Sat, 13 Aug 2005

Grading the Princess Cut

AGS
has given us cut-grading standards for Princess cuts. Designs having
pavilions with 2 chevrons came first and, just recently, those with 3
and 4 chevrons. I prefer designs with minimum facets because they give
stronger ‘bang-bang’ on-off reflections/contrast that attract the eye;
they also take less time to cut and offer less chance of cutting
errors…

Their charts show the grades according to
combinations of the slopes of the main facets. We know that this is a
good guide – as with round brilliants – but that the ‘secondary’ facets
may contribute significantly to the appearance of the stone.

I
saw a broker looking at FireScope or IdealScope image simulations by
software to see what these cuts would look like. However, the image he
saw will not be what he is getting if the secondary facets are not cut
the same way as they are represented in the computer model that is
making the image.

There are many ways to cut the facets of a
Princess cut. These variations will affect the actual FireScope and
IdealScope images and may affect the beauty of the stone in relation to
the cut grades.



The Princess Crown

Fig.2
shows the Princess crown with beveled corners. This is pretty
straightforward. The only data listed by AGS to find the cut-grade in
their charts are the slopes of the two main facets, C1 and C2, and
table size. These do not completely define the crown.

AGS assumes that the azimuth of the ‘star’ facets is 6? *;
this defines the table. They also assume that main facets C1 & C2
are of equal width at the mid-side. With these assumptions the crown is
completely defined; the height, and the slopes of the corner and star
facets can be determined from these data.

If a cutter uses
different assumptions the crown will have a different height and the
slopes of the corner and star facets will be different. This will
produce a real image which is different from that generated by the
computer-model and may not comply with the AGS grade.

Fig.1 The shape of the Princess retains much
weight of a well-formed octahedral crystal.
Fig.2 The crown initially cited by AGS.

 


* GemCAD
data listed by AGS in one publication cite the index as 1/64 circle, =
azimuth of 5.625? but the other data in the same chart agree with 6?
azimuth (= index of 1/60 circle).

 

The Princess Pavilion

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.

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 5.625?, 5?, 3.75?, 3?
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.

 

The Whole Gem

Recall that the cut-grading charts provided by AGS are a grid of BOTH
crown and pavilion slope combinations. The probability of the grades
and virtual images matching what you actually get is therefore
doubly-questionable – you must find out if the software models and the
charts match the actual cutting configuration.

B. L. Harding 2005 Aug 05