Tue, 30 Nov 2004

A review of the “foundation” of GIA’s new cut grade system


Tabulation of the 15 stones presented as examples of 5 Categories

The article concludes with photos and proportion data of 15 stones given as
examples of 5 Categories of cut quality. The Categories are presented
as the ‘foundation’ of an eventual grading system, but a vague
qualification is added; they may not be the basis of the final grades.
It seems their foundation is incomplete which means that this analysis
of it must also be incomplete.

In this review we will examine
each of these stones using various tools designed and / or used every
day by the author. There are advantages to reading on-line because
there are several links included. However you may find having a printed
copy at hand will improve comprehension, and be convenient for
comparing images and discussions.

HCA
proportions were chosen to cover the most widely available diamonds.
The colored part of the chart shown below shows the pavilion on the
vertical chart scale with a larger scale (39.5°-43°) because it is the
proportion factor with the greatest impact on diamond appearance. The
horizontal axis represents crown angle (28°-40°). The third most
important variable, table size, is represented on 13 different charts
ranging from 53% to 65%. In hind sight, it appears diamonds with
shallower crowns and pavilions may become more common in the future.

In
the chart shown here, an interpretation is made of some ranges of
proportions given by the GIA based on my assumption that the GIA would
concur with previous work done by MSU, Bruce Harding and Jasper
Paulsen’s re-interpreted Tolkowsky calculations. But it should be noted
that three of the 15 example stones fall outside these oval
predictions.

The accuracy of the scanning devices used at the
time of the GIA studies has resulted in significant rounding of table,
crown and pavilion data. Pavilion lower girdle half facet length and
crown star half facets length are quoted in 5% steps. All the
proportions are considered as averages; angular deviations within the
facet grouping for each measured parameter will be downgrade for
diamonds with symmetry grades below Very Good. This approach is, in my
opinion, less than perfect.

 

This
HCA 55% table chart (extend to include some additional GIA proportions)
has black ovals representing our interpretation of ranges of crown and
pavilion angles for categories 1, 2 and 3 mentioned in the article. In
white is the new AGS 0 ‘candidates’ proportions for 55% table. The
black numbers represent the crown and pavilion (only) coordinates for
each of the 15 stones in the 5 categories (except 4.3).

Where
additional specifications are shown in the charts below (next pages),
it is because there was a difference between the relevant data quoted
in the 2001 Fire article. The earlier information was not rounded as
broadly as that in the Foundation article.
GIA Category 1

RD01 HCA 0.6

RD08 HCA 2.2

RD20 HCA 1.9

Ideal-Scope Lighting

Ideal-Scope Lighting

Ideal-Scope Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Disco Lighting

Disco Lighting

Disco Lighting

GIA photo 0.61ct

GIA photo 0.50ct

GIA photo 0.62ct

DiamCalc Scores

DiamCalc Scores

DiamCalc Scores

Light Return Mono

1.00

Light Return Mono

0.99

Light Return Mono

0.98

Light Return Stereo

0.99

Light Return Stereo

1.00

Light Return Stereo

0.98

Contrast

0.96

Contrast

0.97

Contrast

0.92

Specifications

Specifications

Specifications

Table Size

54%

Table Size

57% 58

Table Size

61%

Crown Angle

34.0° (34.3°) [i]

Crown Angle

33.5° (33.4)

Crown Angle

34.5° (34.3)

Pavilion Angle

40.6°

Pavilion Angle

41.2°

Pavilion Angle

40.8° (40.7)

Star Length

50% (53.8%)

Star Length

55% (54)

Star Length

55%

Lower Girdle

75% (81%)

Lower Girdle

85% (84)

Lower Girdle

80% (79)

Girdle Thickness

Thin-Med (2.9%)

Girdle Thickness

Med (3.9)

Girdle Thickness

Med (3.2)

Culet Size

None

Culet Size

None

Culet Size

Non

Total Depth

61.2%

Total Depth

61.1%

Total Depth

59.6%

Polish

VG

Polish

VG

Polish

VG

Symmetry

VG

Symmetry

VG

Symmetry

VG

COMMENTS: GIA’s top grade appears looser than the proposed new AGS system

An
obviously good choice; slightly shallower than Tolkowsky proportions
and excellent symmetry (as shown in the photo) give the appearance of
bigger facets, with bigger but less frequent flashes of fire. This is
enhanced by short lower girdle facets (LG’s).

