Selecting a Diamond

When selecting a diamond, we are typically told to investigate the 3 C's: Cut, Clarity and Color. When you receive a certificate from GIA, for example, the stone clarity and color are rated. Cut, however, is a much more complicated issue to define. It turns out, that unlike clarity and color, it might not be possible to establish the ideal cut. Yet, cut is without doubt, the most important of the factors. Brilliance, dispersion and scintillation (differing ways of explaining a diamond's sparkle) are determined by the cut. The cut determines how light enters the top of the stone, and is then is bounced back.

In 1919, Marcel Tolkowsky, a master cutter, tried to establish the ideal cut of a round diamond. He established proportions of the diamond as a percentage of the diameter. For example, the table is 53% of the height of the diameter, the the crown 16.2% and the pavilion 43.5%.
A Tolkowsky cut diamond can sell for a substantial premium (>50%) to that of another diamond of same color and clarity.

Computer modelling today shows that is far from easy to determine the ideal cut. A cut that produces the best dispersion, does not produce the best brilliance. A GIA certificate today will in fact not provide a "cut" rating. It will however give you the proportions.

So how then do you pick a well cut diamond if the certificate doesn't specifically rate cut? You can use Tolkowsky as a guide, but best yet, if you are choosing a diamond, ask the dealer to show you a bunch of diamonds and compare them, face up (as you would see them in a ring) while next to each other. Close your hand and let them rest in the cracks on the back of your hand. Move your hand around under good lighting. Very often, you will note that one diamond stands out from the rest. The dealer will want you to do this under bright lights. That's fine, but also try the exercise in a dimmer area and see which stones sparkle in the lower light.

Finally, another little trick is to take a diamond and hold it, face up, over writing. If you can read any of the writing through the diamond, then the light is not being bounced back enough and there is a "dead spot" in the diamond. A beautifully cut stone will bounce back so much light so that you cannot see through the stone.