Recommended Proportions

[Garry H (Cut Nut)]

About a year ago we had a discussion and a thread (which I can not find).

These proportions were the outcome. I have added and adjusted the the words (not the proportions).
What do you other Cut Nut's think?

 

[prs]

About a year ago we had a discussion and a thread (which I can not find).

Garry, is this the thread you couldn't find? LINK

 

[diagem]

About a year ago we had a discussion and a thread (which I can not find).

These proportions were the outcome. I have added and adjusted the the words (not the proportions).
What do you other Cut Nut's think?

Since you are on rounds (again...), I would add 3D info like both CH & PD, after all..., the important factors to your adjusted wordings?

 

[Paul-A]

Garry, I understand your desire, as per the original thread to stop guiding newbies to a very narrow proportion-set, rather to broaden their view.

But with only 3 average measurements, making 'definite' statements on very diverse results, brightness, fire and even scintillation, while also predicting usability in kind of jewel, seems at the very least a bit of a stretch.

It also seems to contradict more elaborated PS-tutorials, articles and the more elaborated advice, generally presented here in the forum. I fear this leads to more confusion, rather than more knowledge.

Live long,

 

[oldminer]

Paul, I agree that a small number of average measurements is not the way to select a diamond, but these measurements used for SCREENING and REJECTING have proven time and time again, to be useful and reasonable. All the diamonds CBI and the rest of the premium cut diamond trade world cut diamonds with score in the top categories; the green and yellow columns. Many of GIA 3Ex will not score well even if a majority of consumers are completely content to buy them. The mass of cutters continue to be content to retain weight over light return and the perfection of beauty.

I don't especially like round diamonds with less than 33 degree crown angles and doubt I could be convinced that 31 and 32 degree angles have a place among the best cut and performing stones. They might perform well, but somehow I think claiming them to be well cut is pushing the envelope of what my training has been. They fail to "look right" to me. It is very difficult to see it otherwise. Maybe such diamonds for Pendants and Earrings constitute a very good alternative for performance and price in secondary diamond jewelry items. I can see that as acceptable, but not useful when shopping for a engagement ring's main diamond. It muddies the waters of consumers primarily shopping to select an engagement ring diamond.

 

[Garry H (Cut Nut)]

Since you are on rounds (again...), I would add 3D info like both CH & PD, after all..., the important factors to your adjusted wordings?

No no no!
Yoram those are a given based on table size, crown and pavilion angles unless the diamond has no girdle or a very thick girdle which grading reports ding for.

I wish they removed those two useless percentages from all reports.
And BTW - a diamond with Tolkowsky proportions and a 5% thick girdle looks great and is way cheaper because the grade will be VG not X. This is a wonderful way for under paid E ring shoppers (if they can find one) to keep the outlaws happy.
(table sizes need to be bigger as girdle thickness gets thicker)

 

[Garry H (Cut Nut)]

Paul, I agree that a small number of average measurements is not the way to select a diamond, but these measurements used for SCREENING and REJECTING have proven time and time again, to be useful and reasonable. All the diamonds CBI and the rest of the premium cut diamond trade world cut diamonds with score in the top categories; the green and yellow columns. Many of GIA 3Ex will not score well even if a majority of consumers are completely content to buy them. The mass of cutters continue to be content to retain weight over light return and the perfection of beauty.

I don't especially like round diamonds with less than 33 degree crown angles and doubt I could be convinced that 31 and 32 degree angles have a place among the best cut and performing stones. They might perform well, but somehow I think claiming them to be well cut is pushing the envelope of what my training has been. They fail to "look right" to me. It is very difficult to see it otherwise. Maybe such diamonds for Pendants and Earrings constitute a very good alternative for performance and price in secondary diamond jewelry items. I can see that as acceptable, but not useful when shopping for a engagement ring's main diamond. It muddies the waters of consumers primarily shopping to select an engagement ring diamond.

