Complete Explanation of AGS Cut System

Transcription

Foundation, Research Results and Application
of the New AGS Cut Grading System
By Peter Yantzer, Jim Caudill, Dr. Jose Sasian
© 2005 American Gem Society
This presentation is divided into three
sections: Foundation, Research Results, and
Application / Methodology.
The Foundation information will detail our
understanding of diamond cut.
The Research Results will show you what we
found with respect to the Round Brilliant and
the Square Princess Cut.
The Application / Methodology section will
explain the new AGS Cut Grading System.
Foundation
The new AGS System will take into account the following
factors: an observer, a close viewing distance, obscuration,
contrast, appearance as distance varies, brilliance, fire,
leakage, scintillation, weight ratio or ‘spread’, tilt, girdle
thickness, length-to-width ratio, polish, symmetry,
durability and taste.
In April 2002 at the AGS Conclave in Vancouver B.C. we
proposed:
“An Ideal Cut Diamond performs better than other similarly
cut diamonds over the broadest range of usually
encountered lighting and observer conditions. Because the
lighting and the observer circumstances so greatly effect the
perception of a diamond’s beauty, they must be considered
in relation to the diamond’s proportions when assessing cut
quality.”
Premise: In order to build grading systems and teach them,
assumptions and simplifications must be made.
During the course of this presentation you’ll be seeing images:
Our ray tracing software gives us various values and can also color
code the angular ranges from which a diamond draws light.
The ASET is our Angular Spectrum Evaluation Tool and we
pronounce it like the word ‘asset’. It allows us to take color-coded
photographs of actual diamonds.
DiamCalc is a software program created by the OctoNus Company
at Moscow State University in Russia. It has an abundance of
features that allows us to model virtual diamonds, as well as model
‘skin’ maps or wire frames of real diamonds.
Here are images that show the angular ranges and their
color code:
Top or Bird’s Eye View
Side View
Result
Assumptions for Average Human Being and Closest Observation Point
We have defined a human observer as the average of a 5th percentile
female and a 95th percent male. Reference MIL-STD-1472D:
We set the close observation point at 25 cm. This is the “distance of most
distinct vision.”
References:
http://www.books.md/N/dic/nearpointoftheeye.php
http://www.swc.cc.ca.us/~jveal/PHYSICS/Phys274/thin_lenses.htm
http://badger.physics.wisc.edu/lab/manual2/node19.html
Here is the geometry of our average human observer viewing a
diamond:
Because of the difference between whole head obscuration and the
distance to each eye, we assume a cone of 30 degrees at 25 cm. We also
evaluate a diamond’s appearance using a cone of 40 degrees at 25 cm.
Obscuration
The act or operation of obscuring; the state of being obscured; as,
the obscuration of the moon in an eclipse.
Reference:
http://dictionary.reference.com/search?q=obscuration
In order to simplify the complex relationship of a viewer, the
surrounding environment, and the diamond’s proportions, we
propose the concept of obscuration. There is no doubt that an
observer greatly influences a diamond’s appearance when he or
she is viewing it. Additionally, the surrounding environment can
obscure.
Premises:
Obscuration is the primary producer of contrast.
The observer’s head is the most common cause of obscuration.
Diamonds do not possess much inherent contrast. Here’s a
simulation of the Tolkowsky model ( p4075t53c345s50lg78 ) in
isometric lighting without the presence of a viewer at 25 cm:
Face Up
5 degree tilt
10 degree tilt
Looking at the same stone, we’ll add a 30 degree cone of
obscuration:
The Observer’s Head
Our ‘average’ human being’s head obscures a cone of
30 degrees or plus/minus 15 degrees from the normal
(perpendicular to the table facet) at 25 cm.
Our ‘average’ human being’s head is circular in shape.
Assumption: The ability to discern differences in
diamond cut quality decreases as the distance from the
observer’s eyes increases.
The Observer’s Body
The observer’s body also obscures a diamond.
In a typical viewing scenario, the closer a diamond gets
to an observer’s body, the less the effect produced by
the observer’s body.
Or to say this another way, as distance increases, the
effect of an observer’s body increases.
This may seem paradoxical, so let’s look at a typical
viewing scenario.
These viewers are looking at a diamond at about 10 inches from
their eyes.
As you can see, the observers’ bodies obscure somewhere
between 30 and 40 degrees of lighting that may be coming from
behind them.
Here’s what happens to that angular relationship at a viewing
distance of 16 inches.
