Recommended PriceScope RBC proportions - project to establish some guidelines

[Garry H (Cut Nut)]

I lost it a few times when proportion recommendations were very narrow. The seem to have been based on super ideal branded #'s.
(I am not intending this as guide for those companies - if you are buying from our esteemed branded vendors you do not need HCA or guides like these)
So I thought it would be good to put together a chart to help helpers of newbies.

These are no way final numbers and everything is open to debate and discussion.
This may not even be the best way to present such info.
I have left off depth % - we can debate it - but my PoV is that a VG Tolkowsky with a thick girdle is a great way to get a 1ct RBC and save some money - and apart from a small diameter it makes little difference to the beauty of the diamond.


When I imported the table it messed up a bit:

Visual type:	Less fire Bright	More Brightness	Bright balanced	All round (Tolkowsky)	Firey Balanced	More Fire & scintillation
Crown Angle	31°- 32°	32°- 33°	33° - 34°	34° - 35°	35°- 36°	36. 5°
Pavilion Angle	41.4°	41.2°	41.0°	40.8°	40.6°	40.5°
< Risk chipping in rings Suggested Table size range for Carat Weight Small for weight >​
< 0.50ct	55-61%	55-61%	55-61%	55-61%	55-60%	55-60%
0.50 – 0.99ct	58-60%	58-60%	55-60%	55-60%	55-60%	55-60%
1.00-1.99ct	57-60%	57-60%	54-59%	54-59%	54-59%	54-59%
+2ct	56-59%	56-59%	55-58%	54-58%	54-58%	54-58%
Best for:	Earrings and Pendants	Bigger Spread – Rings – Smaller Spread​

 

[Karl_K]

I only recommend what I would buy. I would not generally recommend pretty much everything in

Less fire Bright

More Brightness

categories.

These categories are highly dependent on the minor facets being in the right range.

Firey Balanced

More Fire & scintillation

 

[Karl_K]

The nice thing about the near modern tolk range is the wide range of minor facets that work well with them.
Once you move out of that range you end up having to specify the minors and table% to a narrower range to avoid marginal combinations.

 

[Texas Leaguer]

Garry,
Could you explain your statement about thick girdles?

"I have left off depth % - we can debate it - but my PoV is that a VG Tolkowsky with a thick girdle is a great way to get a 1ct RBC and save some money - and apart from a small diameter it makes little difference to the beauty of the diamond."

This seems to contradict one of the underpinnings of your HCA tool that tends to reward spready stones with better scores and/or ratings.

For instance, last time I checked super ideals rarely get an 'excellent' on you spread rating.

 

[diamondseeker2006]

Garry this is good and I will print it out and keep it handy. But like Karl, I would likely only recommend a range that I am comfortable with: Bright balanced, Towlkowsky, and Firey balanced. Once I looked at a bunch of stones that were 36/40.6 (with IS or ASET) and almost all of them had a ring of light leakage. So that's why I need ASET or IS if the crown angle is more than 35.5 with a 40.6 pa. That would be true on the other side of Towlkowsky, as well.

I think a great idea would be if you could do a video for us to show us 2-3 diamonds from each category with magnified face up image and ASET image. Then we would be able to associate how these numbers look in a stone. An in person training would have been fantastic back in the day when we were in Vegas (although I imagine most vendors didn't travel with a large number of diamonds)! I will tell you one thing I hate due to first hand experience, and that is a very flat crown with a large table. Those don't look good in a ring, IMO.

 

[Garry H (Cut Nut)]

The nice thing about the near modern tolk range is the wide range of minor facets that work well with them.
Once you move out of that range you end up having to specify the minors and table% to a narrower range to avoid marginal combinations.

I agree and I like the idea of adding that Karl.
Any suggestions?

 

[Garry H (Cut Nut)]

Garry,
Could you explain your statement about thick girdles?

