How important are HCA factor grades in assessing diamonds?

HCA only takes in 4 inputs, 3 of which are average and rounded somewhat, that accounts for 17 facets, the table, crown and pavilion main facets, out of 57 facets. So a rough outline and takes no account of minor facets or the positioning of the facets.

Thus IS/ASET images are more useful.

The HCA score is like drawing a chalk outline of a person. Grading report numbers are like knowing their height, weight and clothing measurements. ASET (all shapes) or Ideal-Scope (RB) is like looking at the person's photo. A good 3D scan can add more insight, depending on software and operator experience. But going on a live date is the only way to judge total personality.

I've done a little checking on this lately myself. I picked an EX cut grade round that I feel is one of the nicest I've seen lately. It scored over 5 on the HCA

It might give you the stone Garry likes best- but it offers no useful information for many people who's taste might be different. For those people with differeing taste, the ratings provided may be extremely misleading.

Thank you for your explanations, Stone and John. You both are always so helpful. Thanks for your patience with my newbie questions.

John, you are really good at explaining things in a way that is not only easy to understand, and also entertaining.

I realize it's always best to look with your own eyes after checking everything John talked about (grading numbers, ASET, Idealscope, 3-D scan if possible). But for the sake of discussion, let's say that seeing a diamond in person isn't possible before a purchase. If you were trying to choose between two stones, sight unseen, and the ASET and Idealscope on both stones is good, would you give the edge to one that scored EX, EX, EX in Fire, Scintillation, and Light Return over one that scored VG, VG, and VG in those same factors in the HCA?

by John Pollard » 08 Oct 2010 11:37
The HCA score is like drawing a chalk outline of a person. Grading report numbers are like knowing their height, weight and clothing measurements. ASET (all shapes) or Ideal-Scope (RB) is like looking at the person's photo. A good 3D scan can add more insight, depending on software and operator experience. But going on a live date is the only way to judge total personality.

John, That was SOOOOOO helpful for me. Thanks for making it easier to understand.

I was glad this question was asked, as I really have been doing some comparisons with live stones.

The stone scoring over 5 was particularly telling to me- as it was also GIA triple EX- and looked amazing in person. Particularly with regards to Fire and Scintillation. I'd strongly suggest looking at some stones in person- and running them through hca to see if it matches what your ideas of fire and scintillation are.
Things like LGF% will also make an easily perceptible difference
My feeling is that short of looking in person, photos and videos are the most useful in determining what a person likes.

RD do you have a video of the diamond?

Written by slg47:RD do you have a video of the diamond?

This is the stone.

http://rockdiamond.com/index.php/jewelry/loose-diamond-436ct-round-brilliant-h--vs1-gia-stunning-stone-r3558

The largish lgf results in not much leakage. Note that the stone is placed too deep into the ASET.

stone, would you say that the ASET for the "H&A" diamond is better on the video? It looks to exhibit the 'ring of death' to me and also both ASETs seem tilted, or off in some way?

RD is really comfortable in his skin, and feels comfortable going out on a ledge. I've read here casually a long time, and from having done so, do believe in the reasonableness of the science embedded in the HCA results. Fortunately, SC is a smart guy, and provides useful cover.

Otherwise, I have little clinical experience, did make my wife's purchase selection 5.5 years ago based on the HCA alone, and I like this question, and look forward to hearing comments for it...

Written by blueiris:Thank you for your explanations, Stone and John. You both are always so helpful. Thanks for your patience with my newbie questions.

John, you are really good at explaining things in a way that is not only easy to understand, and also entertaining.

I realize it's always best to look with your own eyes after checking everything John talked about (grading numbers, ASET, Idealscope, 3-D scan if possible). But for the sake of discussion, let's say that seeing a diamond in person isn't possible before a purchase. If you were trying to choose between two stones, sight unseen, and the ASET and Idealscope on both stones is good, would you give the edge to one that scored EX, EX, EX in Fire, Scintillation, and Light Return over one that scored VG, VG, and VG in those same factors in the HCA?