Now the observers are obscuring somewhere between 40 and 50
degrees.
Contrast
Premise: Contrast can produce positive and negative optical effects.
Humans are ‘hard wired’ to detect edges. Contrast provides us with these
edges. Here’s an example of a shape with virtually no contrast and the
same shape with contrast. Humans find the image on the right to be very
appealing compared to the left. Other researchers on diamond cutting
have pointed out that the presence of contrast enhances our perception
of diamond brilliance. We agree.
Humans are sensitive to the amount and distribution of
contrast.
Examples of too little and too much contrast:
Poor distribution of contrast:
Frequency
We are also ‘hard wired’ to find certain frequencies appealing.
When shown this frequency chart, the majority of people will pick a spot
close to the area marked in red as being the most appealing. We
instinctively find the frequencies to the left as being too broad and the
frequencies to the right as being too narrow. In addition to contrast,
frequency has implications for scintillation as well.
Let’s apply a 30 degree cone of obscuration to different round
brilliant cut diamonds:
41.1 Pavilion Angle
Negative –
no primary contrast
Tolkowsky
Positive Effect
Nail Head
Negative – too much
We have named two types of obscuration induced contrast – primary and
secondary. Primary contrast is the face up and/or stationary view and therefore
static in nature. Secondary contrast is the light/dark pattern as the stone moves
and is dynamic in nature.
Below is an illustration of how the primary contrast at 25 cm varies with amount of
the angular obscuration for the Tolkowsky model.
~21 degrees
No effect
~25 degrees
Positive effect
~44 degrees
Still Positive
~47 degrees
Negative effect
It is evident that the Tolkowsky model handles a large range of obscuration in a
positive manner. Most importantly, it handles this obscuration in the range a
human observer’s head (30 degrees) provides. Or, you may say that it
accommodates, in a positive manner, a wide range of different head sizes.
Examples of secondary contrast using 30 degrees of obscuration at
25 cm:
4 degrees of tilt
5 degrees of tilt
6 degrees of tilt
8 degrees of tilt
9 degrees of tilt
10 degrees of tilt
7 degrees of tilt
11 degrees of tilt
1) Very little happens to the light/dark pattern in the first 4 to 5 degrees of tilt.
2 As the optical ‘windows’ or compound mirrors go from light to dark and back to light,
you see fire.
Assumption: Contrast also enhances our ability to see fire.
As you have seen, contrast is a two edged sword – too little, too much, or
poor distribution is ‘bad’. The right amount with pleasing distribution is
‘good’. Contrast is a very important aspect of diamond appearance.
Fortunately, contrast in the standard round brilliant is independent of
table size. It’s a function of crown and pavilion angles, influenced slightly
by star and lower girdle height. This allows us to create an obscuration
chart:
In fancy shapes, the contrast produced by obscuration is unique to each
cut. It is, however, very important to assess the effect of obscuration for
each shape.
Diamond Appearance as Distance Increases
Assumption: The ability to discern differences in cutting quality decreases as
distance increases. Our starting point is 25 cm or 9.84 inches.
Premise: As distance increases, the effect of an observer’s head decreases.
Tolkowsky at 25 cm
at 40 cm
at 80 cm
p4075t53c345s50lg78
Inferior make at 25 cm
at 40 cm
p393t55c330s50lg80
at 80 cm
The previous slide demonstrates that an
inferior make can look quite good as
the distance increases between your
eyes and the stone.
Great makes look great both ‘up close’
and far away. Inferior makes are
readily discernable as inferior ‘up
close’.
Brilliance
Assumption: The world is lit from above.
The terms ‘brilliance’ and ‘brightness’ are not interchangeable in
this presentation.
By color-coding angular ranges ( green = 0 to 45 degrees, red = 45
to 75 degrees, blue = 75 to 90 degrees from the horizon) we can
model where diamonds gather light.
Tolkowsky
p4075t53c345s50lg78
Fish Eye
p370t62c345s50lg78
Nail Head
p440t62c345s50lg78
Well made round diamonds gather a large portion of light from the
angular range of 45 to 75 degrees from the horizon. This is the area
where a diamond is most likely to find direct sources of light and that light
will miss the observer’s head and body. In the above illustration, you see
that a fish eye also gathers a large portion of light from this range and yet
it is universally accepted to be an inferior make. So it’s obvious that
brightness alone does not complete the description of brilliance.
Many fancy shapes don’t gather a large portion of light from this range and they tend to
leak a lot of light. Here are examples of an oval and a princess cut. The color-coded
images here are backlit to show the white leakage areas. The photo real images are not
backlit.