The weakest of the 3 stones.

Shorter
LG’s could improve the stone. LG’s and what appears as the weakest
symmetry of the 3 stones adds to the appearance of more facets.

Note: slight ideal-scope table leakage does not greatly reduce light return.

The second best stone. Very few 61% table diamonds will also be given AGS’s top grade.

The slight ideal-scope table leakage has little impact on light return.

Note the thin looking pavilion mains caused by the larger sized table.

[i]This
is additional data listed in (brackets). In the GIA G&G Fire
article, Fall 2001, 28 stones are listed with precise numerals. The
later Foundation article includes these stones in the 45 used in that
study; however they have been rounded – crowns to 0.5°, pavilions to
0.2° and minor facets to 5% steps.

 

GIA Category 2

RD16 HCA 0.7

RD07 HCA 4.8

RD03 HCA 2.4

Ideal-Scope Lighting

Ideal-Scope Lighting

Ideal-Scope Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Disco Lighting

Disco Lighting

Disco Lighting

GIA photo 0.82ct

GIA photo 0.76ct

GIA photo 0.55

DiamCalc Scores

Improved

DiamCalc Scores

DiamCalc Scores

Light Return Mono

0.98

1.00

Light Return Mono

0.82

Light Return Mono

0.97

Light Return Stereo

0.94

0.98

Light Return Stereo

0.89

Light Return Stereo

0.97

Contrast

0.91

1.03

Contrast

1.25

Contrast

0.90

Specifications

Specifications

Specifications

Table Size

53% 54

53%

Table Size

53%

Table Size

63%

Crown Angle

33.5° 33.8

33.5°

Crown Angle

36.5° 36.4

Crown Angle

32.0°

Pavilion Angle

40.6° 40.4

40.6°

Pavilion Angle

41.4° 41.5

Pavilion Angle

41.0° 40.9

Star Length

50% 51.9

50%

Star Length

55% 59.4

Star Length

60%

Lower Girdle

75% 76

85%

Lower Girdle

90% 89

Lower Girdle

80%

Girdle Thickness

Thin-Med 3.3

3.3

Girdle Thickness

Thin-Med 3.1

Girdle Thickness

Med-Stk 3.7

Culet Size

VSM

Culet Size

None

Culet Size

None

Total Depth

61.2%

Total Depth

64.1%

Total Depth

58.6%

Polish

G

Polish

VG

Polish

G

Symmetry

VG

Symmetry

VG

Symmetry

G

COMMENTS: A pattern has developed; the second stone in grades 1 to 4 appear to be the worst

This stone was downgraded because of ‘Good’ polish.

GIA
claimed combined proportions make the pavilion mains look very dark;
with a small table the lengthening the LG’s (to 85%) makes a diamond
with these proportions perform very well.

An interesting stone. Very
high contrast and scintillation contribute to many small firey flashes;
some are only visible because light return is poor. A contentious
choice; if it were the same diameter as most of the smaller samples,
would it have been selected? (See discussion)

The
large table lets this stone down. Diamonds with large tables benefit
from shorter LG’s, which add a little more contrast. ‘Good’ polish
& symmetry also lower this stones grade. The table and upper girdle
facets were said to be dark; this is not evident in the photo in
G&G Fire, page 194

 

GIA Category 3

RD22 HCA 2.4

RD11 HCA 4.1

RD06 HCA 4 ? (off scale)