What RD posted is a 60/60 stone. Stones like this can have excellent IS and ASET images. As a reminder, ASET and IS trump the HCA score. I believe it is possible for some 60/60 stones to score high on the HCA, so the fact that a stone is a 60/60 cut does not automatically mean it will not score high on the HCA. Conversely, some ideal cut stones with high HCA scores may not have great ASET or IS images. A wise consumer gathers as much information as possible.

60/60 stones are indeed very *bright* stones, as David (RD) notes. I've seen many in person; your average jewelry store will carry them, and you will see many of them on women who were engaged from the 1960s - 1990s. I do not prefer them as they tend to have a large amount of table reflection which, to me, just looks like glare. So it is a preference.

See this article written by John Pollard for more information
[url]http://www.pricescope.com/journal/laboratory_cut_grades_what_report_doesn’t_show[/url]
on different cuts and their respective visual appearances. Note diamond #4, a 60/60 cut with an excellent IS image.

See this article by Karl for more information on how longer lgfs can minimize light leakage
http://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

If you are shopping for a well-cut round brilliant, you may prefer the more balanced look of an ideal-cut diamond or you may prefer the bright, spready look of a 60/60, that is for your own eye and preference to determine. But preferring one "style" over another does not mean that you need to sacrifice good cut, and it does not mean that you should forego an ASET or an IS or allow a vendor to tell you that those objective tools don't matter.

Written by slg47:stone, would you say that the ASET for the "H&A" diamond is better on the video? It looks to exhibit the 'ring of death' to me and also both ASETs seem tilted, or off in some way?

Yap, tilted stones, but if adjusted, the ring of leakage is not very serious for the H&A stone.

Thanks all.
I have looked at a lot of diamonds- well cut diamonds, of many different proportion sets.
The stone in question performs equally as well or outperforms the best "near tolk" under many varied lighting environments- and
of course this is my opinion.
However that same can be said of HCA- or interpretation of any aset image.

Thank you Ira- I am comfortable stating what I see.
As in other discussions, its important to add context.
HCA is virtually unheard of outside PS.
The buyers for the finest stores you can name don;'t use it.
That, in itself does not invalidate it.
But the reference to it's "scientific nature" are misplaced IMO

I had been looking for a while- for what I considered to be "the ideal 60/60".
This is it.
Many of the other 60/60's I have seen recently- including the one used for comparison- lacked the look I prefer.
What I like is: non patterned, large spread, with incredibly bright visuals.
The stone is no longer available for sale on the site, so hopefully we can use it for comparison


eta- it's important ( for me) to state my motivation here. I strongly believe that consumers are better served having context to allow a better understanding of the tools, and how they are used, interpreted, and viewed by experts who do not post on PS.

I don;t have some secret stash of 60/60's I'm looking to sell- but I do wish that more cutters would offer such stones.
I can ( and do) stand in a protected position in that GIA ( not me) called the stone in question EXCELLENT in Polish, Symmetry, AND cut grade

Written by Regular:RD is really comfortable in his skin, and feels comfortable going out on a ledge.

Well that stone really is on the ledge!

At the very ledge of GIA Ex grade, if the average pavilion angles were 0.1 degree steeper it would get a cut grade deduction.
AGS 2-3 for light performance or lower if graded by AGSL.

This stone really highlights the weakness in the width of the GIA cut grade and the reason why averaging eight pavilion angles really can be inaccurate.

Its not just Garry's HCA that would penalize this stone, AGSL would and the supplier discounted it.

Not exactly a good example highlighting a weakness in HCA.
Let us call something cut to save precious weight what it is, don't call an apple an orange.

HCA score advises caution and it was the correct call.

Take home message though:

Use HCA for rejection and to select the most promising stones.
Use ASET and IS to confirm then your eyes for final selection
Don't worry about outliers, save your time, shipping and insurance costs, you don't need to find every stone you find beautiful, you only need to find one stone you find beautiful.

All due respect ccl- you have NO idea as to how or why the stone was cut as it was.
Generally, stones that are on the cusp weight wise- such as a 4.01, might be attributed to weight savings.
But the stone in question was far from that.

