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AN ANALYSIS OF INACCURACIES IN THE HOLLOWAY CUT ADVISER (HCA)

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dymonite

Rough_Rock
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AN ANALYSIS OF INACCURACIES IN THE HOLLOWAY CUT ADVISER (HCA)

Authors: Derek Louey

Study Type: Qualitative

Aim:

To determine the cause of discrepancies between HCA reports and actual diamond appraisals for HCA scores > 2

History:

Various cut-grading systems have been developed to assist buyers in evaluating diamond quality. These systems are still based on deviationsfrom a single set of proportions defined by Tolkowsky more than 80 yearsago. However, recent research from the GIA and MSU has cast significant doubts on the validity of existing systems in commercial use and have clearly shown that there are multiple sets of equally ideal proportions that have not been previously considered. The HCA is a grading system that has incorporated these findings to achieve a more accurate estimation of diamond appearance. The author states that HCA is an accurate guide to selection in at least 80% of reasonable symmetric diamonds. Possible reasons for inaccuracy include asymmetry and minor facet proportion variations. Other factors may be due to inter-individual variation in subjective perception and viewing diamonds
that are illuminated from below the plane of the girdle. This study attempts to analyze in further detail the factors that impact on diamond appraisal.

Method:

A poll will be taken on an internet-based open discussion group, Rocky Talky diamond forum on Pricescope. Requests will be made for subjective evaluations on diamonds that participants believe have been given wrong scores by HCA.

Garry Holloway (the author of the HCA) or other participants will make available Ideal-Scope and GemAdviser files of the diamond utilising DiamCalc (a proprietary 3-D imaging program) accompanied by any relevant comments.

Participants on Rocky Talky will be invited to provide useful comments on the reason for the discrepancy in the examples given.


The following information should be provided by the submitter:

Proportion data
Full HCA evaluation including light return, brilliance and fire (as per https://www.pricescope.com/cutadviser.asp website)

Name of submitter
Including brief outline of credentials e.g. consumer, professional appraiser, jeweller, gem-cutter, amateur gemologist, appraiser's personal assessment etc.

Opinion of the diamond
Utilizing the terminology of HCA (e.g. excellent, very good, good, fair, poor), if possible Ideal-Scope photo, full Sarin report (including symmetry details where available) and certificate gradings.


Period of study:

Indefinite

Declaration of interest:

As above




:wavey:
 
Dymonite...

I am not trying to combative, but in the spirit of academic style information I was little confused about your post above, as to who should be acknowledged for the design of the proposal to audit HCA results.


Your post mentions Derek Louey as being the author. Are you and Derek L. one and the same.

Are you also a diamond seller in Australia with Joglia Diamonds ?

Just thought it would be more credible and meaningful for readers to know these facts, if they are true.


While I do not do HCA analysis for many reasons, I do have a lengthy and comprehensive database of actual proportions of the stones that come in here for testing. If this would be a help - then let me know.

Rockdoc
 
Hi RockDoc,

Reasonable questions. Yes, I write under the same pseudonym here as in Diamond Talk. I have no affiliation with the trade. I bought my one diamond from Jogia - that's all.

Here is an excert of a letter I posted to DT to indicate my position:

'I have no connection to the gemmological industry and have no imminent plans to do so. My interest in diamond research only began 4 months ago when I was looking for an engagement ring.

My initial intention was only to educate myself a little, prior to making a clearly expensive purchase. In the course of my research, I became aware of Garry Holloway's work on the HCA and his ideal-scope. I was surprised at the effort made in the area of diamond cut theory but was also quite drawn and intrigued by it all.

As I have alluded to, I am medical specialist in Emergency Medicine living in Australia. Although I have very little use for it in my profession, mathematics and physics have always fascinated me. In addition to this, my training has given me a fairly solid grounding in human physiology. I think it is obvious to see why research such as yours holds such interest for me.

Unfortunately, the level of knowledge amongst jewellers in my state is sadly lacking. To illustrate, none even knew what a Sarin machine was, let alone a Fire-scope. The best that I have seen demonstrated was a Hearts and Arrows viewer showing a perfectly symmetrical stone but appalling light leakage on ideal-scope. Apart from being provided the two GIA articles on brilliance and fire (kindly copied by the person who eventually sold me the diamond), DT is one of the few places where I can obtain answers in this dynamic area. I have visited your web-site on a few occasions but have been unable to read all the information there. I have to admit that the layout makes it a bit hard for a first-time novice such as myself to navigate. The articles do not seem to be arranged in a progressive manner that starting from basic principles and moving up to advanced. This is probably why I have ended up asking some fundamental questions on DT which have already been addressed on your site.

