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Which Range of C/P Angles & Table% Work With Longer Lower Girdles?

purplecats2495

Rough_Rock
Joined
Oct 22, 2019
Messages
9
Hi All!

The discussions on this forum have some great threads on lower girdle lengths and fire - thanks! From everything I've read so far, I understand that longer lgl will result in more pin-fire flashes and shorter lgl will result in wider pavilion mains and therefore more bolder flashes (and whether pin-fire flashes or bold flashes are better is a personal choice). However, it was also mentioned somewhere that certain crown/pavilion angles and table sizes would work better with longer lower facets, but there was no information on what those numbers are.

If I like pin-fire but also want some bold flashes, what should I look for in terms of angles and table sizes to go with a longer lower girdle length? The diamond I'm considering has the following proportions. Would it exhibit a balance of pin-fire and bold flashes, or would there mostly be pin-fire flashes? Is there anything alarming about the associated angles, etc?

Size: 0.52 carats
Color: F
Clarity: VS2
Pavilion Angle: 40.9
Pavilion Height: 43.1
Crown Angle: 34.6
Crown Height: 15.4
Total Depth: 61.3
Table: 55.4
Star length: 54
Lower girdle length: 80
Girdle: 2.8


Thanks in advance for your help!!
 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
8,961
Very complex question.
I will try to post the long answer soon.
The short answer is that combo can work.
Do you have images?
 

purplecats2495

Rough_Rock
Joined
Oct 22, 2019
Messages
9
Thanks for your help, Karl!

There aren't any ASET/Idealscope or H&A images. I did check with a handheld ASET scope in the store and couldn't see any obvious light leakage, but don't have any pictures.

There is a video of the diamond as light passes through it, but the (.mp4) file can't uploaded. The best I could do was take screenshots at different parts of the video - please see the images below.

Thanks again!


1581663723383.png

1581663788817.png
 

sledge

Ideal_Rock
Premium
Joined
Apr 23, 2018
Messages
4,399
To upload video, create YouTube account. Upload MP4 there. Once done, simply post the link here, and it will show up as an embedded video.
 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
8,961
Video looks like cgi to me, asked some others to look at it.
 

purplecats2495

Rough_Rock
Joined
Oct 22, 2019
Messages
9
Thanks for taking a look at the video!

Aside from this particular diamond, do you have recommendations on how to evaluate diamonds with longer girdles, in general? In the bright lights of a jewelry store, diamonds with longer lgl should perform well, but would it be possible to evaluate or estimate whether they would still perform reasonably well in diffuse lighting?

From the Whiteflash website:

Arrows Diamond

In this view even differences between 78%, 80% and 82% LGF are distinct.
78% - Robust hearts with slight separation from arrowheads above (pointing to the culet).
80% - Narrower hearts with notable separation from arrowheads. Slight splits appear at the Vs.
82% - Narrow hearts with significant separation from arrowheads. Notable splits at the Vs.

Summary
As splits become larger in the Vs of the hearts, some performance qualities are lost in lower lighting conditions. Different configurations behave differently but in general terms our recommendation for best performance through a broad range of lighting conditions are for lower girdle lengths (GIA & AGS reports) 75-80%.



Questions:

1) What performance qualities are lost? (Assuming that different contrast patterns and broad vs pin-fire flashes are matters of preference, not performance)

2) How much difference would there be between say a 77 lower girdle length and 80 length, in diffuse lighting? I know it sounds silly to try to quantify and might not be possible, but to get an idea of what the trade-off would be, are we talking <5% difference? 10%? 50%? Potential dull rock in everyday lighting?

3) What configurations work well with longer lower girdle length?


Thanks again!
 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
8,961
Questions:

1) What performance qualities are lost? (Assuming that different contrast patterns and broad vs pin-fire flashes are matters of preference, not performance)

In theory larger arrows/mains return more light in low soft diffused lighting. Which is true when your looking at an OEC vs an MRB but small variations in lgf% its more theory than fact.

2) How much difference would there be between say a 77 lower girdle length and 80 length, in diffuse lighting? I know it sounds silly to try to quantify and might not be possible, but to get an idea of what the trade-off would be, are we talking <5% difference? 10%? 50%? Potential dull rock in everyday lighting?

Otherwise well cut 0% and depending on specifics the one with 80% lowers could be brighter. It is all with in the range of personality difference. The long lowers could even be better, see next answer.


3) What configurations work well with longer lower girdle length?

Pretty much any that has a matched crown/pavilion/table combo, but it really helps slightly shallow and slightly steep pavilions.
In a shallow pavilion stone it reduces and counteracts the increased response to obstruction some what.
In a steep pavilion stone it reduces the lower girdle angle which can reduce under table leakage.


Thanks again!

 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
8,961
Thanks Garry
It makes some pretty....... but useless images.
 

purplecats2495

Rough_Rock
Joined
Oct 22, 2019
Messages
9
Thanks Garry
It makes some pretty....... but useless images.
Yes, it has been very difficult trying to interpret this video - no idea how it relates to the ASET, Idealscope and H&A images we're used to. I ended up buying a handheld ASET scope from AGS to take to jewelry stores.
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
Joined
Aug 15, 2000
Messages
14,828
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