Also- using AGSL parameters can open an entirely different can of worms.
Both GIA and AGS are brands. GIA and AGSL offer different products- and arrive at cut grades differently.
Which do you prefer?
Sounds like AGSL.
However many in the trade prefer GIA.
Please, before we go to questioning their motivation, many of the people I'm familiar with, and speaking of are incredibly concerned with quality of cut.


In terms of describing this stone as "just making the grade" as a way of saying it's not well cut, I disagree.
GIA's cut grade is like a plateau.
Unlike a an D colored diamond that really should have been E due to a soft grading on a particular day, GIA's cut grade slopes very gently at it's borders.
There are going to be cases of VG cut graded stones many of us will prefer to other EX cut graded stones

The stone in question was amazing. Incredibly well cut- again- GIA made the grade, not I.

eta- the stone was not discounted- it was offered as a full price "triple ex"- you're not going to get far trying to get a cutter to reduce their price based on HCA......

Written by ChunkyCushionLover:

Written by Regular:RD is really comfortable in his skin, and feels comfortable going out on a ledge.

Well that stone really is on the ledge!

At the very ledge of GIA Ex grade, if the average pavilion angles were 0.1 degree steeper it would get a cut grade deduction.
AGS 2-3 for light performance or lower if graded by AGSL.

This stone really highlights the weakness in the width of the GIA cut grade and the reason why averaging eight pavilion angles really can be inaccurate.

Its not just Garry's HCA that would penalize this stone, AGSL would and the supplier discounted it.

Not exactly a good example highlighting a weakness in HCA.
Let us call something cut to save precious weight what it is, don't call an apple an orange.

HCA score advises caution and it was the correct call.

Take home message though:

Use HCA for rejection and to select the most promising stones.
Use ASET and IS to confirm then your eyes for final selection
Don't worry about outliers, save your time, shipping and insurance costs, you don't need to find every stone you find beautiful, you only need to find one stone you find beautiful.

Play nice CCL

David the sone is on a cusp that many experts would say is a little generous. But not a bad stone. I agree HCA is a little tough.
The stone has been helped by the long lower girdles but probably lacks a little contrast.
The spread is about 4% better so it appears as a 2.5ct.

The chart is from http://www.octonus.ru/oct/mss/gia-agspgs.phtml

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Written by Garry:The stone has been helped by the long lower girdles but probably lacks a little contrast.

It also lacks the balance of virtual facet sizes that make a well cut RB an all around performer in a wide range of lighting.
There is no free lunch and making that deep a pavilion work has some significant trade offs.

Written by Anne:John, That was SOOOOOO helpful for me. Thanks for making it easier to understand.

Blueiris and Anne, you're welcome. And the feedback is appreciated.

Written by blueiris:John, you are really good at explaining things in a way that is not only easy to understand, and also entertaining.
I realize it's always best to look with your own eyes after checking everything John talked about (grading numbers, ASET, Idealscope, 3-D scan if possible). But for the sake of discussion, let's say that seeing a diamond in person isn't possible before a purchase. If you were trying to choose between two stones, sight unseen, and the ASET and Idealscope on both stones is good, would you give the edge to one that scored EX, EX, EX in Fire, Scintillation, and Light Return over one that scored VG, VG, and VG in those same factors in the HCA?

I'd want to know more. With experience the measurements can be correlated with ASET or Ideal-Scope to make some valid determinations about fire and scintillation... I will say that, while based on intuitive principals, the HCA's fire and scintillation predictions are less robust than the numerical score returned (0-2, etc). That score is based on the laws of physics and reports how effectively the diamond's primary measurements facilitate the critical angle necessary for optimum light return.

Written by Rockdiamond:All due respect ccl- you have NO idea as to how or why the stone was cut as it was.
Generally, stones that are on the cusp weight wise- such as a 4.01, might be attributed to weight savings.
But the stone in question was far from that.

I imagine CCL is correct here: Factories cutting millions of carats per year will preserve weight wherever possible through the total range of production. An explanation may help:

The finished diamond's proportions are 60 table, 60.1 depth, 41.8 pavilion angle, 32.0 crown angle.