Nevertheless, with the information I was able to glean, I ended up obtaining my diamond using an ideal-scope in combination with the HCA. I was satisfied that these two tools were significantly useful for me to intelligently purchase a good stone. Without the benefit of these two objective assessments, I was doubtful that I had sufficient experience of gem appraisal to immediately identify an excellent diamond by sight. Nevertheless, I continue to be interested in knowing both their capabilities as well as their limitations.

As much as I am interested in the science, I also a realist and wish to take the perspective of a lay-person. Through my endeavours, I want to achieve an adequate degree of knowledge and ability to evaluate diamonds in a confident and meaningful manner. I would also hope that others would also benefit from what I have learnt.'




My poll rather than study was provoked out of academic interest. I stand no profit. I did correspond with Garry Holloway to see if he was willing to assist with this idea.


Nevertheless, your data would be invaluable. If you do not wish to present the HCA score - I can always do that with the information you provide.

Sincere Regards,


Derek Louey
:))
 
Hi Derek

Thanks for your reply. I understand now what it is you want to do.

Yes, gemology and valuation sciences are fascinating. And YES you are extremely correct in stating the level of gemological expertise is sadly lacking. In fact, that's probably an understatement.

What needs to be realized however is that consumer totally misunderstand the relevance of proportiion to actual appearance. Many believe that if the numbers are "right" then the stone will automatically be a great stone. This absolutely is not true. While it is "likely" to be a nice stone, the hair splitting separations of analyzing the appearance is still most accurately done with the trained and experienced eye. Technology is making great strides in assisting the evaluation of cut, as well as light return - but there is still a long way to go.

This is the primary reason that I oppose consumers getting information based on only calculation and formula. Helpful yes, but barely.

Facet placement in relationship to each other is also vital particularly in the fancy shape stones. A couple of hundreths of a millimeter in the length of the facet has serious result in the light return of a stone. It is NOT just the averaged angles and analysis needs to consider the variance between all the facets types on a stone, not just the table, crown mains, and pavilion mains.

Also consider that we really don't have an excellent cut grading standard yet.
Garry's standard of rating favors shallow stones, and none of the cut grading systems prefer this type of stone to others, so in that sense, the HCA's rating conclusions are actually askew.

There are a myriad of acceptable proportions, but in the final conclusions consumers expect that good proportions equal good light performance, and the fact is - THIS IS NOT TRUE.

The reason I don't use the HCA is because it severely lacks the analysis of many important characterstics of the cutting. It is a start, and I understand Garry and Moscow will be enhancing it, but before its really complete - I think it will be a significant amount of time. THE HCA IS BETTER than not having anything to help with the blind selection of the a stone offered for sale sight unseen, but sight unseen just doesn't fly in the sky of accurate evaluation and conclusion.

It takes YEARS to really understand the details of the metamorhasis of the manufacture (cutting) of diamonds taking a rough rock and transforming it to a work of art and beauty. Knowledge of cutting details and analysis of the rough diamond is in itself a lifetime study, and very very few members of the retail community understand these factors.

Yes, you're right ... it's sad, but its equally as sad once you begin to learn all the factors of this, why the major labs don't report the negatives, of a stone in a method whereby ANY reader of their reports can be duly informed. Hopefully, this will take place in the future, but for the moment its being ignored in their reports, and as such do a huge injustice to consumers making or trying to make prudent purchasing decisions.

Again, thanks for your reply, and I'll certainly try to help with your undertaking. However, for some stones where the proportions of the 40 mystery facets and proprietary information, these "numbers" I can't always disclose.

Regards

Rockdoc
 
Dear RockDoc,

Thank you for your comments and your co-operation. I agree that numbers should not outweigh the naked eye appraisal of a diamond. I am aware of the ongoing discussions on this subject but I do not wish this thread to be the forum for this. I only see the HCA as a means of ensuring consumers are not likely to purchase 'lousy, ugly diamond' rather than hair-splitting which are the best ones. Given the GIA study's finding that about 3%% are cut well (and my personal experience that this is probably true), I see the tools such as HCA and the ideal-scope a means to prevent consumers getting a 'lemon' before a formal evaluation.

As you can see from the design of the study, I am primarily interested to see how often the HCA incorrectly excludes poor performers (throwing out the baby with the bathwater). I would never dispute (and I am sure that Garry would say this) that a diamond receiving a excellent HCA score (the remaining 3% of diamonds) should still need to be personally evaluated. We could always perform a separate poll later to see how often the HCA overrates a bad diamond.

:))
 
Let's get down to business. How about I kick off the poll?
- it doesn't quite meet the requirements i.e. HCA 1.1 which is < 2 but it might give an idea about what I am after.

This is the diamond I got my fiance.

0.75 ct

5.98-6.03 x 3.52
Table 61.5%
Crown angle 32.7
Pav angle 40.9
Culet none

I gather this is what one would consider a shallow stone and traditionally largish table.

