GIA EX Symmetry = tight variance in cut angles?

[hytso]

Hi all, after reading hundreds of post in PS and articles on cut of diamonds I finally post my first post in PS!

I have been looking for a 1ct RB diamond for my proposal and after months of surfing I finally find one that looks perfect on paper which at the same time fits my budget. Below are the proportions:

GIA Cut/Polish/Symmetry: 3 EX
Table: 57%
Depth: 61.4%
Crown angle: 34.5 degree
Pavilion angle: 40.8 degree
Star: 50%
Lower Half: 80%

The vendor cannot provide anything other than the GIA cert. No IS image, no ASET and no Helium / Sarin report. However I still think this is a safe purchase because:

1. Yes I do not have a Sarin report and GIA is round up the angles, but the range of the true crown angle (34.25 - 34.75) and the true pavilion angle (40.71 - 40.89) would still give me an excellent proportion for max light return.

2. Yes there is no upper girdle / lower girdle angle given but a GIA EX cut should have elminated diamonds with Painted Girdle / Dug Out Girdle so there should be no problem?

3. Yes I know if the diamond has strong strain or graining this can impact the optical properties of the diamond in a negative way but I believe this is a rare occasion?

My only doubt would be on the variance on the cut angles as the cut angles shown on a GIA cert is only an average of all the angles. So, does a GIA EX symmetry guarantee a tight variance in the cut angles?

Sorry for the long post. Appreciate for all your professional comments. A month ago I couldn't distinguish which is the crown and which is the pavilion of a diamond so please bare with me if any of the above comment is wrong

 

[WishfulThinkingOne]

Hi all, after reading hundreds of post in PS and articles on cut of diamonds I finally post my first post in PS!

I have been looking for a 1ct RB diamond for my proposal and after months of surfing I finally find one that looks perfect on paper which at the same time fits my budget. Below are the proportions:

GIA Cut/Polish/Symmetry: 3 EX
Table: 57%
Depth: 61.4%
Crown angle: 34.5 degree
Pavilion angle: 40.8 degree
Star: 50%
Lower Half: 80%

The vendor cannot provide anything other than the GIA cert. No IS image, no ASET and no Helium / Sarin report. However I still think this is a safe purchase because:

1. Yes I do not have a Sarin report and GIA is round up the angles, but the range of the true crown angle (34.25 - 34.75) and the true pavilion angle (40.71 - 40.89) would still give me an excellent proportion for max light return.

2. Yes there is no upper girdle / lower girdle angle given but a GIA EX cut should have elminated diamonds with Painted Girdle / Dug Out Girdle so there should be no problem?

3. Yes I know if the diamond has strong strain or graining this can impact the optical properties of the diamond in a negative way but I believe this is a rare occasion?

My only doubt would be on the variance on the cut angles as the cut angles shown on a GIA cert is only an average of all the angles. So, does a GIA EX symmetry guarantee a tight variance in the cut angles?

Sorry for the long post. Appreciate for all your professional comments. A month ago I couldn't distinguish which is the crown and which is the pavilion of a diamond so please bare with me if any of the above comment is wrong

You have asked a lot of questions about fine details but I don't know what your goal is with the purchase.
The average numbers are right within the very pinnacle of the Tolkowsky Ideal range and GIA/AGS ideal proportions but that doesn't really help answer your questions.

Do you have to have the finest cut or are you trying save money by getting a "Near Ideal"?
Have you compared near HA to Ideal branded diamonds with perfect images?
What makes this particualr diamond the one and such a deal?

There are many fine vendors here who will provide for you pictures and images on HA and near HA and they are usually quite competitive in price as well. So the question becomes what are your alternatives and at what price?

Do you need to have images and support that your diamond is an "Tolkosky Ideal" cut? If so you need to go to a vendor that will provide this information.

In addition to your listed parameters girdle thickness, visibility of inclusions, Color, and the vendors tradeup and buyback and return policies should be considered in the purchase as well.

I don't think anyone would be able to give an informed opinion until they know what your budget and goals are.

 

[Unknown]

If it is a eye clean stone and the vendor has good return/refund policy. I think it is worth to give a try.

 

[yssie]

Hi all, after reading hundreds of post in PS and articles on cut of diamonds I finally post my first post in PS!

I have been looking for a 1ct RB diamond for my proposal and after months of surfing I finally find one that looks perfect on paper which at the same time fits my budget. Below are the proportions:

GIA Cut/Polish/Symmetry: 3 EX
Table: 57%
Depth: 61.4%
Crown angle: 34.5 degree
Pavilion angle: 40.8 degree
Star: 50%
Lower Half: 80%

The vendor cannot provide anything other than the GIA cert. No IS image, no ASET and no Helium / Sarin report. However I still think this is a safe purchase because:

1. Yes I do not have a Sarin report and GIA is round up the angles, but the range of the true crown angle (34.25 - 34.75) and the true pavilion angle (40.71 - 40.89) would still give me an excellent proportion for max light return.
No - GIA averages around eight sections of stone first, then rounds (crown up/down to nearest 0.5deg, pav up to 0.2deg). So we only know that the average crown rounds to 34.5 and the average pav rounds up to 40.8, we still have no idea what sorts of ranges went into those averages (34.5/34.4/34.6... or 33/34/35/36?) and how that affects optical symmetry. See my two posts in this thread for specific examples - [URL='https://www.pricescope.com/community/threads/gia-excellent-cut-but-5-5-hca-what-gives.169515/']https://www.pricescope.com/community/threads/gia-excellent-cut-but-5-5-hca-what-gives.169515/[/URL]

2. Yes there is no upper girdle / lower girdle angle given but a GIA EX cut should have elminated diamonds with Painted Girdle / Dug Out Girdle so there should be no problem?
Yes, GIA is not lenient with what it considers "excessive brillianteering", but it's certainly possible to have an overly steep or shallow ugf/lgf combination without "excessive girdle treatments". So no, GIA's stance on brillianteering doesn't guarantee of what you're asking about, specifically.