"I have left off depth % - we can debate it - but my PoV is that a VG Tolkowsky with a thick girdle is a great way to get a 1ct RBC and save some money - and apart from a small diameter it makes little difference to the beauty of the diamond."

This seems to contradict one of the underpinnings of your HCA tool that tends to reward spready stones with better scores and/or ratings.

For instance, last time I checked super ideals rarely get an 'excellent' on you spread rating.

Hi Bryan,
Yes that is true Bryan.
At https://diamond-cut.com.au/ there are several factors
https://diamond-cut.com.au/17_girdle.htm explains the penalty system for girdle thickness (the text warning is not engaged any more)
And at https://diamond-cut.com.au/14_spread.htm this is the rule:
We choose to give a zero penalty for a diamond with a 40° pavilion, 32.5° crown, 58% table and a 1% girdle, diamonds with better spreads also rate zero. The worst score of 2.0 is given to a diamond with a 10% girdle, 40° crown, 54% table and 43° pavilion. Stones with even worse spreads are not further penalized.
That works out that your beautifully proportioned diamonds would be about on the cusp of VG and Excellent for spread. the main variant would be girdle thickness. Thin would rate excellent. Slightly Thick would drop to VG spread. You can debate my method.

I find it hard to believe that no other labs grading system rewards spread given that it is one of the most desirable consumer features.

 

[Texas Leaguer]

Garry,
Thank you for the references and I will check them out as time allows.

Your last line, though, highlights the contradiction:

On one hand you say that spread is
" one of the most desirable consumer features"

At the same time you argue "a thick girdle is a great way to get a 1ct RBC and save some money - and apart from a small diameter it makes little difference to the beauty of the diamond."

 

[Garry H (Cut Nut)]

36/40.6 (with IS or ASET) and almost all of them had a ring of light leakage.
I think a great idea would be if you could do a video for us to show us 2-3 diamonds from each category with magnified face up image and ASET image.

Good call-out DS
How is this for a start. But I am always reminded of my conversation when I first met Bruce Harding (who passed just over a year ago, sadly).
He argued that my proportions were a bit on the shallow side based on his 1970's head obstruction work and G&G article (still way under valued). But when we discussed it he got the contrast bang bang concept - the scintillation in a round diamond.

 

[Garry H (Cut Nut)]

Garry,
Thank you for the references and I will check them out as time allows.

Your last line, though, highlights the contradiction:

On one hand you say that spread is
" one of the most desirable consumer features"

At the same time you argue "a thick girdle is a great way to get a 1ct RBC and save some money - and apart from a small diameter it makes little difference to the beauty of the diamond."

Of course it is a contradiction Bryan, but i would sooner a person who MUST impress with SIZE as the diamond industry has trained people rather than dimensions, or as Looks Like factors in - the apparent size.
Your diamonds do get a boost for apparent size based on the peripheral light return compared to diamonds a little bit deeper&steeper.
The 1ct Steeper Deeper can look like a 0.90ct So too does a Tolkowsky with a 5% girdle will look like your 0.95ct diamonds.
I am way happier for Joe and Jo to have that diamond with a GIA VXX and 20% cheaper than a GIA XXX that is dead and looks like a 0.90ct.

 

[Texas Leaguer]

Garry,
I will have to ponder the logic in your foundational assumption for the HCA - that what “Joe and Jo” really want is a diamond that is very spready and costs less, even though a thin girdle may make the diamond more susceptible to chipping. And by extension, you would have the cutters of super ideals to start cutting thin girdles in order to get an Excellent on the HCA tool.

Yet, at the very same time you are now advocating for thick girdled Tolkowsky cuts as a way to (paraphrasing) get a 1 carat and save some money and have just as beautiful a diamond.

I can’t reconcile those two positions. They are diametrically opposed.