In a textbook case rough crystals are sawed along what-will-become the table facets for the finished diamonds. A single crystal will often yield two, or more, diamonds (these are illustrations from an Octonus mapping/planning manual):


After sawing it is likely that the crystal in question had 12% crown height available above the maximum possible girdle diameter. By planning a table of 60% the crown angle became 32.0 degrees. One can easily look up the deepest pavilion angle to accompany that plan and still receive EX... 41.8. Viola. Not a coincidence.

Simply put, it's most likely that this was a well-executed plan to finish at the limit of EX while preserving the most weight. And if not for this stone specifically (crystal could have been odd, makeable or splittable) it was the plan for 99 out of 100 others faced with the same choices.

Another option would have been to cut it right to the performance-center of GIA's chart for 60% table (41.2 PA and 32.8 CA). That would have been the optimum plan on the GIA chart - and would have earned AGS 0 too - but with the available crown height it would have required a 4% reduction in girdle diameter and a whopping 10% reduction in weight... If you project 10% loss of yield (or even 1%) over a million carats of production it's easy to understand why cutting to the deep edge of what's-allowed has become popular.

Written by Rockdiamond:Both GIA and AGS are brands. GIA and AGSL offer different products- and arrive at cut grades differently.
Which do you prefer?
Sounds like AGSL.
However many in the trade prefer GIA.

It's a more profitable preference. Factually, GIA allows a wider set of cutting targets. This represents millions of dollars in profit that would be lost if major manufacturers confined themselves to a stricter set of tolerances. In the upstream business there has been a huge proliferation of diamonds cut with c/p angles at the weight-permissive end of GIA's ranges since that system was introduced in 2006. As a part-time consultant for factories in the world's two busiest cutting centers I say this with great confidence.

Written by Rockdiamond:Please, before we go to questioning their motivation, many of the people I'm familiar with, and speaking of are incredibly concerned with quality of cut.

No argument. Factories, however, are motivated to preserve yield. Diamonds sell. Diamond dust does not. It is simply logical to take the crystal and create a plan which preserves the most weight possible inside the targeted grade.

Written by John:Diamonds sell. Diamond dust does not.

Now how would you cut Diamonds that sell without buying Diamond dust ?

Good to read ya

John's scenario is a possibility of many.

I have another scenario which has nothing to do with yield.

Mass production factories must work efficiently and fast..., cutters have a range of allowed numbers they must attempt to stay within.
A bit more time and details go into cutting 3X's within the pool of products these factories produce.

Reflecting that are boutique type cutting houses which usually cut to completely different systems.
Usually every stone is cut to specifics, and for example on my turf, most Diamonds are started and completed by one cutter alone vs. an assembly line model the majority of the factories work according. This brings other factors rather than just yield. One huge factor is time and patience and knowledge!

Generally there will not be such a big difference in weightloss between cutting an RB to 3X's or 3vg's (a vg's yield will always bea bit higher) as both are cut at very similar main angles. The huge difference is in the small details.

A 3X diamond that is cut in boutique type cutting house can take a few time-folds if comparing the time it takes to complete a 3X stone in the non-boutique type of factory. The number of scans during the process is incomparable resulting in a much longer timeframe but a much more precise cut.

And that is just in short as the details involved are too many to write right now .

Written by DiaGem:John's scenario is a possibility of many.

Definitely. In fact my full-time job is with such a boutique.

Two points.

1. Regardless of "textbook" or not, it was possible to finish this diamond at the center of the GIA EX chart, the optimum target per GIA (and an overlap with AGS0). But it was cut to the limit of the deep border of EX; which added notable weight.



2. Maximizing yield is the rule, not the exception, for operations pouring the most diamonds into the polished pipeline - especially with triple-EXs being sold to China hand-over-fist right now.

Written by DiaGem:And that is just in short as the details involved are too many to write right now .

That is certain.

