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.
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 is the largest most prolific lab, yet it has never graded the proportions of diamonds on its reports (GIA gives the least amount of proportion information on its reports). GIA only list the table size and depth % (depth in mm divided by average diameter = depth %), so in the absence of good quality information, 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?)