Dave one of your pet hates is shallow crown chipped diamonds. I am advocating these stones are used for pendants and earrings.
1. they are in a safe environment
2. the show dirt less and do as we all know - spend 99% of their life very dirty

 

[prs]

Why does the PA of 40.4° match only to a CA of 36.5° and not 36°-37°?

 

[prs]

Dave one of your pet hates is shallow crown chipped diamonds. I am advocating these stones are used for pendants and earrings.
1. they are in a safe environment
2. the show dirt less and do as we all know - spend 99% of their life very dirty

You could add the word "Only" to "Earrings and Pendants" to make sure you are not recommending for rings.

Why did you add "Bracelets and Bands" to the bottom?

 

[DejaWiz]

Why did you add "Bracelets and Bands" to the bottom?

Bracelets and bands extends into the first three columns, whereas earrings and pendants extends into the first two columns.

 

[Paul-A]

Paul, I agree that a small number of average measurements is not the way to select a diamond, but these measurements used for SCREENING and REJECTING have proven time and time again, to be useful and reasonable.

Dave, you are right but you are missing my point about the chart.

The chart makes a prediction about scintillation, based upon 3 average measurements. That is impossible. I cannot even imagine that a relationship between these averages and potential scintillation has ever been studied.

In the same way, the prediction about fire is debatable. Yes, all other aspects equal, a higher and steeper crown may increase the potential dispersion of a diamond. But other factors may have a bigger impact on dispersion, while the requirement of 'all else equal' is highly uncommon. On top of that, having such fire-prediction in a screening-tool seems in conflict with a more detailed PS-article like this.

I thus find the predictions for fire and scintillation problematic.

Live long,

 

[DejaWiz]

This thread is sparking an amazing discussion!

 

[Paul-A]

@Garry H (Cut Nut) , can we simplify things with just one question?

What is the basis for declaring a 36.5 - 40.4 giving more scintillation?

Live long,

 

[Texas Leaguer]

Grading scintillation is a dubious proposition, especially without working from a 3D scan. But even if you accurately measure the scintillation events, the question of what "flavor" of scintillation is best is a thorny question. A lot of small, fast sparkle events like 'crushed ice' or the large, slow flash of a more 'chunky' facet design?

I think the highly subjective nature of what is ideal in terms of scintillation is why AGSL did not include it in their light performance reporting. It is my understanding that they were not ready to build a grading metric for this aspect at the time they released their LP system (brightness, contrast, leakage, fire), though their research continued and one day may include this key component of performance.

 

[Paul-A]

Thank you for the agreement, @Texas Leaguer

Live long,

 

[DejaWiz]

Been thinking on this quite a bit.

So far, we have definitive ranges/pairings for just about every measurement, depending on where one looks.

Just some examples (in no particular order):
Whiteflash ACA
GCAL 8X
Prosumer/Diamond Experts websites

And one that I am really intrigued by: the AGA set of categorical-based measurement gradings from none other than David Atlas (@oldminer ).

Here's the source that I gathered the criteria to put into a spreadsheet:

AGA Cut Grades | Diamond Source of Virginia

www.diamondsourceva.com

The source states "Old AGA Cut Grades - Round Diamonds", plus the PA's and 4A/4B categories are either not listed or ignored.
Mr. Atlas, are these ranges still the current ACA cut grades, and are there pavilion angles plus Category 4 ranges available?

I would like to post the spreadsheet, but I will only do so with expressed permission from David (and/or ACA Laboratory).
Credit has been notated for both ACA Laboratory and David S Atlas in the spreadsheet...If I get permission to post the spreadsheet and anything needs to be added or changed for proper credit and citation, then I absolutely will change it and repost it.