Really interesting stuff- thank you John, and Yoram.
I do know a bit about the way this stone came to be- it was cut by a "boutique" factory.
In fact this factory almost never cuts colorless diamonds.
That may be why they achieved this - what I consider to be stunning effect.
It's true that there's less contrast than in many near tolks- but that's also a way of saying less patterned. The lack of contrast, to my eye, gives a more even scintillation. I did look at the stone under a wide variety of lighting environments, and it was a STUNNING performer.

Personally, I believe it's a mistake to downgrade stones "at the edge" of GIA's cut grade for that reason alone. Pointing to the PA alone to say it was too deep is simplistic.
The stone was not cut too deep- which is evidenced by it's spread.
The larger table, again my eye- makes the stone look larger than another stone of the same diameter, and smaller table might.

As far as rule number two -about maximizing weight- I'd say a more accurate phrase would be maximizing profits- the goal of any business.
Sometimes that might mean cutting to AGS 0 - other times producing a GIA EX cut grade. This depends on the market- and how stones will be sold.
With regards to saving weight: The stone weighed 4.36cts and spread 10.65-10.70.
This is larger than any stone on the PS database till we get all the way up to 4.43- but there are far more candidates that weigh more, are EX cut grade, and have less spread.
So where is the weight?
eta- search paramters: 4.30-4.75 D-J color IF-SI2 clarity

But a very important point here, from my perspective, is that it's a mistake to generalize and imply that any given stone was cut primarily for weight savings - as opposed to producing the nicest diamond possible from the rough.
This generalization is not fair- but far more important is that it does not assist consumers- many of whom, I am quite sure would choose this proportion set over others that scored closer to the center of GIA EX, which may be why it IS included in GIA EX.

Thanks Sir John and Diagem I feel it is important for experienced trade with experience in rough planning to chime in on discussions like this and present a comprehensive picture, I appreciate your insightful responses.

While not on the same experience level as you both I do have a little more to add.

This is a 4.36 carat diamond, saving even 0.1 carat is hundreds of dollars.
Saving 10% would be thousands.

With such large and precious rough, larger sized diamonds 3CT+ are most often cut with a greater focus on weight retention. This makes sense, shoot for the highest finished stone value and the largest market segment for saleability.

I beleive planning time and cutting skill should be higher when dealing with potentially $250,000 to $400,000 worth of polished diamond (assuming rough yielding two stones as in JP's example). The cutters that have survived are really smart and efficient at maximizing stone value(mostly by weight) while still producing stones that sell quickly.

A stone like this, the assymetry and proportions seems most logically weight saving, no matter what kind of rough it was cut from or the myriad of reasons why it was done. (Avoiding inclusions, avoid color zoning, etc. etc. etc).

The proportions and Med Blue Fluorescence suggest to me it should be discounted over shallower pavilion GIA XXX without the fluoro.

If this stone wasn't discounted to the consumer someone along the supply chain had an opportunity for a better margin and maintains an interest in keeping all GIA 3Xs as equal value.

Written by blueiris:I'm sure this has been asked before and I tried to search but didn't have any luck. Sorry if it's redundant.

I've often read here that the HCA is a rejection tool, not a selection tool. I've also read that a diamond scoring below a 2 on the HCA is worthy of further consideration. I've also read that a higher/lower HCA score is not an indication of a diamond's performance, and that some people may prefer the appearance of a diamond with a higher (or lower) HCA score. And I've often read that imaging like ASET and Idealscope "trumps" the HCA, which leads to my question.

I often check diamonds with the HCA just for fun, to sort of test my ability to guess a diamond's HCA score just from its numbers. I have seen many diamonds score Excellent in Light Return, Excellent in Fire, Excellent in Scintillation, and Very Good in Spread. But I've also noticed that sometimes diamonds with excellent-looking ASET and Idealscope (and H & A images, if applicable), some of them in what is called the "super ideal" category, score lower in the HCA factor grades of Light Return, Fire, Scintillation, and Spread. For instance, one stone I was looking at today scored Very Good in all four categories, yet is a branded, super ideal, hearts and arrows stone. The HCA score is below 2 (1.8, IIRC), so it would be deemed worthy of further consideration - but wouldn't that further consideration be the ASET and Idealscope imaging, which looked good? And if that's case, is that stone not going to be as fiery, scintillating and good in light return as another branded super ideal stone that does score EX, EX, EX in the first three HCA factors?