3. Yes I know if the diamond has strong strain or graining this can impact the optical properties of the diamond in a negative way but I believe this is a rare occasion?
I can't comment on this, but it seems like something of this sort would be notated in the comments, or on the clarity plot as an inclusion? Perhaps someone with more knowledge can clarify..

My only doubt would be on the variance on the cut angles as the cut angles shown on a GIA cert is only an average of all the angles. So, does a GIA EX symmetry guarantee a tight variance in the cut angles?
No. GIA is evaluating facet-meet symmetry, which is a requirement for (not always, depending, but in basic principle) but not a guarantee of optical symmetry. Neither GIA nor AGS consider optical symmetry, which I believe is what you're asking about (those arrows patterns you see in so many pics here on PS?)... a stone may earn EX symmetry and have poor optical symmetry, or be demoted to VG sym for a number of reasons on the report and still show nice hearts and arrows patterns. This is something you'll need more information for.
http://www.jamesallen.com/diamonds/G-SI1-Ideal-Cut-Round-Diamond-1397149.asp

Sorry for the long post. Appreciate for all your professional comments. A month ago I couldn't distinguish which is the crown and which is the pavilion of a diamond so please bare with me if any of the above comment is wrong
I am not a professional.

None of this means much if all you're looking for is a pretty stone, and you're not too concerned with the details - if that's the case you're not going to wind up with Ugly with a GIA EX, for sure! If you *do* want to investigate the details, though, that's when these sorts of distinctions become interesting...

 

[hytso]

GIA averages around eight sections of stone first, then rounds (crown up/down to nearest 0.5deg, pav up to 0.2deg). So we only know that the average crown rounds to 34.5 and the average pav rounds up to 40.8, we still have no idea what sorts of ranges went into those averages (34.5/34.4/34.6... or 33/34/35/36?)[/b]

Sorry to have confused everyone with my lengthy post and thank you for all your comments/opinions.

In fact my question is nothing about optical symmetry. As Yssie said, GIA averages all sections and give an average crown angle / pavilion angle on their cert. So, there is a variance within all the sections (a 34.5 crown angle on the cert could be the average of 34.5/34.6/34.4..... or 35/ 34/ 36/ 33....)

My question is, whether a GIA EX symmetry guarantees a tight variance within all the sections? (ie. 34.5/34.6/34.4....)

 

[0-0-0]

Wow, great examples Yssie.

GIA averages around eight sections of stone first, then rounds (crown up/down to nearest 0.5deg, pav up to 0.2deg). So we only know that the average crown rounds to 34.5 and the average pav rounds up to 40.8, we still have no idea what sorts of ranges went into those averages (34.5/34.4/34.6... or 33/34/35/36?)[/b]

Sorry to have confused everyone with my lengthy post and thank you for all your comments/opinions.

In fact my question is nothing about optical symmetry. As Yssie said, GIA averages all sections and give an average crown angle / pavilion angle on their cert. So, there is a variance within all the sections (a 34.5 crown angle on the cert could be the average of 34.5/34.6/34.4..... or 35/ 34/ 36/ 33....)

My question is, whether a GIA EX symmetry guarantees a tight variance within all the sections? (ie. 34.5/34.6/34.4....)

IMO your question has everything to do with optical symmetry. In my experience shopping for my stone, tighter variances generally correlate with better optical symmetry. But as you can see from the examples Yssie posted, GIA EX doesn't guarantee better optical symmetry, and I have also seen plenty of GIA EX in person that lacks optical symmetry as well. As such, I don't think GIA EX guarantees a tight enough variance among the sections to make a difference, even if GIA EX takes the variances into account.

 

[Karl_K]

My question is, whether a GIA EX symmetry guarantees a tight variance within all the sections? (ie. 34.5/34.6/34.4....)

No

 

[WishfulThinkingOne]

GIA averages around eight sections of stone first, then rounds (crown up/down to nearest 0.5deg, pav up to 0.2deg). So we only know that the average crown rounds to 34.5 and the average pav rounds up to 40.8, we still have no idea what sorts of ranges went into those averages (34.5/34.4/34.6... or 33/34/35/36?)[/b]

Sorry to have confused everyone with my lengthy post and thank you for all your comments/opinions.

In fact my question is nothing about optical symmetry. As Yssie said, GIA averages all sections and give an average crown angle / pavilion angle on their cert. So, there is a variance within all the sections (a 34.5 crown angle on the cert could be the average of 34.5/34.6/34.4..... or 35/ 34/ 36/ 33....)

My question is, whether a GIA EX symmetry guarantees a tight variance within all the sections? (ie. 34.5/34.6/34.4....)

No.
The GIA symetry grade is referring to physical external symmetry. (facet meet points, table alignment, culet being centred etc.)
It has no correlation with the variance in crown and pavilion angles.