HCA 1.1
LR - exc
Fire - exc
Spread - exc
BUT
Scintillation - vgood

I see good arrows pattern on the stone (looks symmetric)and not too much dark under ideal scope and virtually no total leak except right in the middle at the culet.

Under naked eye, the scintillation and brilliance are exceptional. Why is it slightly penalised for this on HCA?
 
Funny you guys are discussing this. Rock I haven't had a chance to read your article on the HCA but I've been collecting graphics for a commentary on the HCA myself discussing the pros and cons of it since many on the forums are directed to it and use it. From what you've written here I am pretty much in agreement with what you've said. It's better than nothing at all but there are many factors it is not taking into account which affect BOTH appearance and value.

dymonite, there are not many people who can intelligently comment on the HCA since most jewelers or appraisers do not use FireScope/LightScope/IdealScope technology in the diamond analysis. If you have any specific questions please feel free to email me and I can point you to various examples that demonstrate both the strengths and weakness' of the HCA and it's development according to Gary's understanding and interpretation of FireScope images insofar as I understand it from his statements. I'm off the upcoming week so I'll have a little time to play on the boards. :)

Peace,
Rhino
 
Let's get down to business. How about I kick off the poll?
- it doesn't quite meet the requirements i.e. HCA 1.1 which is < 2 but it might give an idea about what I am after.

This is the diamond I got my fiance.

0.75 ct

5.98-6.03 x 3.52
Table 61.5%
Crown angle 32.7
Pav angle 40.9
Culet none

I gather this is what one would consider a shallow stone and traditionally largish table.

HCA 1.1
LR - exc
Fire - exc
Spread - exc
BUT
Scintillation - vgood

I see good arrows pattern on the stone (looks symmetric)and not too much dark under ideal scope and virtually no total leak except right in the middle at the culet.

Under naked eye, the scintillation and brilliance are exceptional. Why is it slightly penalised for this on HCA?
 
Here is a DiamCalc cut quality and Ideal-scope of a symmetrical version of D's stone with 82% lower girdles and 50% stars.
I will ask Leonid how to make a GemFile :)

Sergey also scores the stone a little low on contrast which is a part of scintillation.
The larger table gets you a bigger spread (6% or .05ct compared to a tolkowsky diamond with a 53% table and medium girdle - see the lower right side of the image) but reduces the amount of scintillation because you get less interplay betwen crown and pavilion facets. See my comments on diamond-cut.com.au about FIC's.

Dym stone.jpg
 
Idealscope image varies somewhat from what I saw in reality.

In your image:

The partial leak of the arrow shafts seems more marked. The black trianges between arrow heads larger and more prominent
Partial table leak is more apparent.
The small polygonal culet leak (that I saw) is not apparent


Gemadviser's LR/light leak also score less for brilliance than HCA excellent as you say.

Any comments?
 
serg first attempt.

DimoniteDiamond0_75.gif
 
Serg's image is made with longer star facets and shorter lower girdle facets.

Pretty interesting... I can see the need for recording these two features in order to really have an accurate diamond grading report.

It's kind of funny considering that GIA doesn't even report crown height or pavilion depth. They're beginning to look really archaic. They better come up with something fast or they're going to get lost in the dust.

Rich, GG
Sarasota Gemological Laboratory
 
Here is the GemAdviser file :)

Sergey is that an Ideal-Scope or Firescope?
You have given the stone unusually short lower girdles and fairly long star facets. Why?

BTW folks, 1= Tolkowsky stone, can not remember if I mentioned it?.
 

Attachments

So if 1=Tolowsky ideal, what does 1.1 equal? An excellent T range stone? I suppose on DiamCalc it is possible for certain characteristics of a stone to score highly in the 1 range and lower in other areas...exhibiting Tolowsky ideal characteristics only in certain areas?
 
You are on the money Mara.
Tolkowsky choose a set of proportions he calculated (or estimated) to have good (not best) light return and good (not best) fire.

That is what this page is all about.
http://www.diamond-cut.com.au/23_bicfic.htm
Here is the copy.
The link has the pictures.

Brilliant Ideal and Firey Ideal Cuts
Seeing is believing
The cut adviser system recognizes three distinct types of diamonds with optimum beauty. To qualify as an HCA ‘Ideal Cut’ a diamond must rate as excellent or score less than 2.0.

Brilliant Ideal Cuts (BIC) have the highest light return and larger spreads. Fiery Ideal Cuts (FIC) have more fire or spectral color and appear to have more facets and scintillation because of the increased interaction between the steeper crown facets that direct your view deeper and further into the pavilion.

Those two factors mean lead some people to feel FIC’s have a ‘romantic’ look, even though they display slightly less brilliance. But because they are smaller than BIC’s, but when spread and cost are considered many consumers trade beauty for desirability and switch to TIC’s or BIC’s.