I am very late to fully read this thread BlueIris. Do you know that all the workings of HCA are at http://www.diamond-cut.com.au which is essentially a 10 year old document.
Regarding the extent you are using the word descriptions you should read and understand how it works.

GIA's system is pretty much a direct copy (IMHO they should never have been granted a patent over mine which predates theirs) and works in much the same way by accessing a database of different factors such as my 4 and gives a final grade (except they do not give you the word answers). I have a desk top that gives my numeric data too.

However any system like this is only as good as the charts they look up. If I had been smart enough to commercialize mine I would do better charts knowing what I know and with the tools I have. But this is not easy work, and if or when I do it, I will do it well.

Written by Rockdiamond: The lack of contrast, to my eye, gives a more even scintillation. I did look at the stone under a wide variety of lighting environments, and it was a STUNNING performer.

Personally, I believe it's a mistake to downgrade stones "at the edge" of GIA's cut grade for that reason alone. Pointing to the PA alone to say it was too deep is simplistic.
The stone was not cut too deep- which is evidenced by it's spread.
The larger table, again my eye- makes the stone look larger than another stone of the same diameter, and smaller table might.

David brilliance is a function of contrast and brightness. A chess board has a lot of contrast but half the light return - yet most people see a chess board as brighter than a sheet of copy paper.

It is a human thing and it is possible for you, your eyes and your experiance that you prefer different things to people who are new to the market - in fact it is VERY likely.
For example I did the GIA's cut grading course immediately after the last GIA Symposium and we asked the instructors (who included some of the cut grade development team because there were a few of us who were rather scary for them in the glass). When asked about strongly patterned vs less so stones they said that when non experianced observers first saw diamonds many were put off by the strong stars patterns. But after looking at more stones - they began to change preference and preferred patterned stones.

The stone was cut too shallow by virtue of the large table size and shallow crown angle. There is -2.3% crown hieght and +1.6% pavilion depth - there is more weight saved in the crown than the pavilion.

20 years ago there would have been more +60% table rounds on the market. There is no doubt that was done because of yeild.

Today for 1-1.19ct D-G VS1-2 GIA graded stones listed on Rapnet(r) this is the spread.
Personally I agree that 57% is the sweet spot between performance and spread.
So David it does seeem the world has marched on.

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Written by ChunkyCushionLover:
The proportions and Med Blue Fluorescence suggest to me it should be discounted over shallower pavilion GIA XXX without the fluoro.

If this stone wasn't discounted to the consumer someone along the supply chain had an opportunity for a better margin and maintains an interest in keeping all GIA 3Xs as equal value.

CCL..., this is a general statement.

It might have not been displayed for sale by some PS vendors due to the numbers, but stating a fact that this stone would/should have been discounted is simply to general.

Its still graded triple Ex (good enough for the majority) which is safe for the majority (including me) and the medium fluo. would probably not affect the appearance in a negative way.
Not all fluo stones are discounted..., and (IMO) if they are discounted, they are probably an added value that consumers doesnt pay for.

Yes..., even strong & very strong blue....

Written by Garry:

Written by blueiris:I'm sure this has been asked before and I tried to search but didn't have any luck. Sorry if it's redundant.

I've often read here that the HCA is a rejection tool, not a selection tool. I've also read that a diamond scoring below a 2 on the HCA is worthy of further consideration. I've also read that a higher/lower HCA score is not an indication of a diamond's performance, and that some people may prefer the appearance of a diamond with a higher (or lower) HCA score. And I've often read that imaging like ASET and Idealscope "trumps" the HCA, which leads to my question.

