In 1919, a Belgian mathematician, Marcel Tolkowsky, who came from a family of diamond cutters, published a Masters thesis on the best proportions for round brilliant diamonds. This work formed the basis of the defining proportions for what are now known as “Ideal cut diamonds”. It must be noted however that different grading laboratories allocate different names to the basic Ideal cut diamond.

American Ideal Cut, Superb Ideal Cut or even Excellent Cut all refer to diamonds that match the range of acceptable geometric proportions based on Tolkowsky’s work.

So, what are these proportions?

The diameter of the diamond is considered to be 100%, so all percentages quoted are relative to the diameter (or 100%) of the diamond.

  • Table diameter = 52.4 – 57.5%
  • Crown angle = 33.7º – 35.8º
  • Girdle thickness = 0.51 – 2.95%
  • Pavilion depth = 42.2 -43.8%
  • Total depth = 58.7 – 62.3%

You may be asking what difference these proportions make. When a diamond has been cut in such a way that all proportions are correct, the diamond will reflect all of the light that enters the diamond, irrespective of the direction that the light has come from. All this light is then reflected out through the top of the diamond, leading to the brilliant flashes of light and rainbow colours that we associate with diamonds.

Sometimes, a diamond cutter will sacrifice the proportions of a diamond to fit the diamond into a specific size (carat) category, allowing for the potential of a higher price. If the diamond has been cut too deep, the diamond will appear black in the centre. If the diamond has been cut too shallow, the diamond will look dull and lifeless.

When buying a round brilliant diamond, make sure that the accompanying diamond certificate contains a printed insert providing the full details of the proportions of your diamond. This is the best way to ensure that the diamond you have purchased has been cut to acceptable, ideal proportions.

Katannuta Diamonds is exceptionally particular about the cut of our diamonds and will preferentially supply Excellent or Very Good cuts to our South African clients. Should you be looking for a particular diamond, either to purchase loose, or for settng in an engagement ring, please contact us with the specifications that you are looking for.