There are many cut charts of the angles and percentages that diamonds should be cut to reflect the maximum amount of light back to your eye, however just because a diamond falls within these parameters does not guarantee that the stone is cut well.
The following chart is what the American Gem Society (AGS) calls a zero cut grade. The AGS grades the cut of diamonds on an eleven grade scale from zero (the best) to ten (the worst)
I feel at this point I must reiterate the fact that just because a diamond falls within these parameters does not mean that it is cut well.
The ever vigilant cutters whom I talked about earlier know how to adapt to the public’s new understanding of diamond cut. The cutters also have the chart for AGS 0 ideal cut diamonds and they know how to make it look like the diamond is well cut and still maintain maximum weight retention.
The chart above talks about each angle and each percentage, however it does not judge how those angles and percentages work with each other. The cutter simply cuts the diamond to the highest acceptable crown angle, pavilion angle, pavilion depth and girdle thickness acceptable to AGS for a zero cut grade, and the stone now weighs one carat and still has the diameter of a three quarter carat. This stone can now be sold to you as and “Ideal” cut. No doubt called this because it would be ideal for said person to sell this diamond to you.
How can you be sure your diamond is well cut you ask?
There is another method. A more visual one, not relying on charts to study. This type of diamond is called a “Hearts and Arrows” cut.
The term hearts and arrows refers to a visual pattern that you can see in the diamond (provided it has been cut to precise internal symmetry) when the diamond is observed through a special viewer. This means that the crown and pavilion mains, and the secondary facets are all in alignment. Try to visualize the facets as mirrors. The mirrors need to be placed to reflect the light coming in from the top of the stone back out to your eye. If these mirrors are not placed properly to reflect the light back out though the top, the result would be a very dull diamond.
Hearts and Arrows diamonds may, or may not fall within the AGS 0 cut grade chart. Although most are considered AGS 0 cut grade diamonds, I have observed diamonds that will show a good hearts and arrows pattern and not be AGS 0.
You must also bear in mind that the cutter loses much more rough from the original crystal when he cuts a hearts and arrows diamond and therefore this type of diamond will cost more than a traditional ideal cut diamond.
Therein lies the balance, a diamond does not HAVE to be a hearts and arrows or even an ideal cut to be beautiful and therefore it is up to you to decide if the cost justifies the look.
I do encourage you to buy a well cut diamond, it will ensure that you are not paying for extra carat weight without seeing it in the diameter.