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Crown Angle larger than ideal...

LightBright

Brilliant_Rock
Premium
Joined
Mar 11, 2013
Messages
710
I have some questions about the following MRB cut diamond. (I'm leaving a few things out to avoid lurkers preempting me)

HCA = 1

L/W RATIO: 1.00
DEPTH %: 62.30
GIRDLE: Medium
TABLE %: 57.00
CULET: None
CERTIFICATE: GIA
CROWN ∠: 36.00
CROWN %: 15.50
PAVILION ∠: 40.20
Cut polish symmetry = good
FLOURESCENCE = SBF

So... The crown angle is taller than what is generally recommended as ideal. The pavilion angle is likewise shallower than ideal.

The HCA score is excellent in all three categories (even though it has a crown to suggest that it would favor fire performance over light return, that's not the case).

First question: the girdle is MEDIUM, it looks fairly thick to me from the photo, and I'm wondering if the thickness of the girdle might be throwing the HCA off and the scores would be terrible with a smaller girdle.

I'm an old cut nut and I happen to want the highest crown possible without compromising light return. Does anyone have any experience with a crown angle larger than the max (ideal) recommendation I've seen of 35.5 degrees? (Or have a photo?)

Can anyone tell me if they think a pavilion angle might be visually uglier because of a shallower than ideal angle? Like, in a setting would this type of stone appear top heavy?

The polish/symmetry/cut are all "good". GIA calls this stone's cut a VERY GOOD, not excellent. Is this something that indicates the stone is a "Pass", even though HCA seems good?

Aside from AVR cuts, are there any branded cuts that are known for larger crown angles? As a rule, do brands like ACA or INFINITY have 35 degrees or shallower crowns? I'm asking this out of curiosity mostly.

Regarding SBF, I recall that SBF is only helpful with slightly tinted stones. It doesn't enhance the color of colorless, in fact it might hurt, correct?

Phew that's a lot of questions!

Thanks in advance for making it this far and for answering any of my questions!
 

kenny

Super_Ideal_Rock
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Joined
Apr 30, 2005
Messages
28,128
HCA is step 1.
Step 2 is to get an Idealscope image for rounds with HCA scores of 2 and below.

If your vendor won't/can't provide a properly-taken Idealscope pic I'd find another vendor.
I recommend Whiteflash and Good Old Gold since they do provide such pics and I've been very happy with their quality, price, policies and service.

Fluorescence can only 'help' diamonds look less yellow if they fluoresce blue, and not yellow.
Even then the blue fluor will only be 'activated' when the diamond is in light that has a large ultraviolet component, like sunlight.
In usual indoor lighting the fluor won't 'help' a bit.
 

gr8leo87

Shiny_Rock
Joined
Apr 24, 2015
Messages
381
I think only a picture would be enough to reject that diamond. That shallow a Pavillion angle means a lot of darkness and contrast under the table. Also that crown angle IMO will not be sufficient to compensate for that shallow a pavillion.

HCA compensates bad scores of tall crowns with shallow pavillions. That's why it's only a rejection tool, and perhaps not even great at that. There will be better looking diamonds than this one which HCA will have us rejecting.

Get some picture even for starters.

Girdle thickness medium is fine.
 

foxinsox

Ideal_Rock
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Joined
Jul 18, 2015
Messages
3,127
I'm curious - OP, this is an old cut, like OEC or similar?
Is the HCA the right tool for this?

I know OECs are rounds so I think yes but I've also seen them referred to as essentially a fancy cut because they weren't cut to favour the same sort of performance as MRBs so I don't know?
 

LightBright

Brilliant_Rock
Premium
Joined
Mar 11, 2013
Messages
710
Thanks for the replies.

I'm going to post a couple (ugly) screenshots that will be recognizable to diamond seekers familiar with pricescope. I don't want to include a link to the stone because it's not on hold. Be aware that I recognize this diamond has an black inclusion under the table. That's a compromise I'm willing to make due to favorable factors such my very tight budget and desire for a whiter stone.