We know from experience that well made rounds are brilliant. We also know that
fancy shapes can appear brilliant as well. So it seems paradoxical that we can
perceive both as being brilliant when the round gathers a majority of high quality
light ( red ) and some fancies gather a large amount of low quality light ( green ).
Brightness is relative. Our brain innately and constantly adjusts levels of brightness
to make sense of what it ‘sees’. This is just like the auto exposure control on a
camera, except humans do it better and faster. We can observe in the above
examples that differences in brightness set up areas of contrast. You might call
this ‘brightness contrast’. It’s this effect that can produce a pleasing ‘look’ and the
stone will appear to be brilliant to us.
Therefore, in our opinion, there are three things that affect our perception of
brilliance:
1) Brightness
2) Contrast caused by obscuration
3) Contrast caused by changes in brightness
It is not so important to measure brightness but more important to understand that
brightness is relative, and if combined with positive obscuration contrast effects
and/or positive brightness contrast effects, the stone will appear brilliant.
We are defining brilliance as: brightness with positive contrast effects.
The Illumination of Great Diamond Design
The most desirable angular range is 45 to 75 degrees.
Why?
1) Because it misses the observer’s head and body.
2) This is where the diamond will most likely find
direct sources of illumination.
Fire
The perception of fire is accentuated by the presence of multiple spot
light sources.
The perception of fire is diminished by the presence of broad diffuse light.
Since lighting environments change constantly, we measure
dispersion.
First, we divide the diamond into three zones of equal area:
Next, we ray traced hundreds of thousands of virtual and wire frame models and
averaged the dispersion for each zone at a viewing distance of 25 cm.
Here are the dispersion values ( in millimeters ) for the Tolkowsky, Fish Eye and
Nail Head stones shown earlier in this presentation.
Tolkowsky
Fish Eye
Nail Head
Table
3.7
1.6
0.8
Inner Bezel
2.8
2.7
4.4
Outer Bezel
3.8
2.6
6.0
Average
3.4
2.3
3.7
Finally, we analyzed the data looking for relevance. Indeed, we did find relevance.
Industry accepted fine makes produced high values across all three zones while
inferior makes suffer in one or more zones.
As an aside, we initially looked at the average dispersion across the entire crown.
What we found was that a very high reading in one zone could skew the average.
We concluded that an entire crown average was useless.
Here’s an example:
p420t55c385s50lg78
Table
2.1
Inner Bezel
4.0
Outer Bezel
7.0
Average
4.4
We postulate that diamonds with high-dispersionaverages across all three zones have the greatest
potential to exhibit fire.
This becomes more important as viewing distance
decreases.
We look at fire similarly to contrast in the sense that
there are primary (static) and secondary (dynamic)
states.
Leakage
In the bulk of our studies, no light was allowed to interact through the pavilion or
girdle plane. Or, to state another way, light was only allowed to interact through
the crown facets and table.
Assumption: small amounts of leakage is essentially inconsequential. Large
amounts of leakage, typically occurring in some fancy shapes and some standard
round brilliant proportion sets, is detrimental.
Leakage is readily quantifiable and can be factored into a grading system.
In these color-coded and ray-traced examples, leakage is quantified and shown in
white.
Fine Make 5.1%
Fish Eye 11.0%
Nail Head 28.9%
It should be noted that leakage is one of the two vehicles whereby brightness
contrast effects are produced. The other is the size and distribution of areas that
draw light from low angles – the greens in these images. These brightness contrast
effects can be positive or negative.
Scintillation or Sparkle
Here are a couple of definitions of scintillation:
GIA Diamond Dictionary, 1977 Edition
Scintillation in gemstones can be defined broadly as an alternating display of reflections from
the polished facets of a gemstone seen by the observer as either the gemstone, the illuminant
or the observer moves; it is a flashing or twinkling of light from the facets. Comparative
scintillation, or the degree of scintillation in a diamond, is determined by (1) the number of
facets on the stone that will reflect light to the eye as the stone is moved about (i.e., the
number of individual reflections), and (2) the quality of the polish of the facets, since the more
highly polished the facets, the brighter the reflections and hence the stronger the flashes from
them.
GIA Diamond Dictionary Online, http://giaonline.gia.edu/public/cgi/as_web.exe?dia_dic.ask+F
Flashes of light reflected from a polished diamond, seen when either the diamond, the light
source, or the observer moves. Besides diamond's inherent optical properties, scintillation
depends on the number and size of the facets, the precision of the facet angles, and the quality
of the polish. Sometimes called sparkle.
