Brilliants
Because of the standardized forms of cut and the relative constant specific gravity of 3.52, the weight of a cut diamond can also be calculated by establishing the volume. One of the earliest calculations for the brilliant cut We published in the year 1931 by the jeweler SCHARFENBERG with the following formula:
carat weight = height x diameter of girdle2 x 0.0067
However, at that time brilliants were cut much "fuller" with larger volume, s that the formula had to be altered when related to today's usual brilliant CI with flat crown, fine girdle and a total height of 58-60 %, and is now used e follows:
carat weight = height x diameter of girdle2 x 0.0061
Example
Girdle diameter 10.0 mm
Total height
6.0 mm
Carat
weight = 6.0 x 10.02 X 0.0061 = 3.66 ct.
Where the proportions of the total height deviate from 58-60 % standard or where the girdle is very thick, this must be taken into account so that in the case if a lower total height 1-5 % of the calculated weight should be subtracted (= weight x 99 to 95) or in the case of a bigger total height 1-5 % of the calculated weight be added (= weight x 101 to 105).
Fancy cuts
Oval cut
diameter = mean value of length and width
carat weight = height x diameter of girdle2 x 0.0062
Emerald cut
| Emerald cut | Relation of length : width |
| length x width x height x 0.0080 | 1.0 : 1 |
| length x width x height x 0.0092 | 1.5 : 1 |
| length x width x height x 0.0100 | 2.0 : 1 |
| length x width x height x 0.0106 | 2.5 : 1 |
Navettes
| Navettes | Relation of length : width |
| length x width x height x 0.00565 | 1.5 : 1 |
| length x width x height x 0.00580 | 2.0 : 1 |
| length x width x height x 0.00585 | 2.5 : 1 |
| length x width x height x 0.00595 | 3.0 : 1 |
Drops
| Drops | Relation of length : width |
| length x width x height x 0.00615 | 1.25 : 1 |
| length x width x height x 0.00600 | 1.50 : 1 |
| length x width x height x 0.00575 | 2.00 : 1 |
Note: The formula refers to thin to medium girdles. For thick girdles add 2-4 %, for very thick girdles 5-10 % to the calculated weight.
| Weight Tables | Instruments for the Approximate Estimation of Weights of Brilliants |