To answer some of the pps' questions:

This vendor sadly can't produce an ASET or IS.

Regarding darkness under the table-- I don't see any but that might be because I don't recognize it.

This is not an OEC, but it's got some of the OEC form factors like high crown and shorter LGF/fatter arrows.

_2226.jpeg

_2227.jpeg
 

Tourmaline

Brilliant_Rock
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Joined
Nov 17, 2013
Messages
1,795
That inclusion would be an immediate disqualifier for me.
 

diamondseeker2006

Super_Ideal_Rock
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Jan 11, 2006
Messages
55,607
I wouldn't consider a mrb with a very good cut. Excellent cut is such a wide range already. You can find nicely cut stones with a 36 crown angle, but the pav angle at 40.6 means that you need an idealscope or ASET image to check leakage. Many will have leakage, but I have seen some without. I really don't think that stone has anything that would draw me to it. It looks like most mrb's to me. Are the lower girdle facets less than 75 or 76? Because you can find excellent cut stones with 75-76 and I don't really think you will see a huge difference in a 35 or 36 crown angle in reality. Overall cut quality is more important.

The H&A stones generally have crown angles up to 35. I do not recall seeing any above that in the brands we are familiar with here.
 

LightBright

Brilliant_Rock
Premium
Joined
Mar 11, 2013
Messages
710
Thanks Tourmaline and Diamondseeker and everyone who commented above.

This stone is a "bargain" stone with great high color and size, and it had an excellent HCA score so I thought I'd ask. The gemologist I talked to said it likely has great performance but couldn't really tell me why...

I think what we see here is that even if HCA is excellent in all categories, there are red flags that veterans are seeing due to the crown and pavilion being outside of ideal range.

Diamondseeker's comment that 35 crown angle doesn't look too different from 36 is true to my eyes, especially if I find a 56% table, which is what I actually prefer. Yes LGF on this stone is 75.

Having looked at hundreds of stones and plugged their angles into HCA, I think I can find one that conforms to the ideal parameters, and is eye clean to boot. Unfortunately, usually when I find a potentially eye clean one it gets snapped up. And I just looooved the color (E), size, puffy crown, and price of this particular stone. Time to keep looking, I guess!

Thanks everyone again.
 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
9,427
There is no reasonable crown angle in a mrb that will compensate for a 40.20 degree pavilion in a ring and the low % lower halves would make it worse.
What happens is the mains(arrows) being dark up close look neat when you can clearly see them but once you get out past 1/2 arm length(for most people, some eagle eyes may go a bit further) they should be well on the way to not obstructing because beyond the distance that the arrows are clearly visible dark arrows just make the diamond look less bright.

In an earring or pendant with the longer viewing distance this is much less of an issue which is why the hca rates it highly it would be an excellent earring or pendant stone and the distance would even help with the inclusion not being visible.

btw this issue will not be visible in IS or ASET images
 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
9,427
for the cut nerds around a 38 degree crown angle would be required and brightness would drop off around 8% compared to tolk.
With long lowers it can come closer to tolk in brightness.
 

LightBright

Brilliant_Rock
Premium
Joined
Mar 11, 2013
Messages
710
Thank you Karl_K for the information on too prominent arrows with this kind of angle combo... Fascinating!
 

gr8leo87

Shiny_Rock
Joined
Apr 24, 2015
Messages
381
The picture doesn't look as bad as those proportions warranted.
 

LightBright

Brilliant_Rock
Premium
Joined
Mar 11, 2013
Messages
710
Initially, these images had a GIA report stored with it, which was incorrect. It was a full report with different inclusions. I asked the vendor to correct it. The new GIA report is basic, with just table and depth and it doesn't include the "map" of inclusions so I can't link it to the image by the inclusion.

I've always liked the look of the diamond for the price, but I'm wondering if maybe the image is incorrect. Like, for example, does that look like a medium girdle? Just curious.
 

gr8leo87

Shiny_Rock
Joined
Apr 24, 2015
Messages
381
I get your concern about the medium girdle. It does look thick for a medium girdle. What's going on.
 
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