This is an example of indexing the upper halves in the 'wrong' direction ( from a performance aspect ):
The upper halves are cut on an index of 3, or just over halfway between 64 ( or 0 ) and 4. The angle of the upper halves is 45.38 degrees.
The girdle thickness is the same at the upper half junction as Diamond #1 but thicker at the mains because of the indexing.
A 'hearts and arrows' simulation of the above stone:
A FirescopeTM simulation of the above stone:
Simulating the American Gem Society's ASET ( Angular Spectrum Evaluation Tool ), here's a color-coded map of how the diamond utilizes light:
White is leakage, green is from 0 to 45 degrees from the horizon, red is from 45 to 75 degrees, and blue is 75 to 90 degrees.
The upper girdle facets draw a large percentage of light from the green area ( 0 to 45 degrees ). Because of the indexed upper halves, their angle is too steep and optical performance suffers.
Lastly, a brightness simulation:
In certain lighting environments, this diamond will optically look smaller because the upper halves will darken. This stone will not earn a '0' cut grade in the new AGS Grading System.