 1.01 1.05
Medium to Thick = Slightly Thick 1.03 1.02 1.01
Medium to Very Thick = Thick – 1.035 1.025 1.015
Medium to Extremely Thick = Thick + 1.045 1.035 1.025
Slightly Thick 1.03 1.02 1.01
Slightly Thick to Thick = Thick – 1.035 1.025 1.015
Slightly Thick to Very Thick = Thick 1.04 1.03 1.02
Slightly Thick to Extremely Thick = Thick + 1.045 1.035 1.025
Thick 1.04 1.03 1.02
Thick to Very Thick = Very Thick – 1.07 1.05 1.035
Thick to Extremely Thick = Extremely Thick – 1.09 1.075 1.05
Very Thick 1.09 1.065 1.045
Very Thick to Extremely Thick = Ex. Thick – 1.09 1.075 1.05
Extremely Thick 1.11 1.08 1.06
The above table of girdle thickness adjustment multipliers (GTA) will be used shortly in weight estimation formulas. Please refer back to this table when studying estimation examples.

 

A very thin to thick girdle. Some parts are medium so the average would be slightly thick.