GIA Diamond Cut Research Technical FAQ

[pricescope]

Authors of the Fall 2004 Gems & Gemology article on diamond cut, “A Foundation for Grading the Overall Cut Quality of Round Brilliant Cut Diamonds,” have created a list of common technical questions and answers related to the article and the fundamental research used in creating the upcoming GIA Diamond Cut Grading System: http://www.gia.edu/research/29324/diamond_cut_faq.cfm

Some of the interesting Q&A''s:

Did GIA confirm that Tolkowsky’s cut proportions give the “best” grade? Or are there sets of proportions that look as good or better than Tolkowsky’s?
"...a “modern” diamond with Tolkowsky’s proportions for most parameters but with a thicker girdle, longer lower half facets or halves, etc., would in most cases do very well in our system (as long as it was well-crafted). ... Our research revealed that many combinations of proportions produce a diamond that deserves a top grade, and we documented many cases where proportions other than Tolkowsky’s were preferred by observers. However, the observations did not support any single set of proportions as “best"

If a diamond looks better in one of the proprietary viewers (such as one of the “hearts and arrows” viewers), does it get a better grade in the GIA System? Did observation tests confirm that such diamonds look better?
"...We found that although many diamonds with optical symmetry received high observation scores, other diamonds (with very different proportions and, in many cases, no discernable optical symmetry) were ranked just as highly"

How does “optical symmetry” relate to the symmetry that GIA currently reports, one aspect of finish?
"...there is not always a direct relationship between optical symmetry and graded (surface) symmetry, so that a diamond with high optical symmetry does not necessarily also have excellent graded (surface) symmetry, and vice versa"

What is the effect of the mounting and the background against which the diamond is viewed on the appearance of a round brilliant cut diamond?

"...the trade usually uses a white tray or folded white business card to make judgments in appearance, certain judgments could be impaired by the use of the white background. For instance, some pattern features that are dark due to light leakage are less obvious when diamonds are observed on a white background."

Bonus link: The Optics of a Dirty Diamond

 

[strmrdr]

I find this interesting:

"The GIA fire metric is modeled using a single point light source; however, there are lots of lights in modern jewelry stores (e.g., multiple light sources in display windows, rows of spot lights above or within jewelry cases plus fluorescent tubes on the ceiling), and many lights in the GIA common viewing environment. Doesn’t this difference create a discrepancy?

A model is a mathematical approximation of reality—not a duplication—that is useful for a particular application. For our research, the usefulness was judged by how well the model’s predictions matched observations of actual diamonds in these environments. In this case, the fire metric uses one light and many observer positions, while the observation data were gatheredusing many lights and single observers who rocked the stone while observing it. We used early observations to choose the best fire metric, and found that later observations matched the model’s predictions well."

That reads too much to me like finding the lighting environment that matched their preconceived model vs. finding a model that matches the real world.

This is a problem I have with all the computer simulation based systems.

 

[strmrdr]

I find this interesting and would like to see a similar study done with consumers only it also brings to mind the lighting environment.
Did it enable the pattern being seen?
If you want the judge brightness, fire and patterns, You need 3 different lighting environments they can not be all judge under the same lighting conditions.




If a diamond looks better in one of the proprietary viewers (such as one of the “hearts and arrows” viewers), does it get a better grade in the GIA System? Did observation tests confirm that such diamonds look better?

Many people in the trade use the term “optical symmetry” in referring to “branded” diamonds that show near-perfect eight-fold symmetry by displaying eight “hearts” in the pavilion-up position, or eight “arrows” in the face-up position, when viewed in specially designed optical viewers. To investigate the possible benefits of optical symmetry, we included several such diamonds in our observation testing. We found that although many diamonds with optical symmetry received high observation scores, other diamonds (with very different proportions and, in many cases, no discernable optical symmetry) were ranked just as highly.

An interesting by-product of the testing was that those trade members who emphasized this type of diamond in their business generally chose such a diamond as the highest ranking (although not always). Those who did not market this type of diamond chose it as best about as often as they chose other diamonds we have placed in the top grade categories. It appears that these types of diamond could be likened to an “acquired taste” or “learned bias.” This doesn’t mean that some of them shouldn’t rank highly—it just means that not everyone agrees.

Finally, there are a variety of proportions that yield these patterns, some of which cause the diamond to appear darker to many observers. When a diamond had such a dark appearance, even though the pattern was considered a very good representation of a diamond with “optical symmetry,” many observers did not place it in the top category. This has been accounted for in our system.