 

[Karl_K]

I think what Garry is getting at is that instead of cutters cutting steep/deep to force 1.0ct out of a rough that they cut a near tolk with a thicker girdle instead and consumers buy accordingly.
The thicker girdle near tolk will look bigger than the steep/deep despite having the same mm size.
I would rather see neither but agree with Gary that the thicker girdle is the lesser of 2 evils compared to being extremely steep/deep.

 

[OoohShiny]

I have nothing of great value to add to this excellent thread, other than to say Thank You to everyone for excellent and informative discussions that help us noobs!



EDIT: Actually, I'm going to ask a question that is probably of no relevance, but...

Q: should there be some sort of metric for 'best for performance on-axis' vs 'best for performance off-axis'?

(ref: Serg's discussions about slightly deeper pavilions potentially providing different/better(?) performance when under binocular viewing, but exhibiting a touch of leakage under head-on and slight-tilt 'scope imaging?)

 

[Garry H (Cut Nut)]

Q: should there be some sort of metric for 'best for performance on-axis' vs 'best for performance off-axis'? (ref: Serg's discussions about slightly deeper pavilions potentially providing different/better(?) performance when under binocular viewing, but exhibiting a touch of leakage under head-on and slight-tilt 'scope imaging?)

That is what I mentioned about Bruce Harding.
Sergey and I have long argued that point. I can agree with him on clean diamond to a point - i.e. there can be more fire, and there is less dark star = brighter on close examination.
But once a diamond has been worn for a day or two it will be dirty on the pavilion side. Diamond gets dirtier than any other known material.
That kills any slightly deeper benefit.

 

[Halogirl1130]

Good call-out DS
How is this for a start. But I am always reminded of my conversation when I first met Bruce Harding (who passed just over a year ago, sadly).
He argued that my proportions were a bit on the shallow side based on his 1970's head obstruction work and G&G article (still way under valued). But when we discussed it he got the contrast bang bang concept - the scintillation in a round diamond.

My crown is 34 and pavilion is 41... I’m trying to learn about The ASET and IS and what the colors mean... do you have a link for where I could learn more about this in a simple way I will understand?

 

[Texas Leaguer]

My crown is 34 and pavilion is 41... I’m trying to learn about The ASET and IS and what the colors mean... do you have a link for where I could learn more about this in a simple way I will understand?

There is a wealth of information on this forum. Here is a pretty good article discussing ASET and IS.

What ASET reveals that Ideal-Scope does not

Ask 10 jewelers about one diamond and you might hear 10 different stories. Ask 10 independent appraisers and the clinical analysis may agree, but terminology and methodology can vary. But […]

www.pricescope.com

 

[Halogirl1130]

There is a wealth of information on this forum. Here is a pretty good article discussing ASET and IS.

What ASET reveals that Ideal-Scope does not

Ask 10 jewelers about one diamond and you might hear 10 different stories. Ask 10 independent appraisers and the clinical analysis may agree, but terminology and methodology can vary. But […]

www.pricescope.com

Yes, I know there is a TON of info here. I just get really overwhelmed sometimes! I will look at that, thank you! I bought my diamond using the parameters suggested here but I just want to continue learning more!

 

[Texas Leaguer]

Yes, I know there is a TON of info here. I just get really overwhelmed sometimes! I will look at that, thank you! I bought my diamond using the parameters suggested here but I just want to continue learning more!

The folks that stick around to learn more are the ones who end up helping newbies. And that is what makes this forum so special!

 

[flyingpig]

We can debate on shallow crown, girdle, etc..

But we can all agree on that steep/deep combo is bad (unless we are talking OEC-like VG/G cut with short LGH and smallish table).

 

[prs]

We can debate on shallow crown, girdle, etc..

But we can all agree on that steep/deep combo is bad (unless we are talking OEC-like VG/G cut with short LGH and smallish table).

So how would you change those proportions for OECs with, for example, a 50% table and 60% lower half length?

 

[Garry H (Cut Nut)]

Folks is this my project alone? I thought there might be some contributions?
Does this help? Do you need more graphics?