Here's a snippet that highlights the crediting:

 

[oldminer]

You are free to post the old AGA Cut Class charts. HOWEVER, these are discontinued and not state of the art. This is a relic of when things were far less commonly measured and most diamonds were primarily cut for retaining weight and not for scientific light return. Garry Holloway was working on the HCA and I was working on these sets of numbers at my AGA Lab at the same time, half way around the world, and we did not even know one another back then. Since things had advanced so far in the technology of cutting and measuring light performance, I stopped promoting the charts several years ago. I told Pricescope to remove them from the Pricescope website where they were available and somewhat controversial on occasion. No doubt, they do have some value in screening diamonds, but it is a dead end issue since there are simply better methodologies today.

Note that technically correct studies showed my predictive charts hit the AGSOOO round diamonds pretty much right on target, and yet the fancy shape charts may be of greater current interest still since no major Lab is promoting their own solution even after nearly 25 years. It must be a big problem for them to get any further than I did so many years ago. We don't want to confuse anyone, so let's not promote these charts as a suddenly discovered solution to anything. They are a piece of diamond cutting history, but not today's answer to what we need. Wish they were, but it isn't so.

 

[DejaWiz]

Thank you for granting permission and for providing a disclaimer that the data available is of an outdated nature, @oldminer.
I still find value in it and it's also pretty neat to compare and contrast to what Garry's findings are!

 

[Paul-A]

The good thing about the AGA-system for round brilliants, kudos to Dave Atlas, is that it clearly states being a 'cut brilliance grading system'. There is no claim regarding any prediction of fire or scintillation.

In fact, most existing systems, whether lab or non-lab are similar brilliance grading systems.

It would be good if that were clearly expressed.

Live long,

 

[oldminer]

Paul, you are correct. My opinion then, and still is, that if you take the time and care necessary to perfect the CUT of a diamond you will end up with the most brilliant diamond which by definition will have the proper and inherent degree of fire and scintillation nature and crystallography provide. Both of these added elements are lighting dependent. The better the craftmanship, the more of each important feature associated with finely cut diamonds will be visible. It never seemed to mean much to me to attempt to quantify fire or scintillation since they come with properly cut and crafted diamonds.

There are direct correlations, but it seems so overly complex that we make choosing a diamond quite a difficult task. It is supposed to be about love and romance, not an engineering project. Then again, I am not a mathematician.

I'd love to see more work put into being able to designate the best cuts in the fancy shapes. My feeling is that we have what is essential for round diamonds well in hand.

 

[Texas Leaguer]

Paul, you are correct. My opinion then, and still is, that if you take the time and care necessary to perfect the CUT of a diamond you will end up with the most brilliant diamond which by definition will have the proper and inherent degree of fire and scintillation nature and crystallography provide. Both of these added elements are lighting dependent. The better the craftmanship, the more of each important feature associated with finely cut diamonds will be visible. It never seemed to mean much to me to attempt to quantify fire or scintillation since they come with properly cut and crafted diamonds.

There are direct correlations, but it seems so overly complex that we make choosing a diamond quite a difficult task. It is supposed to be about love and romance, not an engineering project. Then again, I am not a mathematician.

I'd love to see more work put into being able to designate the best cuts in the fancy shapes. My feeling is that we have what is essential for round diamonds well in hand.

Just to point out for viewers of this thread who may not be aware - the AGSL light performance evaluation methodology, based on ray tracing a 3D model of the diamond (mathematically quantifying brightness, contrast, leakage and fire) can be and is being applied to fancy shapes. This is a core value proposition of the approach they have taken.

Currently AGSL can provide light performance based grading reports on princess, emerald cut, ovals, some cushion and proprietary cut styles. Other shapes are possible, but since each fancy shape has a unique appearance and appeal, the process of developing metrics for assigning grades is painstaking. As the demand for light performance based cut grading increases and expands to other shapes, those metrics can be developed within their existing system.

 

[Karl_K]

Fire and scintillation are design points when creating a design and require a stone to be cut to verify as no computer based system is well suited to the job.
However arguing about fire and scintillation in well cut near tolks is an exercise in futility.
Even on the ranges of that chart its futile.

