Mon, 01 Nov 2010

Making the Grade – GIA Cut Grading

Has the GIA’s cut grading system for round brilliant diamonds made lower cut grades

Polish Lines DiamondPolish Lines Diamond
(left) Polish lines seen through the table – (right) Actual lines on the pavilion girdle facet
Images courtesy of the Russian Gemmological Center
click here to see a larger version

In 2006 the Gemological Institute
of America (GIA) added – to
colour, clarity, etc – a five-grade,
proportion-based cut quality
system for round brilliant
cut diamonds.

What impact, if any, has this had
on the market?

Certainly, having a quantifiable
cut grade has given the round
brilliant cut diamond greater
marketability and fuelled the boom
in diamond branding. Market reports show a clear
financial incentive for diamond
manufacturers to produce round
brilliants in ‘Excellent’ and ‘Very
Good’ cut grades.

Based on a sample of more
than 1,000 GIA-graded, 1-carat
non-fluorescent diamonds in a
range of colour and clarity grades,
those with ‘Very Good’ and ‘Good’
cut grades were priced at 5.4%
and 16.4% less respectively
than ‘Excellent’ cuts. There were
insufficient ‘Fair’ and ‘Poor’
cut graded diamonds for any
statistical analysis.

The GIA’s cut grade system
is based on proportions, with
deductions for finish factors of
polish and symmetry; the grade is
lowered by one if the polish or the
symmetry is two grades lower.

Other round brilliant proportion
deductions include:

• Spread – a grade lowering
penalty for depths greater than
63% (7% of the sample in the table)
and being too shallow (two stones
in the sample were below 58%)

• Girdle – the top grade is within
‘Thin’ and ‘Slightly Thick’– less
than 1% in the sample were below
‘Thin’ and less than 2% were
‘Thick’ or ‘Very Thick’

• Culet – of the 311 stones with
data listed, 304 had none (or were
pointed), six were classified as
‘Very Small’, one was ‘Small’ and
none were ‘Medium’ or ‘Large’.

The table (below) demonstrates
that achieving the top cut grade
has not challenged manufacturers
since the introduction of the
cut grade on certificates: 78% in
the sample managed to achieve
‘Excellent’ cut, with 80% and
77% respectively for those with
‘Excellent’ polish and symmetry

GIA Table Data from RatNet for Round Brilliants Princess Cuts and Cushion Cut Diamonds

But has cut quality really
improved? Assuming that it has,
is the overwhelming popularity
of gem reports pushing quality
improvements in the market?

To date, the GIA has not been
able to offer a cut grade for fancy
cuts (those other than round
brilliant) due to the high number
of proportion variables. Overall,
there are far fewer stones in
fancy shapes that would achieve
an excellent grade for polish
and symmetry.

There are two arguments for
why this might be. The first is
technical: most fancy-shaped
diamonds have facets that are
close to the octahedral plane and
polishing in that direction is near
impossible, so ‘brillianteers’ must
accept a lower polish grade or
adjust the facet angle, resulting in
a lower symmetry grade. The second is about money:
some believe the absence of cut
grades for fancy shapes means
manufacturers get less reward for
higher standards; ie, they get away
with higher yields.

Some in the industry are
adamant that the GIA’s round
diamond cut grade has helped
improve diamond cut quality. Yet
others argue that improvements
are a natural by-product of
technological advances in cutting
and polishing equipment, and
the move away from cottage
industry to industrial production,
largely in India.

Cut is the most important
factor in bringing out the overall
beauty of a gemstone. Whether it
can be quantified – and therefore
certified – across all diamonds,
and what impact this could have
on the marketplace, is the next
big question.

by Garry Holloway
HCA and Ideal-scope developer –

Holloway Diamonds

* Republished with permission of Jeweller magazine

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