Recommended PriceScope RBC proportions - project to establish some guidelines

[prs]

There being no great objections I propose this as is.
True that at the extremes lower girdle and star facets can be an issue, but since it is likely most people will stick to the green zone - for now I think the KISS rule should apply.
Next question - where should this sit so it is accessible?

I noticed on this latest version of your chart the 40.5° PA was changed to 40.4°. To me the obvious question is why isn't there a 40.2° option? Adding that option and explaining on the top line why it's not desirable might be informative.

I would also move the bottom line of the chart to be directly under the colored section. When I first looked at the chart I found myself wondering why certain table sizes were better for Earrings and Pendants!

 

[Garry H (Cut Nut)]

I noticed on this latest version of your chart the 40.5° PA was changed to 40.4°. To me the obvious question is why isn't there a 40.2° option? Adding that option and explaining on the top line why it's not desirable might be informative.
I would also move the bottom line of the chart to be directly under the colored section. When I first looked at the chart I found myself wondering why certain table sizes were better for Earrings and Pendants!

I think most experts here would say this is going too far.
This was made with all locked on 40.2.

Can you explain what you mean by drawing on the chart please prs?

 

[Karl_K]

40.4 even with 36.5ca is pretty boarderline in itself.
it needs very long lgf% to avoid issues in a ring.
58+ tables are also an issue.

 

[Garry H (Cut Nut)]

40.4 even with 36.5ca is pretty boarderline in itself.
it needs very long lgf% to avoid issues in a ring.
58+ tables are also an issue.

As Karl has mentioned before -at the extreme combinations one needs to be more careful with lower and upper facet lengths as well as table sizes.

 

[prs]

Shooting a ray into the lower girdle facets with a very very shallow stone - now the lower girdles are black and the mains have gone red again - does this help PRS?

Garry, thank you for this very informative and interesting post. Please correct me if I'm wrong but as I see it, with a constant CA of 34.5°, at a PA of 40.8° only the mains show obstruction, but as PA decreases below 40.4° both the mains and lowers show obstruction and the entire area under the table becomes dark. (Is this called a nail head?) As PA continues to decrease both facets remain dark until the mains start to come out the other side into the late Bruce Harding's "Area B" At this point we have the exact opposite effect where the mains are light and the lowers are dark!



I'm not sure if 40.4° is the obstruction PA to be avoided no matter the CA, or if the obstruction PA varies along with CA.

 

[prs]

Can you explain what you mean by drawing on the chart please prs?

Sorry Garry, but I don't understand your question.

 

[Garry H (Cut Nut)]

I wish Bruce was still here to help explain, but great point Peter.
Consider these 2 videos
https://youtu.be/DsZ2yUxuThQ 39.2 to 41.2 pavilion variation locked on 57% 34.5
https://youtu.be/nmMRwuUip2Y
tilted to show right eye view

 

[Garry H (Cut Nut)]

Sorry Garry, but I don't understand your question.

You wrote "I would also move the bottom line of the chart to be directly under the colored section. "

I don't know what your asking?

"When I first looked at the chart I found myself wondering why certain table sizes were better for Earrings and Pendants! "
Larger table sizes sacrifice fire a little for brightness. Brightness is the key to great looking earrings and pendants. It is hard to get great lighting for fire into earrings and pendants - in my experiance I see fire way more in rings because of the angles of common lighting incidence.
Also shallower diamonds are way more available in large table sizes.

 

[Karl_K]

I wish Bruce was still here to help explain, but great point Peter.
Consider these 2 videos
https://youtu.be/DsZ2yUxuThQ 39.2 to 41.2 pavilion variation locked on 57% 34.5
https://youtu.be/nmMRwuUip2Y
tilted to show right eye view

Those videos are only valid at one small distance range; change the distance and it changes as does changing the size of the object doing the obstructing.
Put on a black cowboy hat and how your diamond looks changes.

Basically at ~40.45 degrees for the pavilion angle the distance from the viewer that the mains stay dark greatly increases then it reverses itself if you go even lower.
A steep crown angle can help a little and long lowers make the arrows smaller so its a smaller area that stays dark.