 

[flyingpig]

So how would you change those proportions for OECs with, for example, a 50% table and 60% lower half length?

OECs and Circular Brilliants are a completely topic. This thread is about Round Brilliants. Maybe I will join the OEC thread you created.

 

[Mlh]

Folks is this my project alone? I thought there might be some contributions?
Does this help? Do you need more graphics?

Love graphics!

 

[Garry H (Cut Nut)]

Tilted to show what your right eye sees

Tilted and rotated 22.5 degrees (360/

 

[mwilliamanderson]

Love graphics!

I worry about the PA only having one value as it seems it could be confusing. Could there be ranges in the PA row like there is in the CA row?

 

[Garry H (Cut Nut)]

There is a

I worry about the PA only having one value as it seems it could be confusing. Could there be ranges in the PA row like there is in the CA row?

The order of importance in terms of affect on diamond proporfamnce:
Pavilion angle, Crown angle, table size, lower girdle, star facet length
A 1 degree variation in pavilion angle requires an inverse 5 degree crown angle variation.
So essentially the variation in crown angle seems more prcatical.

 

[OoohShiny]

That is what I mentioned about Bruce Harding.
Sergey and I have long argued that point. I can agree with him on clean diamond to a point - i.e. there can be more fire, and there is less dark star = brighter on close examination.
But once a diamond has been worn for a day or two it will be dirty on the pavilion side. Diamond gets dirtier than any other known material.
That kills any slightly deeper benefit.

Thank you for the reply!

It is cool that there is room for both viewpoints and a discussion about them - I think that's what makes PS great; it's not just an echo chamber of 'buy these MRB proportions only', even though we do tend to stick within a certain range generally... lol

I can certainly vouch for the dirty diamond issue - the good lady's is filthy 99 days out of 100 as she just doesn't bother cleaning it...

(See also: her car...)

Tilted to show what your right eye sees

Tilted and rotated 22.5 degrees (360/

Oooooh, I like the Tilt ASET stuff I think it's useful for assessing 'real life' performance more thoroughly!

Personally speaking (of course!) I think that more can be made of it - that could also counter off some of the thoughts about ASET only being 'one dimensional' / monocular, for example, and could encourage design innovation in MMD rough cutting

 

[Garry H (Cut Nut)]

Thank you for the reply!

It is cool that there is room for both viewpoints and a discussion about them - I think that's what makes PS great; it's not just an echo chamber of 'buy these MRB proportions only', even though we do tend to stick within a certain range generally... lol

I can certainly vouch for the dirty diamond issue - the good lady's is filthy 99 days out of 100 as she just doesn't bother cleaning it...

(See also: her car...)





Oooooh, I like the Tilt ASET stuff I think it's useful for assessing 'real life' performance more thoroughly!

Personally speaking (of course!) I think that more can be made of it - that could also counter off some of the thoughts about ASET only being 'one dimensional' / monocular, for example, and could encourage design innovation in MMD rough cutting

Here is an overlay of left and right eye view (7 degree/14 degrees) with a twist

 

[Mlh]

Here is an overlay of left and right eye view (7 degree/14 degrees) with a twist

Wow
That is great!! Which combo is this one?

@Garry H (Cut Nut) , I love these discussions! I have spent a good amount of time staring at these charts since joining PS...dizzying!! I see that the 33.5/41 looks as good as the 34.5/40.8, but then I recall @Karl_K , @sledge warning about the issue of GIA rounding and the dreaded 41.2 cliff for leakage and the 40.5 cliff in the other direction for obstruction. (Hope I got that right). That's when I go back to the safe zone.

 

[OoohShiny]

Here is an overlay of left and right eye view (7 degree/14 degrees) with a twist

That is cool! Thank you!

Shows how complex a 'simple' MRB ASET becomes when viewed binocularly and how areas of slight leakage (to one eye) might be able to be overcome with brightness (to the other eye)!

(Ha, I didn't even know binocularly was a word lol)