Ideal-Scope Lighting

Ideal-Scope Lighting

Ideal-Scope Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Disco Lighting

Disco Lighting

Disco Lighting

GIA photo 0.81ct

GIA photo 0.71ct

GIA photo 0.59ct

DiamCalc Scores

DiamCalc Scores

DiamCalc Scores

Improved

Light Return Mono

0.83

Light Return Mono

0.81

Light Return Mono

0.99

1.04

Light Return Stereo

0.82

Light Return Stereo

0.84

Light Return Stereo

1.00

1.04

Contrast

0.98

Contrast

1.09

Contrast

0.88

0.89

Specifications

Specifications

Specifications

Table Size

54% 55

Table Size

58%

Table Size

56% 57

56%

Crown Angle

35.5° 35.9

Crown Angle

37.0° 37.2

Crown Angle

23.0° 23.1

23.0°

Pavilion Angle

39.4° 39.2

Pavilion Angle

42.2° 41.9

Pavilion Angle

42.0° 41.9

42.5°

Star Length

55% 54.2

Star Length

45% 49.1

Star Length

60% 60.6

45%

Lower Girdle

75% 77

Lower Girdle

85% 87

Lower Girdle

80% 78

85%

Girdle Thickness

Thin 3.3

Girdle Thickness

Med-Stk 4.3

Girdle Thickness

Med-Stk3.2

Culet Size

None

Culet Size

None

Culet Size

None

Total Depth

60.6%

Total Depth

64.9%

Total Depth

57.2%

Polish

VG

Polish

G

Polish

VG

Symmetry

VG

Symmetry

VG

Symmetry

VG

COMMENTS:
The first and last stones appear to be fair mid grade stones. The
middle stone lacks observer’s presence contrast; it would look worse
than it appears in the photo.

“..somewhat shallow pavilion angle produces dark pavilion mains”. As
with Cat. 2 stone RD16; longer LG’s (90% & 45% stars) improves
table brightness and scintillation. A 2° steeper crown would have even
better light return, fire and a 3% better yield and HCA of 1.3. The
stone is nearly a fisheye.

Should
this be a mid grade stone? Upper girdle facet leakage will result in an
apparent size of a diamond 2/3rds its weight. Bad leakage helps fire
and scintillation (appearance of having more facets). The leakage and
lack of contrast from observer obstruction reduces brilliance.

GIA’s
computer model predicted good brilliance for lower crown angle stones;
they had to test it. With a 0.5° deeper pavilion, 45% stars and 85%
LG’s we get a stone worthy of Cat. 2 and a better yield.

To prevent chipping, very shallow crown diamonds should have a slightly thicker girdle. (See discussion)

 

GIA Category 4

RD19 HCA 7.5 (Fish eye)

RD33 HCA 10? (off scale)

RD37 HCA 6.5 (Fish eye)

Ideal-Scope Lighting

Ideal-Scope Lighting

Ideal-Scope Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Disco Lighting

Disco Lighting

Disco Lighting

GIA photo 0.72ct

GIA photo 0.64ct

GIA photo 0.50ct

DiamCalc Scores

DiamCalc Scores

DiamCalc Scores

Light Return Mono

0.97

Light Return Mono

0.79

Light Return Mono

0.93

Light Return Stereo

0.95

Light Return Stereo

0.82

Light Return Stereo

0.94

Contrast

0.83

Contrast

1.00

Contrast

0.92

Specifications

Specifications

Specifications

Table Size

62% 63

Table Size

56%

Table Size

70%

Crown Angle

29.0° 29.2

Crown Angle

37.0°

Crown Angle

33.5°

Pavilion Angle

39.6° 39.5

Pavilion Angle

44.0°

Pavilion Angle

40.2°

Star Length

50% 51.7

Star Length

55%

Star Length

60%

Lower Girdle

75% 76

Lower Girdle

70%

Lower Girdle

80%

Girdle Thickness

Med 3.3

Girdle Thickness

Thn-Med

Girdle

Stk-Thk Bruted

Culet Size

None

Culet Size

None

Culet Size

None

Total Depth

54.5%

Total Depth

68.0%

Total Depth

56.9%

Polish

VG

Polish

VG

Polish

G

Symmetry

VG

Symmetry

VG

Symmetry

G

COMMENTS: The second stone is so bad it is hard to imagine the need for a lower grade?

This
stone will show a fish eye and has very poor contrast. Its light return
is very good because it has very little leakage, but its pattern is
unattractive. The combination of shallow crown and pavilion angles
increases the likelihood of chipping at the girdle.

This
diamond will appear to be very much smaller than you would expect for
its weight. This is not just because of its smaller diameter – but more
so because of the upper girdle leakage. The table is dull because it
acts like a mirror, reflecting back in the same direction – it is a nail head.

Described
as a slight fish eye with “general darkness”, its main problem is a
lack of contrast and scintillation aided by a pronounced inclusion like
fish eye. Light return should be quite good.

Had the LG’s been shorter by just 1% or 2%, fatter dark main facets would add contrast. Spread is just OK.