MRB vs EOC vs crushed ice radiant vs emerald cut is a much more interesting conversation but very very very deep.

Diamond performance boils down to...
It's all lighting dependant and varies based on lighting and environment.

 

[Garry H (Cut Nut)]

Fire and scintillation are design points when creating a design and require a stone to be cut to verify as no computer based system is well suited to the job.
However arguing about fire and scintillation in well cut near tolks is an exercise in futility.
Even on the ranges of that chart its futile.

MRB vs EOC vs crushed ice radiant vs emerald cut is a much more interesting conversation but very very very deep.

Diamond performance boils down to...
It's all lighting dependant and varies based on lighting and environment.

Thanks Karl,
FYI All - Karl and John Pollard have been working on a soon to be released version adding Karls lower girdle recommendations.
Those recommendations do provide some control on scintillation.

It is an arguable fact that steeper crown angles create more virtual facets in a diamond. More virtual facets mean more scintillation. Diamonds with steeper crown angles are vastly more likely to also have smaller tables thereby magnifying this effect.

Small diamonds make resolving those extra virtual facets is more difficult because of the limitations of human vision. However I doubt many people are using these charts to select diamonds for their tennis bracelets etc.
I can also back this up with evidence from HCA since I see the backend of data entered. There were more +3ct diamonds entered into HCA this past week than those below half a carat. I expect something similar for the charts usage.

On an aside, I wrote Dave a letter asking permission to use his AGA Charts in the late 1980's when I was teaching the Aussie GAA diamond diploma course and honorary national convener. So Dave and me go back a year or three.

 

[Garry H (Cut Nut)]

Why does the PA of 40.4° match only to a CA of 36.5° and not 36°-37°?

PRS the 'sweet line'as Serg calls it, tends to break down at the extreme ends.
There can be very nice looking stones well past there but the GIA rounding makes things much harder to provide good advice in a chart like this used as a rejection tool or missused as it often will be.

 

[Karl_K]

Why does the PA of 40.4° match only to a CA of 36.5° and not 36°-37°?

obstruction below 36.5 gets a bit questionable but with long lowers and a under midrange table(56 or under) can work it depends on what is in the averages. 37 is no problem but not with a large table.
Reality is either way its likely unobtainable anyway.

 

[diagem]

No no no!
Yoram those are a given based on table size, crown and pavilion angles unless the diamond has no girdle or a very thick girdle which grading reports ding for.

I wish they removed those two useless percentages from all reports.
And BTW - a diamond with Tolkowsky proportions and a 5% thick girdle looks great and is way cheaper because the grade will be VG not X. This is a wonderful way for under paid E ring shoppers (if they can find one) to keep the outlaws happy.
(table sizes need to be bigger as girdle thickness gets thicker)

Since you guys are tackling the RB scene - again, I will continue reading from the side lines..

Why take CH% out and rely on 3d calculations when such info is quite crucial IMO (CH more than PD) - or at least for me its is..

The visible difference between XXX/000 and VG is consistent & harmonious levels of play-of-lights..., a visible repeatability of flash/scintillation events which at the end of the day are still very much "light environment" dependent...

It might very well be for the majority.., but I am reminding that "poor cut" standard antique OEC's even though they lack the play of light consistency/repeatability, each offer a different optical offering/taste which most would be considered a windfall for XXX/VG/000's.

Just think its kind of neat that wonky antique cuts give any modern round brilliant a great run for the money.. (literally)

 

[Paul-A]

It is an arguable fact that steeper crown angles create more virtual facets in a diamond. More virtual facets mean more scintillation. Diamonds with steeper crown angles are vastly more likely to also have smaller tables thereby magnifying this effect.

@Garry H (Cut Nut)
This is very confusing, Garry.

