Diamond Cut proportions: 60/60

Diamond Tutorial

Diamond Cut proportions: 60:60 -- updated Feb-18-2009

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The industry has a strange fallacy that if a diamond has proportions of 60% depth and 60% table, then it will be beautiful. It’s a great idea, but it does not work. Prior to better cut quality information from labs, generations of dealers have believed that table/depth and symmetry and polish grades are all you need to grade the cut quality of a diamond. If a stone has 60% depth and 60% table size, then it is perfect! (60 must be an easy number to remember?)

Here are two extreme examples of bad proportion combinations. Both are 60:60, both are ugly diamonds.

The pavilion angles on these two stones are 36 degrees and 45 degrees. This range actually exists. Of all of the diamonds within this range of pavilion angles, only those between 39.7 degrees and 41.4 degrees would be worthy of consideration.

Other things that throw the concept out the window are girdle thickness - a thin girdle 60% table stone could be around 59% depth and a slightly thick girdle could be 61% deep.

In general the thicker the girdle the deeper the stone should be. Smaller table diamonds should be slightly deeper (e.g. 53% table with a medium girdle could be around 62% to nearly 63%).

GIA’s Excellent cut grades include a small number of stones with 60% table sizes and depths around 60%. AGS 0 and the new AGS Ideal also has the possibility of diamonds with tables of 60%, but they are few and far between because there is less possible ‘proportion targets’ for cutters to plan for. To achieve both labs top grades table sizes centered around 55% to 58% offer many more proportion combinations and hence many more beautiful available diamonds.

These virtual diamond images were simulated using DiamCalc for the diamonds with 60:60 table:depth ratio and thin girdle but different pavilion angles.

Updated Feb-18-2009

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