GIA Category 5

RD39 HCA 6.3

RD43 HCA 6.2

RD45 HCA 10? (off scale)

Ideal-Scope Lighting

Ideal-Scope Lighting

Ideal-Scope Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Jewellery Shop Lighting

Disco Lighting

Disco Lighting

Disco Lighting

GIA photo 0.70ct

GIA photo 0.50ct

GIA photo o.54ct

DiamCalc Scores

DiamCalc Scores

DiamCalc Scores

Light Return Mono

0.88

Light Return Mono

0.77

Light Return Mono

0.65

Light Return Stereo

0.92

Light Return Stereo

0.81

Light Return Stereo

0.69

Contrast

1.03

Contrast

1.03

Contrast

0.99

Specifications

Specifications

Specifications

Table Size

57%

Table Size

57%

Table Size

62%

Crown Angle

35.5°

Crown Angle

38.5°

Crown Angle

37.0°

Pavilion Angle

41.2°

Pavilion Angle

41.8°

Pavilion Angle

45.2°

Star Length

55%

Star Length

55%

Star Length

60%

Lower Girdle

80%

Lower Girdle

80%

Lower Girdle

85%

Girdle Thickness

Etk

Girdle Thickness

Thk-Vtk

Girdle Thickness

MedVtk

Culet Size

None

Culet Size

None

Culet Size

None

Total Depth

74%

Total Depth

71.5%

Total Depth

69.3%

Polish

G

Polish

G

Polish

G

Symmetry

G

Symmetry

G

Symmetry

G

COMMENTS: This is a grade for the worst 1% of certified diamonds. (On 11/19/2004 there were only 441 plus 1.00ct rounds with depth % greater than 69% listed on Rapnet).

This one has old the AGS 0 proportions, but with an extremely thick girdle which adds 47% of additional weight.

This 0.70ct diamond has the expected diameter of a 0.54ct diamond which has a ‘list’ of half the cost.

Because
of its very thick girdle, this 0.50ct diamond has the expected diameter
of a 0.37ct stone, which has a ‘list’ price of less than half the cost.

This
is as bad as diamond cut quality gets. Stones like this are classed as
‘promotional’ and are seldom graded by laboratories. Its stated “large
total depth” has nothing to do with its poor performance. (See
discussion)

Categories 1 and 2
Category 1

The
GIA have suggested five grades is the largest number of grades that
they believe observers could discriminate. GIA’s top grade proportions
appear looser than the top grade of the proposed new AGS system; these
GIA category 1 stones would likely fall within the top 3 AGS grades of
0-2. Therefore after both GIA and AGS grading systems are implemented
on grading reports, it is likely that the stricter AGS 0 system will
continue to achieve a sellers premium as noted in the Pricescope Diamond Grading and Pricing Survey and reflected in business to business (B2B) web listings.

Category 2

RD07

This
stones has very high contrast and scintillation; it appears to have
many, very small firey flashes. Also because the stone has reduced
light return, its firey flashes would not be more visible than in a
stone with better light return. Indeed it could be hard to sell a
diamond where a personal preference for fire would influence a buyer to
accept a diamond with such poor light return.

If this stone
were one of the smaller samples, would the size of some of its virtual
facets have been too small for the survey observers to discern the
flashes? This is one of the main problems with ‘parametric’ grading
systems that apply the same grade to diamonds irrespective of their
size.

This stone would also display an more apparent dead zone
inside the table once worn and a film of grease has covered the
pavilion. This would give the stone a similar effect to that of a
Fish-eye because it shows the characteristic I have previously
described as a No Go Zone. I would have given this stone a lower grade.

This
DiamCalc ray trace shows the flash that appears in the upper table
region on the GIA photo is within a leakage region. How were the
diamonds illuminated for these photo’s? Was light able to enter the
pavillions?This OctoNus ETAS map of reverse ray traced light return is
for a model of RD07. It shows stronger than usual firey flashes on a
virtual sphere. You can play with an early version of this software
here. RD03 The table and upper girdle facets were said in the article
to be dark; this is not evident in the photo of the same stone in
G&G Fire, page 194. To test this we examined the DiamCalc light
return scores which are broken down further into the table only; the
‘pop up’ window shows the table light return is very good in both
static mode and when tilted through 30°.

This
DiamCalc ray trace shows the flash that appears in the upper table
region on the GIA photo is within a leakage region. How were the
diamonds illuminated for these photo’s? Was light able to enter the
pavillions?
This OctoNus ETAS map of reverse ray
traced light return is for a model of RD07. It shows stronger than
usual firey flashes on a virtual sphere. You can play with an early
version of this software here.