Earlier in this presentation we stated that the light/dark areas changed
very little over the first 4 to 5 degrees of tilt. Here’s an example:
0
1
2
3
4
5 degrees
Let’s put the Tolkowsky model in a lighting
environment that allows us to see what happens
with fire:
0
1
2
3
4
5 degrees
Depending on relative lighting conditions, you may not be able to resolve
these as fire, only as a sparkles. That’s because the brightness may be
overpowering your ability to discriminate.
As you can see, the changes are abrupt and dramatic. They occur
over very small changes in angular displacement.
We believe that any definition of scintillation should include white
and colored sparkles. We also believe that fire-in-motion or
dynamic fire is a strong component of scintillation.
Adding fire-in-motion as a component of scintillation requires that
a diamond have a high potential (dispersion) to produce fire in
order to generate high scintillation.
From a common sense and practical standpoint, this consolidation
of white and colored sparkles into the definition of scintillation
probably best describes what diamantaires have called ‘life’.
We propose the following as a definition of scintillation:
The sparkle of white and colored flashes seen as the stone and/or
the observer and/or the light source(s) move.
Scintillation - Continuing Research
We continue to research scintillation. Here’s what we believe at this
point in time. The trick is in measuring it or creating a metric for it.
Scintillation is a function of the double reflection pattern of a faceted
diamond. We call these compound mirrors.
You can change the amount of scintillation in a faceted
gemstone in two ways that we know of:
1) Add more facets.
2) Change the size of the facets, thereby changing the
compound mirrors.
This is an example of adding facets ( facet arrangement
on left, compound mirrors image on right):
These examples show the compound mirrors for the Tolkowsky
proportion set but with changes in the length of the stars and
height of the lower girdle facets.
35% Star
60% lower girdle
Image 1
50% star
80% lower girdle
Image 2
50% star
90% lower girdle
Image 3
Image 1 is a classic Old European cut. These stones were known for
big, broad flashes of fire with less scintillation than today’s
modern round brilliant.
Looking at the compound mirrors, you can see why.
Image 2 is a modern round brilliant. Over time, cutters lengthened
the stars and lower girdle facets. The net effect is more
scintillation with good perception of fire. Research by scientists
at Moscow State University postulate that a well cut stone should
have a nice balance of large and small compound mirrors.
Image 3 shows the effects of too long lower girdle facets. The
compound mirrors are similar in size and the lower girdle facets
overpower the table area. The net effect is that scintillation may
be higher but there is lower dispersion in the outer bezel and
contrast is adversely affected.
With excellent performance to begin with, you might say:
Big compound mirrors = big fire but small scintillation
Small compound mirrors = small fire but big scintillation.
Dynamic contrast + dynamic fire = scintillation.
The new AGS Grading System handles scintillation passively, and depending
on the outcome of our continuing research, probably sufficiently. Here’s
how: if the lower girdle facets get too short, contrast becomes a
negative factor. If the lower girdle facets get too long, contrast and
dispersion suffer.
You probably deduced that it is not necessarily the number of compound
mirrors, but the balance and distribution of large and small compound
mirrors in any given cut that matters.
You may also have speculated that different size compound mirrors may
enhance diamonds of different size - .50 ct vs 2.00 ct vs 7.00 ct., for
example. This may lead to new faceting arrangements based on physical
size.
Weight Ratio or ‘Spread’
‘Spread’ is an industry term that refers to a diamond’s face up size
compared to its weight. You can also call this ‘weight ratio’ or ‘millimeter
footprint versus weight’.
The classic example is that a fine make 1.00 carat round brilliant cut
diamond should have a ‘spread’ of about 6.5 millimeters. Naturally, you
would want to purchase the largest millimeter stone that weighs the least
amount and still performs. Why pay for unwanted weight?
For a one-carat diamond, the current AGS Ideal 0 proportions allow a
millimeter ‘spread’ range of 6.30 to 6.57 mm.
Most people would not consider a 1.00 carat round brilliant cut diamond
with a ‘spread’ of 6.30 millimeters to be an Ideal.
Therefore, we are using a 5% ‘spread’ factor for the round brilliant in our
new grading system. We are normalizing to the Tolkowsky cut with a 2.7%
girdle thickness at the mains and 1% at the scallops. This Tolkowsky
model will weigh 1.00 carat at 6.47 millimeters in diameter.
