Throughout this site we mention that the initial stage of our selection process for round brilliant cut diamonds is conducted "by the numbers" and thus people often ask us what the "magical numbers" are for princess cut and other fancy shape diamonds... Unfortunately the same "by the numbers" approach we incorporate for the first stage of our selection process for round brilliant cut diamonds can not be applied to fancy shape diamonds because of variances in the facet designs of each diamond shape.

Differences in how the measurements are determined for various diamond shapes can alter our perception of diamond size and the potential for visual performance and mislead us if we are not aware of the differences in how different diamond shapes are measured and how the measurements for total depth are calculated.  The diamond pictured above is graded by the AGS Laboratory as weighing 1.05 carats and the measurements on the lab report indicate that the diamond measures 5.42 x 5.44 mm in diameter because the diamond was measured from edge to edge and this measurement would be equivelent to the outside diameter of a round brilliant cut diamond weighing about 0.60 carats, however the diamond clearly looks larger when viewed from tip to tip and if measured from that perspective measures 7.60 mm across which is relatively the same size as a round brilliant cut diamond weighing about 1.60 carats.  Is the glass half full or half empty?  It is all a matter of perspective.

Example #1
An AGS-0 Ideal Cut Princess Shape Diamond:
Here are the basic measurements for this diamond, as indicated on the AGS lab report:

Weight:    1.257 carats
Measurements:    5.78 x 5.78 x 4.60 mm.
Total depth:    79.6%
Table %:    59.7%
Crown angle:    32.8°
Crown height:    11.2%
Pavilion angle:    42.7°
Pavilion depth:    64.9%
Girdle    2.7% to 3.8%
Culet:    Pointed

Example #2
This is the same diamond references in Example #1, but with the measurements calculated as they would have been for a round brilliant cut diamond instead of a princess:

Weight:    1.257 carats
Measurements:    8.17 x 5.78 x 4.60 mm.
Total depth:    65.9%
Table %:    49.4%
Crown angle:    32.8°
Crown height:    9.3%
Pavilion angle:    42.7°
Pavilion depth:    53.8%
Girdle    2.2% to 3.1%
Culet:    Pointed

With a round brilliant cut diamond, all percentages are calculated as a percentage of the average diameter, while in fancy shapes, they are calculated as a percentage of the smallest diameter... The key differences once again are "average diameter" and "smallest diameter" and as you can see, a subtle difference in terminology and approach results in a major difference in perception!

Just looking at the difference in the figures between the two sets of results based upon the two manners in which the total depth percentage is calculated based on the fancy shape parameters "by smallest diameter" and the round brilliant cut parameters "by average diameter"... Clearly the measurements outlined in Example #2 are better than in the first example. First, it clearly shows the diameter from point-to-point being 8.17 millimeters and the total depth measurement decreased from 79.6% down to 65.9% which sounds a lot better, especially since we are used to total depth measurements in the neighborhood of 60% in round brilliant cut diamonds.