RD03

The
table and upper girdle facets were said in the article to be dark; this
is not evident in the photo of the same stone in G&G Fire, page
194. To test this we examined the DiamCalc light return scores which
are broken down further into the table only; the ‘pop up’ window shows
the table light return is very good in both static mode and when tilted
through 30°.

 

Category 3

The
first and last stones appear to be fair mid grade stones. The middle
stone lacks observer’s presence contrast; I believe it would look worse
than it appears in the photo.

RD22

This stone
could possibly have been improved by the manufacturer; by lengthening
the lower girdle facets and shortening the star facets (while holding
the other proportions constant) would dramatically improve
scintillation and would reduce the darkness caused by the short lower
girdle facets which are the cause of the thick dark star in the table.

A
second and better possibility may also have existed. Diamonds cut to
these proportions are frequently the only stone polished produced from
the rough. They are called ‘makeables’ because they are made without
sawing from often rounded ‘dodecahedra’ rough. Of course we can not
know if it this would have been possible – but often very steep crown
angle diamonds can be made from such material. If it was possible in
this instance, then making the crown angle 2° steeper would have
resulted in a stone that scored HCA 1.2 FIC which is an abbreviation for Firey Ideal Cut. This stone would have a 4% better yield.

DiamCalc model – GIA data Model with new minor facets New minor facets & +2° crown
Light Return Mono 0.84 0.89 0.91
Light Return Stereo 0.83 0.92 0.93
Contrast 0.99 1.14 1.14
Star facet length 55% 45% 45%
Lower G’s 75% 90% 90%

In
the article, GIA only mentions one stone as being a slight fisheye
(RD37 Cat 4), but infact this stone, even though it has a small table
size, would show a slight fish-eye (as would RD19 Cat 4).

RD11

If
this survey had involved consumers actually purchasing diamonds, spread
and apparent size would have been a very important factor in the grade
setting of this and some other stones. This mid grade stone is a clear
example of the difference between the institutional and the real
worlds. The dark upper girdle facets in this stone (as mentioned in the
article) are a result of leakage that is evident in its DiamCalc
modelled ideal-scope image. Upper girdle facet leakage results in
reduced ‘apparent size’ or ‘spread’ because our eyes loose the ability
to distinguish the edge of the diamond; i.e. it looks smaller than its
actual diameter. RD11 weighs 9% more than we would expect a nice
diamond with this diameter to weigh, but a diamond with the same
‘apparent’ face up spread would about three quarters of the weight.
This diamond would cost half as much again as a better cut diamond with
a similar apparent size.

In my opinion ranking a diamond like
this as a mid grade stone could result in retailers and consumers
accusing GIA of pandering to large manufacturers and dealers, some
donate generously to the GIA’s League of Honour Fund.

This
diamond raises the possibility of a weakness in the lighting
arrangement that GIA used. The DiamCalc ray trace shows that two facet
sets are illuminated by light coming predominantly from the same light
source. If we imagine placing this stone on a Brilliancescope®,
with its annular or circular light source that moves up and down
perpendicular to the diamonds table, we would expect the diamond to
remain dull unless one of the 5 Brilliancescope® reading positions
happens to fall in that region. In the DiamCalc ASET™ model, employing
a variation on an Ideal-scope / Gilbertsonscope * approach, we see that the bright portion of the stone in the GIA photo are predominately pink.

The
ray trace diagram shows that the center of the table, and most of the
crown main facets, are both illuminated from the same direction; this
shows as the red on the AGS ASET DiamCalc lighting model. The stone
would appear dead if light from those directions was unavailable. (The
ASET blue illumination would be largely obscured by an observers head.
The green colour indicates light that has come from close to the
horizon, the least likely source of bright illumination.)

RD06

GIA’s
computer model predicted lower crown angle stones would have better
brilliance; naturally they needed to test this with visual
observations; stones were polished especially for this purpose. The
computer scientists must have been disappointed by the results. On page
225 two reasons are given for mediocre performance. Firstly they
mention low values for crown height and angle and secondly they refer
to the stones lack of contrast and scintillation. I believe they made
the pavilion angle too shallow. (It would be good if GIA stopped
referring to “Crown Height”. A diamonds crown height is determined by
two factors; crown angle and table size. Crown height per se has little
or no determining impact on a diamonds appearance.)