A tight ‘spread’ tolerance is a beautiful thing because it self corrects a lot
of cutting faults.
Cutters know how to ‘swindle’ our existing proportion sets to maximize
weight at the expense of beauty. We hope that the ‘spread’ component
will go a long way in furthering the world diamond community’s and
consumer’s perception of fine make. It’s also reasonable in today’s world
of precision diamond cutting. Lastly, it’s easy to teach and understand.
Indexing the Upper Half Facets on a
Round Brilliant Cut
‘Normal’ cutting produces equal girdle thickness
at the junction of the mains and the half facets.
Indexing the Upper Half Facets
Cutting the upper halves on a non-normal index results in
girdle thickness that is different at the halves than at the
mains.
Example 1
Thicker at Mains
Example 2
Thicker at Halves
Example 1 can result in better weight retention
but at the expense of optical performance.
AGS ASET
‘Hearts & Arrows’
Brightness simulation
Fire Scope
Example 2 can result in less weight retention but
face up leakage is eliminated.
AGS ASET
‘Hearts & Arrows’
Brightness simulation
Fire Scope
Example 1 can be an AGS 0 in our existing system.
It will not be an AGS 0 in our new system.
Example 2 can be an AGS 0 in either system.
The AGS ASET provides, at a glance, much more
information than other types of viewers.
A complete article on this topic is included on
this CDRom disk.
Tilt
George Kaplan wrote a letter to Gems and Gemology and it was
published in the Summer 2002 issue. In that article he made a
case for the concept of the ‘Cone of Beauty’. He said that a well
made round brilliant can stand a larger amount of tilt before the
girdle is reflected in the table of the stone. As an example, his
firm cut two identical diamonds except one had a 55% table and
the other a 65% table. With a tilt of 10 degrees, the 65% table
stone started to show dull girdle reflections in the table. On the
other hand, the 55% table handled a tilt of 18 degrees. Therefore,
he said that the 65% table stone had a ‘Cone of Beauty’ of 20
degrees. The 55% table stone had a ‘Cone of Beauty’ of 36
degrees.
We support George Kaplan’s astute observation and think it should
be part of our new grading system. It’s another factor that helps
to separate fine makes from inferior makes. It’s also another
factor that adds to the concept of an Ideal.
Tilt Examples
It’s easy to make tilt charts for the standard round brilliant. We
started with the Tolkowsky model and verified what it looked like at
18 degrees of tilt. In order to build a grading system, we needed to
‘relax’ that number while still maintaining the same ‘look’. 14
degrees of tilt seems to be realistic.
Tolkowsky @18 degrees of tilt
59 table at 14 degrees of tilt.
Girdle reflection shown in green
Tilt Charts
Table % Minimum Pavilion Angle
47
39.0
48
39.1
49
39.2
50
39.4
51
39.5
52
39.6
53
39.7
54
39.9
55
40.0
56
40.1
57
40.2
58
40.4
59
40.5
60
40.6
61
40.7
62
40.9
63
41.0
64
41.1
65
41.3
66
41.4
67
41.5
68
41.6
69
41.7
70
41.9
Girdle Thickness
Our research has shown that the only good things about a girdle are:
1) it defines the shape of the stone.
2) if it is sufficiently thick, it helps to prevent chipping.
Other than that, the girdle is an area that allows detrimental infiltration and
leakage of light.
Here are color-coded, face up simulations of the Tolkowsky model and photo real
simulations of their profiles.
very thin
thin
medium
sl. thick
thick
very thick
ex. thick
Taste
With respect to the standard round brilliant, the three areas of taste are:
Table reflection
Width of Pavilion Main facets
Table Size
Some fancy shapes can offer a wide variety of equal performance with different
‘looks’.
Here are some Princess cuts.
55% Table
65% Table
70% Table
The new AGS grading system allows for taste factors.
Insights Gained Using the AGS ASET
Why Older Cuts Have Short Star Facets
Before the invention of the diamond saw, the diamond’s table was
fashioned by grinding away one of the points on an octahedron. In
order to save weight, small table sizes were the rule.
Cutters are very, very smart. These ASET images show you why
cutters shortened the star length when making older cuts.
P410t47c358s50lg60
Crown
ASET
Brightness Sim
Profile
It’s readily apparent that the 50% star length makes the upper
halves too steep. Performance suffers.
See what happens when the star facets are shortened to 35%:
P410t47c358s35lg60
Crown
ASET
Brightness Sim
Profile
Shorter star facet length lowers the angle of the halves so they draw
light from the red area. Performance is greatly improved.