1. Is the 'fact' you are describing arguable, like you say, or not? If arguable, is it even a fact? Or is this a typo?

2. More virtual facets mean more scintillation? I wonder about that. The problem is that the definition of scintillation is unclear, possibly incomplete. Yes, sparkle due to movement is one of the pleasures of a gemstone. But what about it is actually most pleasing?

Is it the number of sparkle-events only, which would mean that so-called crushed ice, caused by a very high number of very small virtual facets, mathematically gives more scintillation? I think that there is agreement that this type of scintillation is not considered pleasant.

Or is the 'delta' in scintillation more pleasing to the eye, meaning the difference between a virtual facet being dark jumping to a spark due to movement? My personal opinion is that this is far more pleasing to the eye.

Unfortunately, the definition of scintillation does not clearly answer this question.

3. What about the observation of fire? You state that steeper crown angles create more virtual facets, thus by definition smaller virtual facets. Smaller virtual facets automatically decrease the potential observation of fire. That is in direct contradiction with your chart, where steeper crown angles are declared as more firey. Can you see that this is extremely confusing?

4. Steeper crown angles are more likely to have smaller tables? That is counter-intuitive, isn't it? With the same crown height, a steeper crown angle automatically gives a bigger table. To the contrary, a shallower crown angle gives a smaller table.

To get a smaller table in combination with a steeper crown angle, crown height needs to be seriously increased. Assuming this, the diameter and surface-area of the diamond of the same weight is automatically reduced. More virtual facets on a smaller surface again means a serious reduction of the size of these virtual facets, thus a negative influence on the observation of fire, and to the extreme, going more into the direction of crushed ice.

You generally do not confuse me, my friend, but now you clearly did.

Live long,

 

[Garry H (Cut Nut)]

Why take CH% out and rely on 3d calculations when such info is quite crucial IMO (CH more than PD) - or at least for me its is..
Just think its kind of neat that wonky antique cuts give any modern round brilliant a great run for the money.. (literally)

Crown height percentage is a factor of two variables Yoram. Table size and crown angle. At 14.8% you can have a 30 degree crown with a 49% table or 40 crown and 65% table. Or 34.5 and 57%. It is a misguided dinosaur measure.

Wonky old cuts are wonky. Nice ones like you produce are beautiful. And yes - they can have amazing huge fire flashes (at the expense of some brilliance). I too am a fan.

 

[diagem]

Crown height percentage is a factor of two variables Yoram. Table size and crown angle. At 14.8% you can have a 30 degree crown with a 49% table or 40 crown and 65% table. Or 34.5 and 57%. It is a misguided dinosaur measure.

Wonky old cuts are wonky. Nice ones like you produce are beautiful. And yes - they can have amazing huge fire flashes (at the expense of some brilliance). I too am a fan.

I don't know about you or others..., I personally would never consider a RB with a crown height below 15% (with a preference to higher)..., actually the first thing I look for when I view a RB grading report...., I believe purchasing any RB's with CH's below 15% is participating in an historic design error just to service a mass-production world..., but this is for another time.

In the mean time, I agree to disagree!

I started to study, design & cut them 20 years ago just because I found them much more beautiful than their modern counterparts..., you must remember our old discussions 15 years ago when I advocated for them. Not only were they more beautiful and interesting (IMO again), they also came with built in (hi)stories which most modern's lack.

Take a beautiful old cut into a outdoor shaded lighting environment and it will out-brilliance any RB (different brilliances with slight overpowering for old cuts). At the end of the day, the lighting environments surrounding diamonds ultimately decide what optical property to play.

I am glad you are a fan today..., I am very glad I am a fan of 3D optical symmetry as well.., I just believe we should have been way further then where we are today..., still stuck in the RB world.

 

[akshayk]

As I was informed by another fellow member according to white flash a 34 crown and 40.6 pavilion angle falls in the “ACA” range. Therefore would this not fit in the premium / ideal category ?