A low
crown height needs to be combined with an appropriate pavilion angle.
GIA predictions of high light return for RD06 (WLR 3.01) in their 1998
G&G light return study failed to account for an observers head, a
fact well known and discussed adnauseum. Also well known is the head
obscuration of shallow crown stones has a more deleterious impact than
with other proportion sets. Because they made the pavilion too shallow
f particular crown height, the stone shows what all shallow stones show
– too much darkness. The darkness could have been reduced with
adjustments to minor facets, but a better approach would have been to
cut a deeper pavilion as in the example below (A pavilion angle in
between would probably have yielded a more balanced stone). It appears
that GIA have failed to heed the feedback from critics and have not
made appropriate adjustments to their brilliance software.

The stone on the left is a DaimCalc model of the GIA stone. On the right is this stone
with similar proportions, but 1° deeper pavilion; it scores an amazing
1.11 and 1.08 for DiamCalc light return, but rates only 0.75 for
contrast (questioning the Brilliancescope® results for scintillation).

GIA
have made no mention that shallow crown angle is a durability issue.
Perhaps GIA will follow AGS’s lead and downgrade shallow crown angles
for durability in the finalized system?


* Al Gilbertson is now one of the GIA Cut Study team
Categories 4 and 5

GIA-GTL
would be under considerable pressure from benefactors and clients who
submit large quantities of diamonds for grading to be liberal one the
one hand. On the other hand they are an ethical and honourable
institution charged with protecting the public and promoting the
industry. The second stone (RD33) in this category is so poor that it
is hard to imagine the need for a lower grade?

RD37

Worthy of note: The Liddicoat depth estimation
is a commonly used and GIA taught method of pavilion depth estimation.
By estimating the width of the table reflection compared to the width
of the table, a reckoning of the pavilion depth % is made (add 1/10th of the % to 40 = estimated pavilion depth). In this case the table reflection is 1/3rd
the table width. This would lead us to believe the pavilion depth is
43.3%, but the depth is actually only 42.3%. The Rapid Sight method
works with a relatively small range of table sizes and is also affected
by girdle thickness and crown height. In this case a 1% over estimation
of depth percentage would indicate the stone had a 40,9° pavilion;
knowing the pavilion was only 40.2° might influence a buyers decisions.

Category 5

I question the validity of this grade
that would apply to the worst 1% of certified diamonds based on a
survey conducted on Rapnet on 11/19/2004. There were only 441 plus
1.00ct rounds with depth % greater than 69% listed out of approximately
40,000 round stones searched by the same criteria. Surely the lowest
grade in a 5 grade system should be set to reflect around 10 to 20% of
the goods in the market?

It would also have been instructive to
see more than one example of a diamond downgraded for other than having
an overly thick girdle.

The optical performance of RD39 appears
to be as good, or better, than some stones in Categories 3 and 4. Aside
from the extremely thick girdle (which does not have a major impact on
optical performance) the crown and pavilion angles represent the
steepest and deepest AGS 0 angles; was the inclusion of this stone a
political choice?

 

Discussion
The
GIA study noted during the observational survey that factors other than
proportions played a role in diamond beauty. This table attempts to
simplify those observeations.

Factor No effect Slight effect

Noticeable reduction

Color

D decrease in apparent brightness as saturation increases Z

Clarity

IF to SI1

Grade determining Clouds in SI2

Grade determining Clouds in I1 >

Symmetry

Excellent VG Good

Fair > reduced brightness > Poor

Polish

Excellent VG Good

Fair> reduced brightness and fire > Poor

Girdle

No effect on apparent brightness and fire from girdle condition

Fluoro

No effect on apparent brightness and fire

Factors that showed discernable effects on brightness and fire that were noted by observers during the GIA survey study.

 

Conclusion
There
are a number of instances in the paper where references are made to
diamond performance or appearance based on percentages or preconceived
trade based ideas of how a diamond should look, or what proportions are
considered ‘acceptable’. On page 220 they state “For example, we
downgraded diamonds with pavilion angles that were very shallow or very
deep because these proportions generally changed the face-up appearance
of the diamond in ways that made it less desirable to experienced trade
observers.”

The findings of this GIA study, and other studies
and approaches to cut grading, will fundamentally change industry
expectations. If part of the purpose of this GIA article is to change
those preconceptions to match the findings of human observers and the
realities of the laws of physics, why be considerate of preconceived trade customs
and opinions? As Paul Slegers says, we should not be led by logic
dictated by adages like “my child is beautiful”. This would lead to a
lost opportunity to take full advantage of the laws of nature to
maximise both the beauty and the yield on each and every one of those
“finely crystallized carbon” creations.