In this case, looking at the past helps to validate the present.
Lower Girdle Height
We measure lower girdle length by height. This
is the same as the OctoNus DiamCalc software.
GIA measures lower girdle length by radius.
That way you can measure it with a table gauge.
It can cause confusion because the two aren’t
identical.
Lower Girdle Height – AGS / DiamCalc to
GIA Conversion Chart
Special thanks to Bruce Harding and Jason Quick
Lower Girdle Height – GIA to AGS /
DiamCalc Conversion Chart
Special thanks to Bruce Harding and Jason Quick
Foundation Summary
• Studied the effects of an observer,
surrounding environment, and lighting.
• Studied how humans ‘see’ our world.
• Developed metrics that reflect actual
observation.
• Invented new tools.
• Proposed new definitions.
• New system is three-dimensional in nature.
Research Results
The Round Brilliant
• Tolkowsky’s Five Diamonds
Stone 1 Stone 2 Stone 3 Stone 4 Stone 5 Average Theoretical
Diameter
7.00
7.08
6.50
21.07
9.12
Depth
4.12
4.35
3.61
12.34
5.47
Pavilion Angle
40.75°
40.75°
40°
41°
41°
40.7
40.75
Pavilion Depth %
43.0
42.8
42.1
42.8
42.2
42.6
43.1
Crown Angle
35°
35°
34.5°
33°
34°
34.3
34.5
Crown Height %
15.7
18.6
13.3
15.7
17.8
16.2
16.2
Girdle Thickness
v. thin
v. thin
v. thin
v. thin
v.thin
Est. Table %
55
47
61
52
47
Est. Weight
1.23
1.31
.94
33.26
2.77
Their Appearance with 50% Star Length, 80% Lower
Girdle Height
1
2
3
4
5
Conjecture
• If Tolkowsky had the tools we have, he
might have picked this one for his
Stone #3:
• P411t61c329s50lg80
Why Now?
• Fast computers, accurate measuring
devices, ray tracing software, OctoNus
DiamCalc software.
• Advanced science and technology
allows us to grade the diamond in
three-dimensional space rather than
two-dimensionally.
Explanation of Charts
• In case it’s hard to read them, the following charts
are set up in the following manner:
• Pavilion angle is 43 degrees in the top left corner
and descends in 0.2 degree increments to 39.8
degrees in the bottom left corner.
• Crown angle starts at 29 degrees in the top left
corner and increases in 0.2 degree increments to 40
degrees in the top right corner.
• The charts are for a 6 millimeter diameter stone,
50% star length, 80% lower girdle height, and a 3.5%
girdle thickness at the mains.
Findings
• Combined overlays:
• Tilt, Weight Ratio, Contrast, Durability
• You’ll notice that we’ve defined an area for
potential 0’s for a 55% table without
knowing anything about brightness,
dispersion, or leakage.
Defining the New AGS 0
• The previous chart with the candidates
for a 55% Table.
Candidates
• The reason we use the word ‘candidates’ is because the entire
system is dynamic. For example, contrast changes with size.
• The boundary edges are ‘fuzzy’ in the sense that a cutter can
bring a borderline stone into a higher category by adjusting
star length and lower girdle heights. A cutter can also adjust
girdle thickness to bring some stones into a higher grade if its
grade is being reduced by the weight ratio factor.
• You can also lower a candidate through sloppy cutting or
indexing the facets, especially the upper girdle facets.
• To consistently produce a desired cut grade, cutters will have
to cut to ‘fat’ portions of the charts.
Round Brilliant Cutting
Guideline Charts for Table
Sizes 47 through 70%
All of the following charts are included on this
CDRom disk
Special thanks to Jake Sheffield for the charts
The Old and the New
• 55% Table
• The steep pavilion - steep crown and
the shallow pavilion - shallow crown
corners of our existing two-dimensional
system will no longer be AGS Ideal 0’s.
Star Length
• This chart shows the effects of changing the star length for a
6mm round brilliant, cut normally with a 55% table:
• Shortening the stars to 40% increases the candidates by almost 25%.
• Lengthening the stars to 60% reduces the candidates by over 75%.
Lower Girdle Height
• This chart shows the effects of changing the lower girdle height for a
6mm round brilliant, cut normally with a 55% table:
•
•
Shortening the lower girdle facets to 75% decreases the candidates by about
75%.
By increasing the lower girdle facets height to 85%, the number of candidates
remains about the same, but shifts slightly up and to the left.