I often check diamonds with the HCA just for fun, to sort of test my ability to guess a diamond's HCA score just from its numbers. I have seen many diamonds score Excellent in Light Return, Excellent in Fire, Excellent in Scintillation, and Very Good in Spread. But I've also noticed that sometimes diamonds with excellent-looking ASET and Idealscope (and H & A images, if applicable), some of them in what is called the "super ideal" category, score lower in the HCA factor grades of Light Return, Fire, Scintillation, and Spread. For instance, one stone I was looking at today scored Very Good in all four categories, yet is a branded, super ideal, hearts and arrows stone. The HCA score is below 2 (1.8, IIRC), so it would be deemed worthy of further consideration - but wouldn't that further consideration be the ASET and Idealscope imaging, which looked good? And if that's case, is that stone not going to be as fiery, scintillating and good in light return as another branded super ideal stone that does score EX, EX, EX in the first three HCA factors?

I am very late to fully read this thread BlueIris. Do you know that all the workings of HCA are at http://www.diamond-cut.com.au which is essentially a 10 year old document.
Regarding the extent you are using the word descriptions you should read and understand how it works.

GIA's system is pretty much a direct copy (IMHO they should never have been granted a patent over mine which predates theirs) and works in much the same way by accessing a database of different factors such as my 4 and gives a final grade (except they do not give you the word answers). I have a desk top that gives my numeric data too.

Garry, I was really hoping you would be able to add your input, since you developed the HCA, so thanks! I didn't know about the document you linked so thank you also for that link. I have read everything there now.

Written by Garry:However any system like this is only as good as the charts they look up. If I had been smart enough to commercialize mine I would do better charts knowing what I know and with the tools I have. But this is not easy work, and if or when I do it, I will do it well.

I'm trying to understand this: "However any system like this is only as good as the charts they look up. If I had been smart enough to commercialize mine I would do better charts knowing what I know and with the tools I have." I'm sorry if I'm dense, but can you elaborate?

Written by Garry:

Written by Rockdiamond: The lack of contrast, to my eye, gives a more even scintillation. I did look at the stone under a wide variety of lighting environments, and it was a STUNNING performer.

Personally, I believe it's a mistake to downgrade stones "at the edge" of GIA's cut grade for that reason alone. Pointing to the PA alone to say it was too deep is simplistic.
The stone was not cut too deep- which is evidenced by it's spread.
The larger table, again my eye- makes the stone look larger than another stone of the same diameter, and smaller table might.

David brilliance is a function of contrast and brightness. A chess board has a lot of contrast but half the light return - yet most people see a chess board as brighter than a sheet of copy paper.

It is a human thing and it is possible for you, your eyes and your experiance that you prefer different things to people who are new to the market - in fact it is VERY likely.
For example I did the GIA's cut grading course immediately after the last GIA Symposium and we asked the instructors (who included some of the cut grade development team because there were a few of us who were rather scary for them in the glass). When asked about strongly patterned vs less so stones they said that when non experianced observers first saw diamonds many were put off by the strong stars patterns. But after looking at more stones - they began to change preference and preferred patterned stones.

The stone was cut too shallow by virtue of the large table size and shallow crown angle. There is -2.3% crown hieght and +1.6% pavilion depth - there is more weight saved in the crown than the pavilion.

20 years ago there would have been more +60% table rounds on the market. There is no doubt that was done because of yeild.

Today for 1-1.19ct D-G VS1-2 GIA graded stones listed on Rapnet(r) this is the spread.
Personally I agree that 57% is the sweet spot between performance and spread.
So David it does seeem the world has marched on.

Clearly a lot of the current style - which is trying to be "ideal cut" does follow the smaller table mantra.
Stones like the one I'm using for this example are very rare today.
The part about people preferring patterning after seeing it more- and the general issue of taste and what is new, or "old fashioned" was also interesting. The implication is younger observers will go for patterning in higher percentages than older observers.
What would you suppose the average age of the buyers of four carat diamonds?
IN my case, it's really not what I'm used to- I know what I like.
The contrast in this type of stone is more dispersed- this is elemental in the look.

With regards to GIA "borrowing" your methodology: no question you published first- and probably did influence their work. ( they probably should have at least given you some credit)
However the breadth of the GIA EX cut grade is no accident. This is where I must say- with tremendous respect- where the HCA may fall short for a percentage of buyers.
If you read PS at all, you'll see it referred to as a "rejection tool, not a selection tool" frequently. You have made this point clear on many occasions.
However many people comparing two stones ( sight unseen in many cases), simply assume the lower HCA score is a "better" stone.