Paul also notes that
trade observers are educated in a certain way of looking, judging and
thinking. No matter how ‘blind’ you make their observations by not
giving them any parameters before judging, they can identify certain
parameters and they may inevitably be influenced in their observational
judgements.

This GIA observation based study used diamonds with considerably different weights and diameters;
I believe it is un-reasonable to compare diamonds with noticeably
different diameters. Diameters were not published, but by estimation
with DiamCalc using published proportion data, they ranged from
approximately 4.7mm to more than 6mm. Some of the 15 example stones
included heavier diamonds that had larger spreads, and some of the
deepest diamonds had the lightest weights.

There appears to be no part of the study that included apparent or observed differences in spread.
Stones like RD11 in Cat. 3 and RD33 in Cat. 4 would appear
significantly smaller than their measured diameters because of greater
upper girdle leakage. In my experience this is a major factor in
diamond desirability; it would result in diamonds of these grades being
rejected in favour of stones with a lower GIA grade but larger apparent
size.

For all practical purposes, GIA has designed a 4 Category
grading system. The fifth Category may only apply to the worst 1% of
certified diamonds. It could be argued that there is a larger
percentage of overly deep uncertified ‘promotional’ grade diamonds, but
one can not help wondering if this category was set to appease some
sectors of production and wholesale within the industry? Does it serve
retailers and consumers to propose an irrelevant fifth grade when there
are stones in the fourth category that could already be considered by
many as only a little better than unpolished rough diamonds? Cut
quality is confusing and creates doubts in consumer’s minds. Doubts are
impediments to buying; that inevitably reduce the growth of diamond
jewelry as a whole. Brighter, more firey and sparkly diamonds sell
themselves and keep customers coming back for more.

It would
appear the basis of the GIA grading system will be the proportion data
from a scan. Such a system can be described as a parametric
grading system that employs look up charts similar to HCA (but with
additional minor facet proportions and symmetry and polish grades). The
AGS will shortly introduce a more advanced system using parametric and direct assessment
techniques. Neither system appears to account for differences in
diamond size and accompanying effects on appearance. Neither system is
readily functional for the designing and planning of polished diamonds
by the scanning software that is widely employed in the industry for
rough diamonds. The manufacturing industry will discover that adapting
to these new grading systems is rather difficult. OctoNus is continuing
with the development of a 3D software based grading system. This system
will account for diamond appearance based on size differences, the same
system will work for any shaped diamond and it will function in reverse
by enabling the best yield planning and the production of rough
diamonds into the most attractive gem possible. Eventually this
approach to cut grading could do away with the encumbrance of
predetermined faceting arrangements; imagine non symmetric one of a
kind diamonds with optimal beauty?

 

Symmetry and averaged data

It
appears from the articles and public presentations in Basel and Hong
Kong that the GIA system will use averaged crown and pavilion angle
data. Lower symmetry grades will lead to lessor overall cut grades.
There are at least three shortcomings from this approach.

Firstly
consider the idea that there can be ‘sweet spots’ within a grade. If
all the measured proportions are such that the stone is well within
grade boundaries, then symmetry deviations may have little impact on
the diamonds appearance. The GIA authors have qualified this by the
fair claim that a diamond in the top grade should also meet certain
crafting quality standards. But consider a diamond that is near the
boundary of a proportion grade that has certain types of symmetry
deviations, like for example, a slight squarish out of roundness that
results in the combined effect of steeper crown facets directly over
deeper pavilion angles. The additive effect of this type of small
symmetry deviation on the ray paths in a diamond can result in far
greater leakage and reduced light return. AGS would be able to
downgrade such stones because of their combined use of parametric and
direct determination using their 3 colored ASET scope.

Secondly,
the GIA article mentioned that diamonds that exhibited excellent
symmetry with Ideal-Scope or Hearts and Arrows viewers were not rated
any higher by observers than those with lessor patterns. I suspect that
their study only focused on diamonds within very good proportions, and
in this respect, Peter Yantzer and I have never subscribed to the idea
that Hearts and Arrows diamonds offer any better appearance to the
naked eye than well cut diamonds with small symmetry deviations. Had
GIA tested diamonds with the ideal-scope with the purpose that it is
designed to be used for, rather than simply as a gauge of symmetry, I
feel they would have discovered benefits form its usage and weaknesses
in their current symmetry grading method.