Opportunities
• Candidates in new table sizes.
• Better weight retention and weight
ratio options.
What’s Ahead
• More research for smaller diamonds
– Diamonds under 15 points not eligible for
grading
• Commercial and Industrial AGS
Software
– Batch processing with industrial version
The Princess Cut
• Our new methodology will enable us to
develop cut grading systems for any
shape and facet arrangement.
Configuration
• New grading system for square princess cuts
– Bezel Corner
– French Corner
Bezel Corner - 45 Facets
French Corner - 41 Facets
Complexity
• Princess cut more complex
– Two pavilion main angles and two crown
main angles.
– Increases combinations exponentially.
Not really exponentially, but a whole lot.
‘Exponentially’ sounds better than ‘a
whole lot’ and I couldn’t think of a better
word.
Example
Versatility
• Very wide range of table sizes.
• When tilted, girdle reflections under
the table are broken up into small
pieces.
Top Performers
• These charts are for a 6 millimeter
square with 2 rows of chevron shaped
facets on the pavilion.
Macro Chart
All of the
macro Square
Princess
Guidelines
Charts are
included on
this CDRom
disk.
55 Appearance
55 Cutting Suggestion
Micro View of 55 Table
Guideline Chart
60 Appearance
Guideline Chart
65 Appearance
Guideline Chart
70 Appearance
Guideline Chart
75 Appearance
Guideline Chart
Top Performer Distribution By
Table Size
AGS Grade Distribution
Application / Methodology
The New AGS Cut Grade System
Introduction
• Software driven system.
• Combination of deduction and net
lowering categories.
• Complex set of criteria is evaluated to
establish a grade.
Expression
• Same AGS cut grade format.
• Modified sub-categories.
Cut Grade
Light Performance
Proportion Factors
Finish
AGS Ideal 0
0
0
0
Sub-Category 1
• Light Performance
–
–
–
–
Brightness
Dispersion
Leakage
Contrast
What’s Going On Inside the
AGS Software?
• The following slides will simulate
grading for a Round Brilliant Cut
diamond with a 55% Table, 50% star
length and 80% lower girdle height
Brightness Deduction Charts
• Face up, tilted 15 degrees and overlay
Dispersion Deduction Charts
Leakage Deduction Charts
Contrast Deduction Chart
• 30 degree obscuration, 40 degree
obscuration and overlay
Overlay Result
• The 55% table round brilliant cut diamond
must fall within the white area of this chart
to be an AGS Zero candidate
Sub-Category 2
• Proportion Factors
– Girdle Thickness
– Culet Size
– Weight ratio or millimeter footprint versus
weight or ‘spread’
– Durability ( Crown Angles less than 30
degrees )
– Tilt ( at what point does the girdle reflect
under the table )
New Girdle Thickness Chart*
* Reflects AGS Diamond Grading Standards Change
Effective May 1, 2013
Net Lowering Chart
Culet Size – same as before
Culet Size
AGS Grade
Pointed, Very small, Small,
0
Medium
Slightly large
1
3
Large
Very large
5
Extremely large
7,8,9,10
Weight Ratio Deduction Chart
Durability Deduction Chart
Durability
Traditionally the industry discounts diamonds with extremely thin and very thin
girdles. We also do the same. The other durability factor that we do not address is
shallow crown angles. Industry wisdom says that a diamond with crown angles
under 30 degrees is more likely to break under normal wear and tear. GIA
currently issues a statement on its reports if the crown angle is less than 30
degrees.
We’ll address shallow crown angles in our new system.
Here’s an example of a round brilliant with high, but not the highest performance
and crown angles of less than 30 degrees:
p416t55c296s50lg78
Tilt Deduction Chart
55% Table
Sub-Category 3
• Finish
– Polish
– Symmetry
Net Lowering Chart*
• Polish and Symmetry
The AGS Ideal® (0) Cut grade is attainable with Ideal or Excellent polish and symmetry grades as follows:
AGS Ideal® (0) Cut
Light Performance /
Proportion Factors
Ideal (0)
Ideal (0)
Ideal (0)
Ideal (0)
Grade
Polish
Ideal (0)
Excellent (1)
Ideal (0)
Excellent (1)
AGS Excellent (1) Cut Grade
Light Performance /
Polish
Proportion Factors
Excellent (1)
Excellent (1)
Excellent (1)
Very Good (2)
Excellent (1)
Excellent (1)
Excellent (1)
Very Good (2)
Symmetry
Ideal (0)
Ideal (0)
Excellent (1)
Excellent (1)
Numeric Designator
0 0 0 The Triple Zero®
010
001
011
Symmetry
Excellent (1)
Excellent (1)
Very Good (2)
Very Good (2)
Numeric Designator
111
121
112
122
This Cut Grade model continues for Very Good, Good, and so forth.