What is considered "ideal" for a near tolk h&a is dark arrows until about 1/2 arm length and then turn bright.
If they stay dark to far away they just make the diamond look dark.
An oec with the same response would look to dark up close due to the big mains.

 

[Serg]

Garry, thank you for this very informative and interesting post. Please correct me if I'm wrong but as I see it, with a constant CA of 34.5°, at a PA of 40.8° only the mains show obstruction, but as PA decreases below 40.4° both the mains and lowers show obstruction and the entire area under the table becomes dark. (Is this called a nail head?) As PA continues to decrease both facets remain dark until the mains start to come out the other side into the late Bruce Harding's "Area B" At this point we have the exact opposite effect where the mains are light and the lowers are dark!



I'm not sure if 40.4° is the obstruction PA to be avoided no matter the CA, or if the obstruction PA varies along with CA.

Bruce Harding used 2D section simplification in his work Faceting limits 1975 year . He did not study raytracing in 3D model, he did not calculate any head obscuration for halves.

Faceting limits

 

[prs]

I suppose that to comment on those proportions one would have to have seen lots of diamonds in each of those six combinations. Likely why you aren't getting much response from non trade prosumers.

I know AGS is highly regarded, and our super ideal vendors use them exclusively. An AGS "Ideal" cut grade is seen as a sign of perfection. So I wonder why their 2008 cut chart includes more "Ideal" combinations than Garry's chart.



Karl has told me that obstruction might be AGS's achilles heel, so maybe that explains the 40.4, 36.0-36.5 combos, but what about their PA 41.5 to 41.8 combinations? I'm no expert, as has been pointed out at great length in my OEC thread. Just trying to play Devil's advocate and hoping to learn something.

 

[Garry H (Cut Nut)]

I suppose that to comment on those proportions one would have to have seen lots of diamonds in each of those six combinations. Likely why you aren't getting much response from non trade prosumers.

I know AGS is highly regarded, and our super ideal vendors use them exclusively. An AGS "Ideal" cut grade is seen as a sign of perfection. So I wonder why their 2008 cut chart includes more "Ideal" combinations than Garry's chart.



Karl has told me that obstruction might be AGS's achilles heel, so maybe that explains the 40.4, 36.0-36.5 combos, but what about their PA 41.5 to 41.8 combinations? I'm no expert, as has been pointed out at great length in my OEC thread. Just trying to play Devil's advocate and hoping to learn something.

Karl has told me that obstruction might be AGS's achilles heel, so maybe that explains the 40.4, 36.0-36.5 combos, but what about their PA 41.5 to 41.8 combinations? I'm no expert, as has been pointed out at great length in my OEC thread. Just trying to play Devil's advocate and hoping to learn something.

AGS over does obstruction - and in their images showing hearts they over over do it.
they use some silly US military vision distance - I think 8 inches as the obscuration test.

Dropping down to the crown angle that sits with 41.8 and 41.9 degree pavilions is risky for chipping girdles.
Diamonds cut to those proportions tend almost always to have thinner girdles and when appraising ones that have been worn for a decade or more we see around half of them have chipped or abraded girdles.
Those diamond usually come from sawn tops.
They can be even brighter for pendants and earrings, but generations later will end up in rings - they are not forever

 

[prs]

Karl has told me that obstruction might be AGS's achilles heel, so maybe that explains the 40.4, 36.0-36.5 combos, but what about their PA 41.5 to 41.8 combinations? I'm no expert, as has been pointed out at great length in my OEC thread. Just trying to play Devil's advocate and hoping to learn something.

AGS over does obstruction - and in their images showing hearts they over over do it.
they use some silly US military vision distance - I think 8 inches as the obscuration test.

Dropping down to the crown angle that sits with 41.8 and 41.9 degree pavilions is risky for chipping girdles.
Diamonds cut to those proportions tend almost always to have thinner girdles and when appraising ones that have been worn for a decade or more we see around half of them have chipped or abraded girdles.
Those diamond usually come from sawn tops.
They can be even brighter for pendants and earrings, but generations later will end up in rings - they are not forever

OK, that's news to me, but now I understand. My wife's diamond has a very thin-medium girdle (2.5%)

However your excellent explanation is not self evident from the chart. Maybe it might be a good idea to add a girdle thickness line. At least that thickness is staring the consumer in the face on every GIA cert, and it will be much easier for them to determine if they have a potential problem.