 

[Garry H (Cut Nut)]

I would like to thank the gIA for dribbling out a few answers to some of our many questions.
There are quite a few things I would like to challenge, but the main one is this:

The dark area in the GIA brightness metric is 23 degrees in radius; this seems too large for a “head shadow.” Doesn’t this make many diamonds appear dark?
The (top) dark area for our brightness metric is 23 degrees in radius, centered at the dome zenith; however, it does not represent a “head shadow.” Observations through hemispheres that re-created the metric environment were used to establish relative rank order for the appearance of brightness, not to match patterns observed in more typical lighting. The rank order for brightness determined from metric calculations using this environment was the best match to that determined by observers in our common viewing environment.

This statement got me thinking as to why it could be that GIA found they needed to block that much light in their ''Metrics'' (metrics is code for computer modeling environment). You see their goal of the observation study (survey) was to prove the validity of their computer modeling (I thought good science was to try to disprove theories?).

Here I have modelled with DiamCalc the effect you would see when you look at a diamond with the gIA lighting ( with 10% light return from the observers body / head) (left) and the bit that was subtracted by eliminating 46 degrees.
Besdie that I have shown the ideal-scope lighting. And to the far right is the essential bit that is missing between GIA and ideal-scope lighting.

Perhaps what happened to GIA was they made a lighting box that kept the observer outside and probably made about a 23 degree angle between the lights in the box and the observers eyes. Apparently the room the viewing was done in was darkened - so I guess with a white coat on - they made the equivalent of a dome with a 46 degree viewing hole.

Could someone please find a picture of the ''common viewing environment'' (big plastic light box)?

 

[Serg]

Date: 3/25/2005 7:45:56 PM
Author: strmrdr
I find this interesting:

''The GIA fire metric is modeled using a single point light source; however, there are lots of lights in modern jewelry stores (e.g., multiple light sources in display windows, rows of spot lights above or within jewelry cases plus fluorescent tubes on the ceiling), and many lights in the GIA common viewing environment. Doesn’t this difference create a discrepancy?

A model is a mathematical approximation of reality—not a duplication—that is useful for a particular application. For our research, the usefulness was judged by how well the model’s predictions matched observations of actual diamonds in these environments. In this case, the fire metric uses one light and many observer positions, while the observation data were gatheredusing many lights and single observers who rocked the stone while observing it. We used early observations to choose the best fire metric, and found that later observations matched the model’s predictions well.''

That reads too much to me like finding the lighting environment that matched their preconceived model vs. finding a model that matches the real world.

This is a problem I have with all the computer simulation based systems.

see: http://www.gemology.ru/cut/english/model.htm

1. Illumination sources.
The illumination model involves a set of up to 100 virtual Lambertian sources of white light. Each of the sources has its own position and angular size. To determine the fire of a diamond, 60 randomly positioned light sources are used. The angular size of these sources is small (2-10°) - to model "Chandelier" light source. To model diffuse illumination, from 2 to 5 separate sources are used, each of these having an angular size of 20-40° - light source "Office" - type. The sources are located at a semi-sphere, from which the contour of the observer is excluded. The positions of the sources are selected so that they do not illuminate the pavilion of the diamond when the latter is inclined by an angle of no more than 30°. The software is capable of modeling not only Lambertian sources, but also light sources with non-uniform directional patterns.

 

[strmrdr]

Garry interesting, that would explain a lot of this:

"Finally, there are a variety of proportions that yield these patterns, some of which cause the diamond to appear darker to many observers. When a diamond had such a dark appearance, even though the pattern was considered a very good representation of a diamond with “optical symmetry,” many observers did not place it in the top category. This has been accounted for in our system."

Also we all know that perfect optical symetry cant overcome the limitations of the main angles to a large extent.
It can help but its not going to help much if the P. angle is 42 with a 34 degree crown.

 

[strmrdr]

Serg,
Where in the real world do those exact light conditions exist?
What viewing position does the person have to be in and how often do real people look at a diamond that way?

 

[Serg]

Date: 3/26/2005 10

4:23 AM
Author: strmrdr
Serg,
Where in the real world do those exact light conditions exist?
What viewing position does the person have to be in and how often do real people look at a diamond that way?

Strmrdr,

Please describe real world. After it will done by you please verify our light condition.