* Reflects AGS Diamond Grading Standards Change
Effective November 15, 2012
Deduction vs. Net Lowering
• Cumulative deductions
– Brightness, Dispersion, Leakage, Contrast,
Durability, Weight Ratio, and Tilt
– Add them up or sum them. I.e. 1+1+1 = 3
• Net lowering deductions
– Girdle Thickness, Culet Size, Polish and
Symmetry
– They only lower the cut grade if lower than the
sum of the deductions
• Software performs calculations
Length-to-Width Ratio
We have had much discussion on what length-to-width ratios should be
included in our new system.
The existing AGS Diamond Standards specify acceptable length-to-width
ratios for various fancy shapes.
Here are the ratios for some fancy shapes:
Oval length to width range: 1.33 to 1.66
Emerald Cut length to width range: 1.50 to 1.75
Marquise length to width range: 1.75 to 2.25
Pear length to width range: 1.5 to 1.75
How Do I Cut Grade a
Diamond?
• Step by step demonstration.
• Mostly software automated.
Step One
• Measure the diamond with a machine.
• Create a three-dimensional model.
• Does require a hardware investment.
Step Two
• Import three-dimensional image into
AGS Software and ray trace.
Pavilion
angles
40.75
Crown
angles
34.5
Table
53
Star
Length
50%
Lower
girdle
78%
T
I
Dispersion T value Dispersion I value
1.0
21.2
1.0
19.9
O
Dispersion
1.0
O value
27.4
15 degree
15 degree
T
15 degree 15 degree I 15 degree I
O
15 degree
Dispersion T value Dispersion
value
Dispersion O value
0.9
21.9
1.3
22.9
1.0
19.0
Blues
21.2
Reds
66.2
Greens
7.2
Leakage
5.4
Blues
11.4
Reds
64.9
Greens
18.5
Leakage
5.2
• Optical performance is measured.
Step Three
• Software will compare values and assign a
deduction grade.
• Software will compare values to net lowering
categories and adjust.
• Grader inputs polish and symmetry grades,
verifies girdle thickness and culet size.
• Software assigns light performance portion
of final cut grade.
Don’t I Get To Do Anything?
• Check or verify girdle thickness and
culet size.
• Evaluate polish and symmetry.
• You may have to assess contrast on
some fancy shapes.
Easy as 1, 2, 3
• The new AGS Cut Grading System is the most
sophisticated system ever developed.
• Even so, it is the easiest AGS system to use –
ever.
• Most importantly, the methodology can be
applied to any shape and facet arrangement.
Wait, There’s More
• Research on rectangular Princess Cuts
and impact of more ‘chevron’ shaped
facets.
• Results in very near future.
Afraid of Diamond Commodities?
Let’s Revisit ‘Taste’
• Most of us probably feel that a diamond commodity
is a bad thing.
• Some are afraid that this new grading system will
make diamond a commodity.
• The research only quantifies what experienced
people already know.
• You and your expertise are still the most important
part of a diamond sale.
• Let’s look at some examples of different AGS Grades
and you decide for yourself.
• The following examples have 50% star length and
80% lower girdle height.
AGS 4 Cut Grade
•
P410t65c298
P426t51c324
AGS 3 Cut Grade
•
P418t49c338
P416t68c304
AGS 2 Cut Grade
•
P400t55c382
P412t63c302
AGS 1 Cut Grade
•
P412t58c322
P402t55c372
AGS 0 Cut Grade
•
P418t47c338
P412t61c328
Conclusion
In this case, seeing is believing. I submit that the new AGS Grading
System does more to keep diamonds from becoming a commodity
than anything else others have proposed.
I believe that the cutting community is looking at vast and broad
opportunities because of the new AGS system.
You are empowered by knowing that equivalent grades are equivalent in
performance but different in look ( taste ).
You can empower your customer, get them into your store, and make
more sales by asking them what their taste is in an Ideal, or a Very
Good, or a Good cut.
The new AGS Cut system will serve you well, if you learn it.
The ball’s in your court.
Thank You!
Special thanks to JCK and all involved in this
historic project

Complete Explanation of AGS Cut System

Transcription

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