These
stones have a 1% out of round squarish profile, yet they would be
graded as having excellent symmetry by all labs. The stone on the left
– four of the eight sets of main facets have crown and pavilion angles
of less than 34.9° and 40.9°, while the section that is leaking with a
less well defined star, has crown and pavilion angles of 35.1° and
41.2°. The HCA scores for each set of proportions are 1.7 and 3.4
respectively. The stone on the right is the same model with a one half
degree shallower pavilion; it is in a “sweet spot” with HCA scores of
0.7 and 1.4 respectively.

Thirdly, another
disadvantage of a parametric system is that it takes no account of a
diamonds size. GIA will give the same grade for different sized round
brilliants with equal proportions[ii] . Tom Moses acknowledged that a 1/3rd
carat diamond with equivalent parameters would have a different
appearance to a 5ct diamond. Better symmetry in a well cut 1/3rd carat
diamond aids in producing easily discernable firey and contrasting
facets. But in larger diamonds, of say 5 carat or more, some types of
symmetry deviations can actually improve a diamonds appearance[iii] by
increasing the frequency of flashes from an observably larger number of
virtual facets, even though both stones have the same number of facets.

The current methods for grading symmetry has evolved from the basis of the capacity to assess or detect deviations. These systems can be shown to be inadequate for the current task.

Rounding errors

As
noted, the GIA has applied a rounding of parameters, presumably because
of the inherent accuracy of existing scanners. Rounding introduces
errors. Accurate measurements of azimuth are also critical for
parametric grading when considering the Indexing; Sarin and Ogi
scanners appear to have limitations in this area.

Upper and Lower Girdle Facet Indexing

GIA published an article 13th
Feb 2004 discussing azimuth and angular adjustments or minor facets
that are achieved with the aid of ‘indexing’. The accounts of the
affect of these polishing practices on diamond appearance ran counter
to the widely held opinions of others in this field. For instance
diamonds that have painted or built up upper girdle facets like
EightStar diamonds® were said to show “little contrast except for the
dark appearance of the pavilion mains beneath the table”. The current
article makes no mention of how painted or dug out facets will be
interpreted in a new grading system, nor of how these azimuth shifts
could be detected with adequate accuracy.


[ii] I asked about this on behalf of Sergey Sivovolenko from OctoNus
at the GIA Basel Gemfest 2004 presentation. Tom Moses replied that they
would give the same grade to a 0.30ct diamond and a 5ct diamond if all
proportions etc were the same.

[iii] Personal communication with Pol Van der Steen (DiamCad, Belgium) who is a specialist manufacturer of very large diamonds.

Future Pricing
The
issue of the effect on pricing of diamonds was not mentioned in the GIA
Foundation article, however I feel that this is one of the most
interesting aspects from most readers perspective. This is my opinion;
the market currently has a comparatively narrow band of price
differentials for variations in cut quality. For instance dealer to
dealer 1.00 ct round brilliant Rapaport pricing can range from ‘list’
down to 50% discount; put simply a good cut can cost twice as much as a
bad cut. Compare this to Clarity and Colour Rapaport pricing; the
average difference between IF and I3 is 9 times and for D to M the
figure is 4 times.

What will the market do with this new
information? It is likely that over time a new market pricing will
evolve. During the year after GIA’s system is released I predict that
the gap will widen to 2.5 times. Over a longer period 3 to 4 times
(67-75% discount) could be common as poor graded goods become more
difficult to sell. Consumers will become more aware of the need for a
grading report with a reliable and respected cut grade.

Possible GIA terminology

Discount to the next lower grade

Short term prediction

10%

15%

20%

25%

Excellent

100

100

100

100

100

Very Good

90

85

80

75

75

Good

81

72

64

56

60

Fair

72

61

51

42

50

Poor

63

52

41

32

40

Finally,
it is our opinion that the foundation of the GIA system is not built on
rock hard science. This most recent article is verbose and vague in
many areas. But it appears that by and large the GIA is moving forward.

 

Acknowledgements
The following people have contributed ideas and or reviewed and made suggestions to this review:

Paul Slegers

Yuri Shelementiev

Sergey Sivovolenko

Pol Van der Steen

Peter Yantzer

Leonid Tcharnyi has spent hours formatting and putting it here on Pricescope.

 

Thank you all.

 

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