Proportions determine a diamond’s brilliance (the amount of light reflected back to your eye), fire (the flashes of colour due to prismatic separation into the colours of the rainbow) and scintillation (sparkling movement of light as you move the diamond). If ideal proportions are used in the creation of diamonds, their overall excellence can be optimised.
In 1919 Marcel Tolkowsky was responsible for discovering the basis of what is now regarded as the “Ideal Cut” diamond. The Mathematician formed a Masters thesis on the proportions for round brilliant cut diamonds.
Supposedly, he conducted his research by asking Londoners to select the most appealing diamond from a small group. He combined these observations with those of his family’s Belgian diamond cutter business and then applied maths and physics to confirm why certain proportions produced the best looking diamonds.
The image shows a single example; in fact Tolkowsky’s theorem predicted a range of proportions with varying combinations of pavilion and crown angles that could enhance brilliance and fire. A variation on this was the Eulitz cut, developed in 1972 and considered to be ‘mathematically perfect’.
American Gem Society (AGS) Proportion Grading
The American Gem Society Lab (AGS) began proportion-based Cut grading in 1996 based on Tolkowsky‘s proportions. The top grade was listed as AGS 0 and the worst was AGS 10.
All diamonds are given an overall grade based on three factors, the lowest score making the highest grade:
- Light Performance: Brightness, Fire (dispersion), Leakage and Contrast
- Finish: Symmetry and Polish
- Proportion Factors:
- The girdle thickness of all shapes of diamonds will now be measured at facet junctions, where the girdle is about 1.7% thicker than the scalloped valleys (as measured previously)
- With regard to culet size: AGS 0 = None to Medium, 2 = Slightly large, 5 = Large, 9 = Very Large.
- Weight or Spread: For rounds the DiamCalc (performance software) factor should must be within + or -5% about 6.35 to 6.6mm for a 1.00ct stone. Princess spread factors should not be worse than -25, or about 5.3mm square.
- Durability: For Round or Princess cuts, no crown angles below 30°
- Tilt (fish-eye): This is the amount of tilt to when you can see the girdle of a stone from the face up position. The Fish-eye calculation on DiamCalc must be higher than 6°
This table shows the percentages used by AGS to attribute grades.
The Gemological Institute of America (GIA) Proportion Grading
The Gemological Institute of America also provides a grading system that offers an ‘excellent candidate’ range, in consideration of proportions and in additions, symmetry, polish and minor facet detail. Having conducted research and lectured on diamond cut for several years, in 2001 Pricescope also launched the Holloway Cut Advisor system. However, this is solely for round diamonds with known Table, Crown, Pavilion and Total Depth inputs, it does not consider diamond symmetry, poor minor facet proportions or particular cutter’s tricks.
Based on all these factors – from Tolkowsky’s paradigm to established Grading systems - it is possible to postulate the approximate proportions to create a round diamond of maximum beauty. A brilliant round diamond cut comprises of 58 Facets in total:
- 33 Crown Facets; 1 Table facet, 8 Star facets, 8 Kite or Upper Main Facets, 16 Upper Girdle facets
- A central Girdle (waistband), sometimes faceted
- 25 Pavilion Facets; 8 Pavilion, 16 lower Girdle, 1 Cutlet
Below, is a diagram depicting proportions which would achieve an excellent balance between brilliance, fire and scintillation.
Of course, as we have seen, there are many possible combinations of proportions which would fall under this ‘ideal; category. Below is a table showing brilliance in percentages of girdle diameter according to the International Diamond Council formed in 1979.