The Tolkowsky Ideal Cuts (TIC) combine a balance of fire and brilliance, but diamonds close to Tolkowsky proportions command a price premium. So again when given the choice it has been my experience that BIC’s have the most demand.

This is rather fortuitous because the yields on BIC are better than the other two types of ideal cuts. In fact FIC’s are few and far between.

Category Crown angles (HCA score must be <2.0)
BIC Brilliant Ideal Cut Crown angle is less than 32.5°
TIC Tolkowsky Ideal Cut Crown angles between 32.5° and 35.5°
FIC Firey Ideal Cut Crown angle is more than 35.5°



In the representation the oval shows the approximate range of each classification. Separating diamonds graded as excellent into these sub groupings will assist cutters and dealers in meeting the demands of different markets and will assist in establishing pricing information.
 
Serg,

Is this the using the same proportion data as in the previous gemadviser pic sent by Garry but with different minor facet percentages?

If so, this suggests significant effects on appearance with these changes.

However, the appearance still does not resemble my observed image - I had this culet leak right in the centre but basically nowhere else.
 
----------------
On 1/7/2003 1:49:16 AM

Serg,

Is this the using the same proportion data as in the previous gemadviser pic sent by Garry but with different minor facet percentages?

If so, this suggests significant effects on appearance with these changes.

However, the appearance still does not resemble my observed image - I had this culet leak right in the centre but basically nowhere else.
----------------

I use your data. I change Square deviation, Star facets, Lower Facets,..., Gamma, Common Lighting brightness.
I think your data is conflicting data.
Can you give full Sarin or OGI report?

P/s
Garry,
Did you check the avg. Pav and Crown angles?
 
Derek you said no culet. The stone must have a culet.

If you can get it to me I can run a full 3D scan which will produce a very accurate Ideal-scope image.

I cant see how this 'study' is going to be a benefit?
 
I cant see how this 'study' is going to be a benefit too.
 
----------------
I cant see how this 'study' is going to be a benefit?
----------------


Let's put "study" in strong quotes. A poll such as this on an unmoderated internet discussion forum has no scientific validity whatsoever.
 
----------------
On 1/8/2003 2:04:36 PM


----------------
I cant see how this 'study' is going to be a benefit?
----------------


Let's put "study" in strong quotes. A poll such as this on an unmoderated internet discussion forum has no scientific validity whatsoever.
----------------

Moderator is not necessary for science. And the presence of moderator does not guarantee science.
 
Lawgem, Serg, Garry,

You are all right in saying that what is being done here is not a proper study in the way that we traditionally view the scientific method.

I know I am not trying to propose a hypothesis that we can prove by repeated experimentation. At the same time, I was hoping to see that by sampling many diamonds with HCA scores > 2, whether we see patterns emerging about the reason why discrepancies may occur. Perhaps we will gain insight into how we perceive diamonds in real life.

For instance, I am not sure if other members of this discussion group were aware how markedly ideal-scope images/gemadviser results can vary by varying minor facet proportions. I was always under the impression that these generally would have lesser effects on appearance. Perhaps, people might offer theories on how we can predict the impact of these changes. i.e. what minor facet lengths are generally recommended, in which proportions etc. My diamond looks subjectively better under my ideal-scope than the first gemadiviser image - that surprised me. I wanted to know if other people experienced the same thing

Note that HCA uses a constant set of minor facet proportions for each of these diamonds. If accounted for, what would the error of the HCA scores vary from? +/- 0.5? +/- 1? +/- 2?

This would be interesting to know.

However, I acknowledge your concerns about the lack of moderator. I was hoping that as the poll progessed, the 'smart people' here - Garry, Serg etc might also give some direction about how to most fruitfully utilise the information and proffer insights to stimulate discusssion and thought.

Regards all
 
----------------
On 1/8/2003 7:21:15 PM



For instance, I am not sure if other members of this discussion group were aware how markedly ideal-scope images/gemadviser results can vary by varying minor facet proportions. I was always under the impression that these generally would have lesser effects on appearance. Perhaps, people might offer theories on how we can predict the impact of these changes.
----------------


The minor facet is not main reason.
I think the main reason is HUMAN Garry mistake. He used Pav. angl=41.4. See Garry file.
Exist some other reasones.
 
I think you are correct Sergey :(
 
.
 
----------------
On 1/9/2003 4:37:56 AM

I think you are correct Sergey :(
----------------

I think you owe me a bottle of very good Shiraz.
:angryfire:
 
This is the image I get for Dymo's stone when using the DiamondCalc default length for star & lower girdle facets.

DymoniteII.jpg
 
OK
Cme here and get it :)
 
Hi Richard,

That looks a bit more like it.

There were small pieshaped wedges around the culet which flashed on/off under the ideal-scope when I tilted the diamond.

Have you got the accompanying gemadviser report?
 
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