 

[Garry H (Cut Nut)]

OK, that's news to me, but now I understand. My wife's diamond has a very thin-medium girdle (2.5%)

However your excellent explanation is not self evident from the chart. Maybe it might be a good idea to add a girdle thickness line. At least that thickness is staring the consumer in the face on every GIA cert, and it will be much easier for them to determine if they have a potential problem.

GIA's grading system is great for girdle thickness under 10ct diamonds Peter.
Very few people are buying less than XXX that come here.
I am very happy if they drop down to VG X X for the left side of the chart - it is only if they drop to GXX or FXX that there will be a problem, and I think those buyers are all shopping in Mauls.

 

[OoohShiny]

Anyone have any preferences?

Are you asking about a preference for a facet pattern?

Yes - for the best range - not too fat - too much darkness, not too little - less fire and less bang bang sparkle,
A range

Garry, DW and I have found that our perception of beautiful facet patterns sometimes changes when we switch from close up to life size view.



In this life size view we prefer the stronger, more bold patterns towards the left hand side of your chart. Perhaps it's because our eyes are not as sharp as they used to be.

Super-late replying here!

I was going to say that some of the patterning on the left hand side of the chart is appealing to my eyes, and prs's comment was far more astute than I was going to be!

Are we excluding them because GIA Excellent limits LGFs to 75-80%? (Well, with GIA rounding... )

If dropping to GIA VG X X (or whatever the order would be) would mean being able to get one of those left-hand-side pattens (and at a lower price than GIA XXX) it would cover the scenario where some buyers desire to aim lower than GIA XXX (because of budget/4Cs confilcts) and 'validate' their aims as being perfectly acceptable, as well as help widen the options available for recommendation on here, which IIRC both you and Serg have said should happen

 

[Garry H (Cut Nut)]

Super-late replying here!

I was going to say that some of the patterning on the left hand side of the chart is appealing to my eyes, and prs's comment was far more astute than I was going to be!

Are we excluding them because GIA Excellent limits LGFs to 75-80%? (Well, with GIA rounding... )

If dropping to GIA VG X X (or whatever the order would be) would mean being able to get one of those left-hand-side pattens (and at a lower price than GIA XXX) it would cover the scenario where some buyers desire to aim lower than GIA XXX (because of budget/4Cs confilcts) and 'validate' their aims as being perfectly acceptable, as well as help widen the options available for recommendation on here, which IIRC both you and Serg have said should happen

OhYes Shiny,
I was promoting this idea for many years when GIA first bought out their cut grade (6 years after HCA and pretty much a dead copy look tables up method).
But so many people howled me down. Not worth the fight.
This works really well for pendants and earrings.

 

[OoohShiny]

OhYes Shiny,
I was promoting this idea for many years when GIA first bought out their cut grade (6 years after HCA and pretty much a dead copy look tables up method).
But so many people howled me down. Not worth the fight.
This works really well for pendants and earrings.

Thanks for the confirmation, Garry!

I shall note it for future search usage

 

[OoohShiny]

I suppose that to comment on those proportions one would have to have seen lots of diamonds in each of those six combinations. Likely why you aren't getting much response from non trade prosumers.

I know AGS is highly regarded, and our super ideal vendors use them exclusively. An AGS "Ideal" cut grade is seen as a sign of perfection. So I wonder why their 2008 cut chart includes more "Ideal" combinations than Garry's chart.



Karl has told me that obstruction might be AGS's achilles heel, so maybe that explains the 40.4, 36.0-36.5 combos, but what about their PA 41.5 to 41.8 combinations? I'm no expert, as has been pointed out at great length in my OEC thread. Just trying to play Devil's advocate and hoping to learn something.

Hey prs,

May I ask if you have a link to the AGS charts like those in your post?

I and @act1980 have been looking for them in another thread


Thank yoooou!

 

[OoohShiny]

There being no great objections I propose this as is.
True that at the extremes lower girdle and star facets can be an issue, but since it is likely most people will stick to the green zone - for now I think the KISS rule should apply.
Next question - where should this sit so it is accessible?

