What is the cut proportions of an ideal cut diamond?

A round diamond has 58 facets, each one is like a mirror and thus the placement and angle of each one in relation to the other 57 has a distinct, though complex, effect on the manner of which light is projected off the diamond.

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I think that every jeweler has his own conception of "ideal cut". A large table (~60%), which makes the entire diamond look bigger is considered "ideal" by many. The "ideal cut" perception of Markov Tolkowsky is obviously still common and regraded by many as the best polish. The GIA has set its class 1-4 grades of round diamond, in which class 1 is Ideal.

In the range between ideal cut and "very fine" cut the round diamond proportions are as follows:

• Table Percentage (ratio between diamond Table diameter and total diamond diameter) - 51.5 - 62%.
• Pavilion Angle (angle between the girdle and a pavilion facet) - 39.7 - 41.7 degrees.
• Crown Angle (angle between the girdle and a crown facet) - 32.2 - 36.8 degrees.
• Total Depth (the ration between the diamond's depth and the total diamond diameter) - 59 - 63%.
• Pavilion Depth (ratio between the diamond's pavilion depth and the total diamond diameter) - 41.8 - 44.8%.
• Crown Height (ration between the diamond's crown height (or depth) and the total diamond diameter) - 13.2 - 16.2%.

Diamonds in other shapes than the round cut aren't measures so "scientifically", since "fancy cuts" are more polished with more flexibility. When it comes to purchasing round diamonds, a diamond grading certificate includes specification concerning all the above parameters. It is advised to ask the jeweler about them and also look for them in the GIA (or other type of grading certificate) report.

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An ideal cut round diamond must not be too shallow, to prevent light from escaping from the side of the stone, and not too deep, to avoid light escaping from the bottom of the diamond.

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Based on my experience I can say that total depth percentage is perhaps the most principle parameter in determining an ideal cut, because it has a huge effect on the sparkle and brilliance that a diamond projects. The range of 59-63% depth percentage is essential for Ideal or Very Fine diamond cuts. One can save money by purchasing diamonds with depth percentage beyond this range, but I think it's an element which must not be compromised upon, since much of the diamond's brilliance is lost. In round diamonds with depth percentage of 64% and more, light refracts through the lower part of the diamond and the diamond appears rather dark and unappealing.

Although total depth is the most important element, I don't claim that a proper depth percentage guarantees an ideal cut. A diamond with a 60% total depth can still have a too small or too big Table, problematic pavilion and crown angles, etc. Still, when the total depth is just right, there's a bog chance that the other parameters will also fall right in place.

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The cut proportions of an ideal diamond are determined by the American Gem Society (AGS). The diamond's table diameter should range between 52.4% to 57.5%, the crown angle should range from: 33.7 degrees to 35.8 degrees, the girdle thickness can range from 'Thin' to 'Slightly Thick' (.51% to 2.95%). The diamond's pavilion depth percentage: 42.2 % to 43.8%, diamond culet: (Pointed) None, Very small to Medium, the diamond's total depth: 58.7% to 62.3%. Good symmetry and polish is also essential in the grading of an ideal cut diamond. The ratios and percentages listed above are exactly what makes an Ideal cut reflect nearly all light- defining it as a highly prized cut.

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