Mon, 28 Mar 2005

Cutting the AGS-0-Princess : the experience with the lab

Some
time ago, I reported you about our experiences while cutting the first
princess-cuts, in order to obtain the new AGS-0-cut-grade.

Last week, we received a first batch of six stones back from AGS. Let me tell you how we experience the grading of AGS.

1. The grading system

Where
the current cut-grade consists of the following three factors,
proportions, polish and symmetry, the new grade will have three
different factors:

  • Light performance
  • Proportion factors
  • Finish

1.1. Proportion factors

This
factor is on the one hand a very important factor, while on the other
hand, it is relatively easy to achieve an Ideal grade. In this factor,
certain detrimental effects are observed:

  • The girdle thickness is checked in order to make sure that the diamond has neither a too thin nor a too thick girdle.
  • The culet size is checked.
  • A spread-factor is observed, in order to make sure that a diamond has a surface, which is in line with its weight.
  • Durability issues are checked, like a too thin girdle or a too low crown angle.
  • Finally,
    tilt is checked, in order to make sure that the girdle does not reflect
    under the table, after minimally tilting the diamond.

For a
cutter, aiming for AGS-0, obtaining Ideal on this factor is
straightforward and easy. For a consumer however, this is a very
important factor, since it addresses very important problems like
durability and spread.

1.2. Finish

Finish consists
of two factors that we already know, polish and symmetry. This grading
basically is the same as in the current AGS-system for rounds.

At
the end of last year, one could deduct from some AGS-presentations,
that the grading for Polish and Symmetry would be loosened to the
extent that the current Excellent-grade would be included in an
Ideal-grade. Apparently, this idea was abandoned, and there still is a
clear distinction between the Ideal and the Excellent grade.

1.3. Light performance

Light
performance consists of brightness (or brilliance), fire, scintillation
and contrast. AGS has developed a complete software-system, based upon
its studies, and they use this software to judge light performance.

In
order to help cutters, AGS has distributed ‘cutting guidelines’, which
gives combinations of proportions, that should yield candidates for
AGS-0, and for all the other AGS-scores. However, it is very important
to understand that ‘cutting guidelines’ are not ‘grading guidelines’.

In
grading, the first step is to take a 3D-scan of the diamond. After the
accuracy of this scan is checked, it is introduced into the
AGS-software. This software performs a series of ray-tracing, and
finally returns scores according to the AGS ASET-system. These scores
are compared to certain values, and if a diamond scores in the
Ideal-range on all these scores, it will be graded Ideal on Light
Performance.

Light Performance-grading will make the big
difference with the current AGS-system. While, in theory, a cutter has
many more possible combinations to cut an AGS-0, each combination needs
such total precision that it is very difficult to obtain this.

Our experience with the grading system

We
have sent a first batch of 6 stones to AGS, ranging from 0.78 Ct up to
1.34 Ct. All 6 were cut within the ‘cutting guidelines’ for AGS-0.
While cutting however, we aimed at very small AGS-0-areas in the
cutting-guideline-charts, since this would ultimately give us the most
information on how the system actually works.

2.1. Proportion factors

As
expected, this factor does not represent any difficulty. All 6 are
Ideal. I do wonder at what point a diamond will start to be downgraded
on Proportion Factors, since it is a very important factor for
consumers. Where a Very Good on Light Performance still is a very
beautiful stone, it might well be that a Very Good on Proportion
Factors should not be considered as a decent purchase.

2.2. Finish

Symmetry
in a fancy shape is much more difficult to obtain than in a round. In
rounds, one has the advantage of the symmetrical round shape, but also
the cutting equipment allows to hold a round stone much better than a
fancy shape.

A similar difficulty can be observed in Polish. When
cutting a fancy shape, one encounters more often a facet that is
difficult to smooth. Because of this, Ideal Polish in a fancy shape is
more difficult to obtain than in a round.

Of our 6 stones, 5
scored Ideal on Polish (1 Excellent) and 3 scored Ideal on Symmetry (3
Excellents). What is weird, is that while 2 scored Excellent on
Symmetry, they still received an Ideal score on Finish.

Looking
at the stones, the reasons why some did not get Ideal are pretty
straightforward, and rather easy to repair. After some experience, a
cutter should be able to get a very high performance of Ideal-scores on
Finish.

2.3. Light performance

Obtaining Ideal Light Performance will be the cutters’ nightmare. We encountered the following problems.

When
choosing for a specific combo of proportions, we used the
cutting-guidelines and Diamcalc, in order to check the appearance of
the chosen combo. It was weird to see that when we compared the
estimated final weight in Diamcalc, with that for the same combo in
Sarin, there was a discrepancy, which often was considerable.

Cutting
according to the chosen proportions was not really a difficulty,
although with the guidelines working with 1°-steps, one is not sure in
which direction one can allow deviations.

The true frustration
starts when one wants to check the measurements of the finished stone.
Both Sarin and OGI were unable to correctly measure the crown area of a
princess. Especially, the measurement of the C2-angle and the
Table-size were completely off. Apparently, the angle of the C2-angle
is so low, and the facet relatively small, so that in the shadow-image,
both Sarin and OGI often miss this facet, thus measuring a much bigger
table size than actually is the case, and almost no C2-facets.

We understand that AGS also works with Helium-machines to measure, and we hope that they are more accurate in this area.

Of
our 6 stones, 2 scored Ideal on Light Performance, the other 4 are
Excellents. Knowing that we were aiming at very small 0-areas in the
cutting guidelines, we are quite happy with these results.

Testing the performance of the stones

Before
and after we sent the stones to AGS, we have tested their light
performance with a number of professional and amateur observers.

The
professionals all agreed that all 6 are princess-cuts, like they had
never seen before. Compared to the traditional princess-cuts on the
market, these offer a play of brilliance, fire and scintillation that
is much closer to that of a round, than one is used to in
princess-cuts.

As for both professionals and amateurs, it is
interesting to note that some preferred stones that graded Excellent on
Light Performance above stones that graded Ideal.

Conclusion

Checking
the stones after their grading, we have decided to repair 4 stones on
either Polish or Symmetry. These repairs are relatively easy, and the
errors are avoidable in the following stones.

At the same
time, we have decided not to repair stones with Excellent Light
Performance, since they are extremely beautiful as they are, and the
direction in which to work in order to obtain Ideal from here, is
either unclear, or brings about a high loss of weight.

This means
that we will be selling stones with AGS-1 (1 on Light Performance only)
from now on, and that, within a few weeks, we will also have stones
with full AGS-0-reports.

 

Discuss…