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Recommended PriceScope RBC proportions - project to establish some guidelines

mwilliamanderson

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There is a

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.
Thanks Gary. I’ve been looking at only AGS stones for a couple of months and I was thinking where are all the other PA’s? I forgot about GIA rounding, lol.
 

OoohShiny

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I have a lot on my mind right now but will try to get some thoughts down!
 

Garry H (Cut Nut)

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These categories are highly dependent on the minor facets being in the right range.
Firey BalancedMore Fire & scintillation
Here is a little help Karl - I kept the star facets at 50% - of course they could be adjusted too: 1599188772864.png

And Lower Girdle length is also table sized dependant but we are splitting hairs now I think?
I wrote this for diamond cutters (who still don't get it) about 18 years ago https://ideal-scope.com/fine-tuning/
 

Karl_K

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Garry sorry for not being more help. Rough time lately.
50% stars is fine in the ranges were discussing and is the most common in the real world.
Probably dont want to go there but shallower crowns benefit from longer stars and steep crowns they can be to long.
The difference in the uppers angle makes for more scintillation on the shallower crown and the shorter stars lowers the uppers angle to prevent problems around the edge of the stone on steep crowns.

Lowers 77-78 is pretty universal but gia in their infinite stupidity made one 75 and the other 80.
The gia stupid gross grounding makes this next to impossible.
 

Garry H (Cut Nut)

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Garry sorry for not being more help. Rough time lately.
50% stars is fine in the ranges were discussing and is the most common in the real world.
Probably dont want to go there but shallower crowns benefit from longer stars and steep crowns they can be to long.
The difference in the uppers angle makes for more scintillation on the shallower crown and the shorter stars lowers the uppers angle to prevent problems around the edge of the stone on steep crowns.

Lowers 77-78 is pretty universal but gia in their infinite stupidity made one 75 and the other 80.
The gia stupid gross grounding makes this next to impossible.
Sorry to hear your not doing so well Karl :(2
The table size is probably the most important for lower girdles - as per my link above - here is the chart (remembering that Lower Girdle depth was the terminology used before GIA decided to change the method of measurement - like almost every confusion they could throw at us).
So subtract about 2%.
And 2 other things - our range is smaller and much tighter than what we saw on the market 20 years ago.
1599433080260.png
 

Karl_K

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something has been bugging me we need to consider.
 

bright&shiny

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Gary, this is fantastic work! I have no expertise so sadly can’t contribute to the effort, but I am curious - how in the world did you get the off axis images? I think this might get my vote for the best thread ever....
 

Garry H (Cut Nut)

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Gary, this is fantastic work! I have no expertise so sadly can’t contribute to the effort, but I am curious - how in the world did you get the off axis images? I think this might get my vote for the best thread ever....
This is done in DiamCalc Pro which Karl also has.
 

Mlh

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Just wondering, what impact does pavillion depth have on a mrb? 42.5 vs 43 for example. I am under the impression that 43 is best.
 

prs

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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.

MRB Facet Pattern PriceScope Garry 25%.png

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. =)2
 

tripleX

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If I can throw this into the mix: http://www.jogiadiamonds.com.au/diamonds/CR-PAV.pdf

I have a 32.5/41.2 stone in my fiancé’s engagement ring and I’ll tell you what it is a winner! But I feel a lot of people on here would dismiss it as the crown angle is smaller.

What might be interesting is to include real life videos of stones in each category? It’s fine on paper, but to actually distinguish in look between these various angles might be beneficial! Thoughts?
 

Garry H (Cut Nut)

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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.

MRB Facet Pattern PriceScope Garry 25%.png

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. =)2
It why some younger folk prefer princess cuts and older people may like old cuts or emeraald cuts
 

Karl_K

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It why some younger folk prefer princess cuts and older people may like old cuts or emeraald cuts
I have some relatives that hate h&a rounds and love princess cuts.
They are strange.
But what I see as pretty little arrows, they see as dark spots that make it look like "a cut up pie with thin slices missing"
I laughed for 20 min the first time I heard them say that.
 

prs

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It's easy to understand the facet pattern is caused by the geometry of the cut, but I don't understand what it is that produces the contrast that highlights the pattern.

Karl's article LINK mentions that the lower girdle facets are cut at a slightly steeper angle than the pavilion main facets, but I've not been able to find any other references to this.

Is it possible the lower half and pavilion main facets are cut at different angles such that the pavilion mains are darker thru obstruction while the lower halves are not, and this produces the contrast?
 

Garry H (Cut Nut)

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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?
1599808248669.png
 

AllAboardTheBlingTrain

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Fantastic effort everyone who’s contributed and thank you so much @Garry H (Cut Nut) for starting this thread! I’m afraid a bunch of it has gone over my head :D is it fruitful to also include something about a range of depth to consider? Because as I see the chart so far doesn’t consider depth. Or is a favourable depth something that’s more of a case to case basis?

Also, I think we could put this chart in the round brilliant guide, or perhaps sticky this thread to the top of RockyTalky?
 

prs

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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?
1599808248669.png
What is the "< Risk chipping" all about? It's not obvious to me.

Also does "Small for Weight >" mean the same as "Smaller Spread>"?
 

Karl_K

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the lower half and pavilion main facets are cut at different angles such that the pavilion mains are darker thru obstruction while the lower halves are not, and this produces the contrast?
Correct in a mrb anyway.
 
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Garry H (Cut Nut)

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Karl's article LINK mentions that the lower girdle facets are cut at a slightly steeper angle than the pavilion main facets, but I've not been able to find any other references to this.

Is it possible the lower half and pavilion main facets are cut at different angles such that the pavilion mains are darker thru obstruction while the lower halves are not, and this produces the contrast?
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?
1599865669610.png
 
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prs

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What is the "< Risk chipping" all about? It's not obvious to me.
It just struck me this was likely a heading for a recommended girdle thickness table that is no longer in your chart.

Come to think of it, as a person who was recently offered an MRB with a 5.5% girdle, it might be worthwhile to put that table back into your chart!
 
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Garry H (Cut Nut)

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It just struck me this was likely a heading for a recommended girdle thickness table that is no longer in your chart.

Come to think of it, as a person who was recently offered an MRB with a 5.5% girdle, it might be worthwhile to put that table back into your chart!
I don't think we need a girdle thickness overlay.
HCA will blow that out of the water.
Even GIA protect you here - although their pre RBC cut grading system was far superior.
This is the current % of diameter system:
1599876007163.png
 
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