 

[strmrdr]

Date: 3/26/2005 12:51:16 PM
Author: Serg

Date: 3/26/2005 10

4:23 AM

Author: strmrdr

Serg,

Where in the real world do those exact light conditions exist?

What viewing position does the person have to be in and how often do real people look at a diamond that way?

Strmrdr,

Please describe real world. After it will done by you please verify our light condition.

Serg,
What I was saying was where do I have to go to view the diamond on my finger under those light conditions and how shall I hold my hand?

If im correct that is impossible because those light conditions only exist in your program.

Im not trying to pick on you specifically but its a problem I have with the computer model based approach.
Id feel better if a study was done on how people actually view their rings and under what lighting.
Then basing a computer model on that lighting and position.
I think you would find that tilt plays a much bigger roll when the diamond is on the finger.
So the most ideal cut may not be the brightest strait on but one that handles real world conditions and angles better.
But I dont know for sure because I havent seen the study results despite asking everyone that I can that is using either simulated or "computer controlled light box" lighting conditions to grade diamond performance if they have the results of such a study and no one has provided one or even said they did one.
Maybe im just dont get it but I would think that in order to study diamond performance you would first have to study how they are viewed in their most important position.

On the finger :}

Just my 2c and no disrespect meant to you Serg.

 

[Garry H (Cut Nut)]

Storm you play devils advocate too hard sometimes.

Surely you are aware that DiamCalc ''looks'' at the diamond as it is rocked through 0 degrees, with two eyes, as well as mono.
What you may not know is that it is a knights move too - not a straight rocking.
Check mate

 

[valeria101]

.

 

[strmrdr]

Date: 3/27/2005 12:54:41 AM
Author: Garry H (Cut Nut)
Storm you play devils advocate too hard sometimes.


Surely you are aware that DiamCalc 'looks' at the diamond as it is rocked through 0 degrees, with two eyes, as well as mono.

What you may not know is that it is a knights move too - not a straight rocking.

Check mate

Im aware of that but it still doesnt solve the fundamental problem of relating DiamCalc performance to on the finger performance.
Rocking a stone 30 degrees or other fancy moves while a good start isnt all there is too it.
I will ask again for the results of a study of the lighting and position of a diamond on a typical persons finger under typical for the sake of simplicity office lighting.
Or if anyone knows of such a study being done?


PS. my question applies to all the other systems also both calculation based and light box and camera based systems not just DiamCalc.

 

[Garry H (Cut Nut)]

Date: 3/27/2005 5:42:59 AM
Author: strmrdr

Im aware of that but it still doesnt solve the fundamental problem of relating DiamCalc performance to on the finger performance.
Rocking a stone 30 degrees or other fancy moves while a good start isnt all there is too it.
I will ask again for the results of a study of the lighting and position of a diamond on a typical persons finger under typical for the sake of simplicity office lighting.
Or if anyone knows of such a study being done?


PS. my question applies to all the other systems also both calculation based and light box and camera based systems not just DiamCalc.

Storm lets ask you to design a good environment? Sergey is no fool and has done his best. If you can do better I am sure he will be happy to install it in DiamCalc. (you have DiamCalc dont you?)

BTW my daughter did a small survey about 4 years ago. She found what people wore -dark : light and how close they held the stone added more variability thatn anything else.

On another issue: there was some discussion a while back about dominant eye vs stereoscopic views. GIA wrote this:

Second, observers were not asked to close one eye. In fact, as part of the set of dome observations, some large domes were built with holes up to three inches in diameter, in which observers could look with both eyes; these gave the same observation results as smaller domes with the same configuration. These tests confirmed what a vision specialist told us—that when looking at most small objects, a dominant eye takes control.

I would like to ask all you h&A''s junkies to hold your rings very still, and look closely at your stars that you can all see very clearly. Then close one eye at a time and see if you can still see the star with either eye, or only with two?

 

[strmrdr]

Garry,
I have a lot of respect for Serg and think his work is better than anyone elses that Iv heard of.
But its not perfect and neither are the other systems.
Some of them in my opinion are close enough to be usable including DiamCalc.
I cant afford a copy for myself.
........
Re. dominant eye:
I was saying the exact same thing as GIA a few weeks ago.
Its what is taught on the issue and verified by experiments in the class I took.
I wish I still had my class notes from that class.

I can see the arrows on my diamond under the right light conditions with both eyes and either eye by itself.
Im strongly right eye dominant.
I will disagree with them a little in someone who is just mildly one eye dominant they would see some difference in my opinion/experence.