This is an example of indexing the upper halves in the 'right' direction ( from a performance aspect ):
The upper halves are cut on an index of 1, or just less than halfway between 64 ( or 0 ) and 4. The angle of the upper halves is 36.79 degrees.
The girdle thickness is the same at the upper half junction as Diamond #1 and Diamond #2 but thinner 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.
Face-up leakage is eliminated.
Lastly, a brightness simulation:
