Fri, 25 Mar 2005

Upper Halves – Counter Point

This response is written to address the article The Effect of Indexed Upper Half Facets written by Mr. Peter Yanzer.

The
opening paragraph of the article presents a concept that indexing top
halves produces different amount of weight retention. This is correct,
although not as dramatic as presented. A diamond cutter knows that
brilliandeering an entire diamond calls for a weight loss of
approximately 4%. This causes one to question the facts presented in
the conclusion of this article Setting a Constant Girdle Thickness at the Mains
which shows a range of finished weights from .866 cts. (Index 3) to
.774 cts. (Index 1) or 11%. As noted, these findings are outside the
parameters of realistic cutting. The results presented in the article
were accurate when they concluded that a fraction of a point was lost
by cutting the top halves in different configurations (stating that the
Index 1 stone was .817cts. VS the Index 3 stone of .808cts.), but the
results change upon further examination of indexing with relation to
diameter, and then change more so in setting constant girdle thickness
at the mains, which in a normally cut stone is not a product indexing
the upper girdle halves. Girdle thickness can only be increased by
adding more girdle (read reducing the diameter of the stone) either by
bruting or faceting or polishing.

Under Setting a Constant Girdle Thickness at the Halves
another discrepancy is that the crown height will change by “indexing”.
It is erroneous to think that indexing the girdle facets to 3 will make
the size of the facets bigger thereby adding to the crown height and
leaving the diamond with a larger weight retention. In actuality, the
thinner the girdle underneath the halves means that more weight was
removed than in either the Index 1 or 2 models. This contradictory
evidence is found in the portion of the article, Setting a Constant at the Mains.

As
a side note, measuring the crown height percentage at these points is
questionable and could explain inconsistencies of the original AGS Cut
Grade System where sometimes a stone with a lower cut grade actually
looked better than one with a higher cut grade. Such inconsistence
sometimes can be explained by comparing data generated scores produced
by the W.R.Bray’s Cut Scoring System for Modern Brilliant Cut Diamonds
(patent pending, aka BrayScore® with AGS cut grades.

Under Indexing and Millimeter Spread at the Same Weight,
another discrepancy in the findings is that this “indexing” of upper
girdle halves will always change the diameter of the diamond. This is a
false assumption because in actual cutting ,if the plane of the girdle
is 90° from the table (theoretically but not uncommon in normal
cutting) or less than 90° (rarely seen) then there would be no removal
of diameter. If the plane of the girdle is greater than 90° (usually
done so the girdle will not be seen in the face up position) then
there’s a possibility of diameter reduction.

Diamond cutting is
a three dimensional world. Sculpture. An upper girdle facet cannot
simply be indexed to one side and achieve the dramatic thinning of the
girdle without also intentionally raising the angle of that facet. This
was pointed out by Yanzer who called it the “resultant change in
angle”. To simply move (index) the half facet to the 3 or 4 position
without making it on a higher angle would result in a disfiguring of
the bottom point of the star facet. This “indexing”is not a simple
directional change made by a click of the dop, but rather a directional
change plus an angular change of correction made by the cutter. These
two elements of facet making are taken into account with the BrayScore
method of quantifying cut.

The models in the section Setting a Constant Girdle Thickness at the Mains
start out with a thicker than normal girdle. By utilizing the BrayScore
system and just analyzing the deductions regarding the upper girdle
facets and not any other data, the 2 index stone scores midway between
models 1 and 3. Compared to the 2 index stone, the 1 index scores about
50 points lower and the the 3 index stone actually scores 50 points
higher because the girdle was thinned to a more desirable thickness. If
the model girdle thickness under the main facets is “normalized” to a
more palatable range of 2 -2.5% found on fine makes, then this thinning
would produce a girdle edge that is too thin, a completely opposite
effect. Under this “normalization” the Index 2 stone would score the
highest with the Index 1 and 3 stones slightly lower by an average of
40 points in a system of 1000 points awarded to a stone with no cutting
mistakes.

The reasoning behind BrayScore deducting more points
for upper girdle facets that are “pasted” on by the cutter (model 1
index) and those facets that are “dug” too much (model 3 index) is
simple. In the model 1 instance, “pasted” halves have less definition
between themselves and the main facet. This produces less of a jump of
surface light from one facet to another when the stone is moved. This
look is similar to that found on a diamond with a real shallow crown,
where there would be less jump of surface reflection from the table to
the stars to the mains. In both instances, both “pasting” and “digging”
produce an uneven girdle around the stone. The straight girdle is a
desirable look that enjoys over 80 years of market share. The diamond
is a piece of sculpture and as such is viewed in more than in just a
face up position. The work of the cutter is mostly measurable and using
present day measuring devices can be evaluated on a very precise level
and certainly on par with the level of precision used in color and
clarity grading.

In conclusion, the errors in The Effect of Indexed Upper Half Facets
show that cut evaluation methodology has its shortcomings when
including only a gemological and academic view. An empirical view that
would include knowledge from the cutting sector would yield a cut
evaluation system with greater weight.

 

Discuss…