Hi Garry,

Just coming back to this question in bold, could it be useful to add the table to the HCA results page?

@act1980 has highlighted in another thread today that it is perhaps not clear whether the HCA tool is giving results for a stone that PS would usually recommend, (e.g. the green boxes and the yellow boxes in your table) or whether it is just confirming that the C/P angles do work together but other options might be preferred by the buyer (in terms of Fire or whatever).

Perhaps having the table on the results page somewhere, and somehow showing where the stone being analysed fits within it, would help HCA users understand if they need to be looking for different angle combinations in the stones they are choosing to submit to the HCA tool?

 

[Garry H (Cut Nut)]

Hey prs,

May I ask if you have a link to the AGS charts like those in your post?

I and @act1980 have been looking for them in another thread


Thank yoooou!

Upper left corner of our master stone list has various comparison charts

OctoNus : List of all MSS diamonds

legacy.octonus.com

 

[Garry H (Cut Nut)]

Hi Garry,

Just coming back to this question in bold, could it be useful to add the table to the HCA results page?

@act1980 has highlighted in another thread today that it is perhaps not clear whether the HCA tool is giving results for a stone that PS would usually recommend, (e.g. the green boxes and the yellow boxes in your table) or whether it is just confirming that the C/P angles do work together but other options might be preferred by the buyer (in terms of Fire or whatever).

Perhaps having the table on the results page somewhere, and somehow showing where the stone being analysed fits within it, would help HCA users understand if they need to be looking for different angle combinations in the stones they are choosing to submit to the HCA tool?

Interesting idea OShiny!
The further from the green you are the more you are giving up one benefit and the more of the other benefit(s) you are gaining.

 

[OoohShiny]

Upper left corner of our master stone list has various comparison charts

OctoNus : List of all MSS diamonds

legacy.octonus.com

I totally missed those links

Thank you!

 

[OoohShiny]

Interesting idea OShiny!
The further from the green you are the more you are giving up one benefit and the more of the other benefit(s) you are gaining.

I think something to help understanding of the trade-offs could be a great addition

 

[prs]

Hey prs,

May I ask if you have a link to the AGS charts like those in your post?

I and @act1980 have been looking for them in another thread


Thank yoooou!

I got the 2008 charts from the AGS website, but currently I can only find the 2005 version. However here's a LINK to another site that has them. They are in pdf format.

 

[OoohShiny]

I got the 2008 charts from the AGS website, but currently I can only find the 2005 version. However here's a LINK to another site that has them. They are in pdf format.

Thank you very much!

@act1980 - prs' link is what you were looking for.

 

[act1980]

Thank you very much!

@act1980 - prs' link is what you were looking for.

Thank you!

 

[act1980]

I have to admit that I'm still confused about table sizes. How is a larger table not good if both the crown and pavilion come within the ideal range?

 

[winetime12]

I have to admit that I'm still confused about table sizes. How is a larger table not good if both the crown and pavilion come within the ideal range?

I think because you see less of the facets, but don't quote me on that

 

[OoohShiny]

I have to admit that I'm still confused about table sizes. How is a larger table not good if both the crown and pavilion come within the ideal range?

The Upper Girdle Facets and the Star Facets that sit between the Table and the Girdle split light exiting the stone into rainbow fans - which is dispersion, which creates 'fire' when your pupil only sees one colour in that rainbow fan.

The larger the table, the smaller the facets that do this, therefore less fire is produced/seen..

The lower the crown, the more light exits the stone perpendicular to the UGFs/Star facets, meaning it is not split into rainbow fans / fire, meaning less fire is produced / seen.

 

[act1980]

The Upper Girdle Facets and the Star Facets that sit between the Table and the Girdle split light exiting the stone into rainbow fans - which is dispersion, which creates 'fire' when your pupil only sees one colour in that rainbow fan.

The larger the table, the smaller the facets that do this, therefore less fire is produced/seen..

The lower the crown, the more light exits the stone perpendicular to the UGFs/Star facets, meaning it is not split into rainbow fans / fire, meaning less fire is produced / seen.

Thanks for that, makes sense! Is there an optimum table size then?