John- your explanation makes all the sense in the world- and if we look at diamonds in bulk, it likely explains a lot of the poorly cut stones out there- and we all agree there are plenty. But the generalization seems not to work in all cases.
Back to weight retention and, the question of if the 4.36ct was too deep- a question to Garry and John ( again)
If there's extra weight on this stone, how does it manifest itself?
That is to say, given that there's sufficient crown and pavilion angle, and more than ample spread ( in fact excellent spread for the weight)- where is this extra weight?
How much should the diamond have weighed?

Written by Rockdiamond:John- your explanation makes all the sense in the world- and if we look at diamonds in bulk, it likely explains a lot of the poorly cut stones out there- and we all agree there are plenty. But the generalization seems not to work in all cases.
Back to weight retention and, the question of if the 4.36ct was too deep- a question to Garry and John ( again)
If there's extra weight on this stone, how does it manifest itself?
That is to say, given that there's sufficient crown and pavilion angle, and more than ample spread ( in fact excellent spread for the weight)- where is this extra weight?
How much should the diamond have weighed?

I have explained previously in another discussion we had. There is a simple, approximate hand calculation using high sch math. Those with DiamCalc will probably be able to generate one with better accuracy.

Using the parameters from this chart.

Your Example stone,
Stone A. Table 60, Crown 32.0, Pavilion 41.8.

Stone situated in the center of GIA Ex cut for 60% table,
Stone B. Table 60, Crown 32.5, Pavilion 41.2.

To simplify calculation, assuming the diamond is made up of conical shapes, no faceting, volume, V = 1/3 Pi R^2 H,

where, Pi = 3.142, R = base radius of cone, H = height of cone.

Volume of a cylinder, used for the girdle, = Pi R^2 H

This diamond is made up of the crown volume, pavilion volume and girdle volume.

The crown volume is obtained from the subtraction of 2 cones, the crown cone, everything above the crown, and the table cone, everything above the table.

Stone A
Crown cone.
Height of the crown cone = 50 * Tan(32) = 31.24
Volume of the crown cone = 1/3 Pi (50)^2 * 31.24 = 81795.21

Table cone
Height of the table cone = (60/2) * Tan(32) = 18.74
Volume of the table cone = 1/3 Pi (60/2)^2 * 18.74 = 17667.76

Pavilion Cone
Height of the pavilion cone = 50 * Tan(41.80) = 44.71
Volume of the pavilion cone = 1/3 Pi (50)^2 * 44.71 = 117037.83

Girdle
Volume of the girdle = Pi (50)^2 * 3 = 23561.95

Total volume of Stone A = 204727.22
Depth = 60.2%

Stone B
Crown cone.
Height of the crown cone = 50 * Tan(32.5) = 31.85
Volume of the crown cone = 1/3 Pi (50)^2 * 31.85 = 83392.30

Table cone
Height of the table cone = (60/2) * Tan(32.5) = 19.11
Volume of the table cone = 1/3 Pi (60/2)^2 * 19.11 = 18012.74

Pavilion Cone
Height of the pavilion cone = 50 * Tan(41.20) = 43.77
Volume of the pavilion cone = 1/3 Pi (50)^2 * 43.77 = 114594.01

Girdle
Volume of the girdle = Pi (50)^2 * 3 = 23561.95

Total volume of Stone A = 203535.53
Depth = 59.5%

By Ratio, given original stone has a weight of 4.36carat, Stone B should weigh about
4.36/204727.22 * 203535.53 = 4.33carat, so about 0.03 carat.

The extra depth is where the weight comes from.
Depth of Stone A = 60.2%, Depth of Stone B = 59.5%.

If the limit is due to the crown height, meaning the crown angle is fixed at 32 degrees, if given a pavilion angle of 41.4, with a largish lower half, the carat weight will approximately come up to 4.32 carat with a depth of 59.5%.