Fig.3 shows a princess pavilion with 2 'chevrons'. To identify the cutting grade, AGS lists only the slopes of the side main P1 and the corner facet P2. This does not completely define the pavilion but does define its height U.
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Fig.3 Note odd chevron azimuths which result
from specifying too many other things. | Fig.4 Similar appearance of 2- ,3-, and 4-chevron
designs, but note location of point P. |
AGS assumes that points P,Q,R are located at distances .05,.15,.25 from the center of the gem (girdle width 1.0). This defines the heights of points P and R but the height of Q is still arbitrary. AGS chose a rule to define this point (now changed). Knowing the height of Q also, it becomes possible to define the azimuths and slopes of the two chevrons
This approach resulted in azimuths which were different for most combinations of slopes P1 & P2. This is impractical in a diamond-cutting shop because it would require a different click wheel for each combination.
New cutting data are in progress where the azimuths are specified to be the same for many cuts and the corner slope P2 is specified; slope P1 of the side main depends on these choices.
This is not a problem; choose the corner slope P2 you want and see what the side main slope P1 will be. The pixels on the AGS grading chart are large and you can find this combination within one of them.
In their new modeling, AGS has chosen the azimuths to be whole degrees. This is not necessary; many common indexes (using toothed wheels) are not whole degrees. It is okay for production cutters of one material, who can have special wheels made, but not for colored-stone cutters, who use wheels full of evenly-spaced teeth – such as 64, 72, 96, and 120 teeth (increments of 5.625º, 5º, 3.75º, 3º respectively).
Fig.4 shows Princess pavilions with 2, 3 and 4-chevrons. 3- and 4-chevron designs have the same problems as discussed above for the 2-chevron design and are receiving similar corrections.
A broker told me he prefers the 3- and 4- chevron designs because they have steeper side main P1 and thus more weight from the crystal. This may be true if the gems are cut as AGS defined them – note that they specified P1 as steeper and the radius to point P greater than in the 2-chevron design. These differences are not necessary; the 2-chevron design could have the same side slope at the same radius as the others. There are so many variables that one cannot make such generalizations.