Girdle thickness chart - meaning of percentages

[pb]

Can someone copy and paste this girdle thickness chart into a new post with the numbers filled in? I can''t seem to find them anywhere. Please make a note of whether it applies to all diamond shapes or not. If it matters what shape it is, I''m interested in a princess cut. If there''s anything else that I might be missing on this subject, feel free to fill me and the other readers in. Thanks.

Extremely Thin = X.X % to X.X %
Very Thin = X.X % to X.X %
Thin = X.X % to X.X %
Medium = X.X % to X.X %
Slightly Thick = X.X % to X.X %
Thick = X.X % to X.X %
Very Thick = X.X % to X.X %
Extremely Thick = X.X % to X.X %

 

[jaz464]

This thread has info on AGS''s girdle chart:

https://www.pricescope.com/community/threads/girdle-size.36651/

 

[stebbo]

In addition to John's comments at that thread, AFAIK the GIA measure the girdle in mm's.

Here's the only table I've ever seen for the GIA:

Extremely Thin = 0.00mm to 0.08mm
Very Thin = 0.08mm to 0.12mm
Thin = 0.12mm to 0.15mm
Medium = 0.15mm to 0.20mm
Slightly Thick = 0.20mm to 0.23mm
Thick = 0.23mm to 0.33mm
Very Thick = 0.33mm to 0.40mm
Extremely Thick = 0.40mm up

Here's a topic for discussion:

I think both systems are flawed, especially AGS's, given that the primary purpose of the girdle is to protect against chipping.

The GIA's doesn't take into account how sharp the edge is between crown and pavilion. They leave that to their comments section.

The AGS's partially takes into account the crown/pavilion junction, but it also implies that a bigger diamond needs a thicker girdle than a smaller one. A larger diamond still has the same sharp junction as a smaller diamond (of equal angles), more of it need not be buffed off. Bigger AGS diamonds can thus store more unnecessary weight around the waist and get away with it, although they will keep a consistent, proportional look.

 

[jasper]

Martin Haske wrote an article about how the GIA's girdle thickness seemed to work a few years ago:
An Easy Way To Estimate Girdle Thickness at http://www.adamasgem.org/cut.html#girdle

I also wrote an article about how the percentage girdle thickness affects Tolkowsky's Diamond Design:
Diamond Girdles: How Adding a Girdle Changes Marcel Tolkowsky's Diamond Design at
http://www.folds.net/diamond_girdle/index.html

A big caveat
When these two articles were written, the GIA measured the overall girdle thickness (at the thick parts), whereas the AGS and the on-line vendors measured the girdle thickness at the thin parts. So Martin Haske's article measures the girdle at the thick parts, and my article measures the girdle thickness at the thin parts. The difference between the two measurements is about 1.7 % of the stone's diameter.

-- Jasper

 

[JohnQuixote]

Cool articles Jasper.

Here is a thread with graphics to explain girdle thickness in terms of range (rather than fixed measurements). It addresses how AGSL and GIA currently differ in measuring but I have an image somewhere that may be useful.

 

[JohnQuixote]

Date: 8/18/2007 12:24:55 PM
Author: stebbo
In addition to John's comments at that thread, AFAIK the GIA measure the girdle in mm's.

Here's the only table I've ever seen for the GIA:

Extremely Thin = 0.00mm to 0.08mm
Very Thin = 0.08mm to 0.12mm
Thin = 0.12mm to 0.15mm
Medium = 0.15mm to 0.20mm
Slightly Thick = 0.20mm to 0.23mm
Thick = 0.23mm to 0.33mm
Very Thick = 0.33mm to 0.40mm
Extremely Thick = 0.40mm up

Here's a topic for discussion:

I think both systems are flawed, especially AGS's, given that the primary purpose of the girdle is to protect against chipping.

The GIA's doesn't take into account how sharp the edge is between crown and pavilion. They leave that to their comments section.

The AGS's partially takes into account the crown/pavilion junction, but it also implies that a bigger diamond needs a thicker girdle than a smaller one. A larger diamond still has the same sharp junction as a smaller diamond (of equal angles), more of it need not be buffed off. Bigger AGS diamonds can thus store more unnecessary weight around the waist and get away with it, although they will keep a consistent, proportional look.

I believe they do take it into consideration Stebbo.Traditionally, 30 degrees was the ‘flagged’ angle but many of us here exercise caution before that! In their new system the GIA won’t give EX to any diamond with a crown angle
Garry has a great post (1/29/2007, 12:31:20 AM in this similar thread) with his rules for HCA.

I’m hear what you’re saying about larger diamonds, but there is more mass to them...I’m not a lab guy, but requiring proportional greater thickness along a larger diamond’s vulnerable edge makes sense to me; like a larger boat should have a thicker hull than a smaller boat. What do you think?

 

[stebbo]

Date: 8/18/2007 1:23:54 PM
Author: jasper
Martin Haske wrote an article about how the GIA''s girdle thickness seemed to work a few years ago:
An Easy Way To Estimate Girdle Thickness at http://www.adamasgem.org/cut.html#girdle

I also wrote an article about how the percentage girdle thickness affects Tolkowsky''s Diamond Design:
Diamond Girdles: How Adding a Girdle Changes Marcel Tolkowsky''s Diamond Design at
http://www.folds.net/diamond_girdle/index.html

A big caveat
When these two articles were written, the GIA measured the overall girdle thickness (at the thick parts), whereas the AGS and the on-line vendors measured the girdle thickness at the thin parts. So Martin Haske''s article measures the girdle at the thick parts, and my article measures the girdle thickness at the thin parts. The difference between the two measurements is about 1.7 % of the stone''s diameter.

-- Jasper

It always amazes me who''s listening!

Thanks for the pointer to Marty''s article - I remember the thread where Marty was trying to ''unwrap'' the girdle into 2d space and quantify it, but hadn''t seen the GIA graph this article showed. It''s interesting that Marty found that the graph converted to mm boundaries independent of the diameter, if as you say Jasper, they were measured at the thick parts. As the thin parts are related to the thick parts by a % of the diameter (eg 1.7% for standard brillianteering), this would mean now the girdle thickness boundaries measured at the thin parts would now become dependent on the diameter. DiamCalc however uses independent boundaries for measurements at the valleys.

I''d thank you for the pointer to your article as well, but I''d already enjoyed that many moons ago

You must have ranked better with google.

There are also differences in the thinner % boundaries shown in the AGS chart John posted to that used by DiamCalc and shown in Pricescope''s tutorial. All appear to be for the latest AGS system and I know the Octonus boys actively followed the changes that happened in this area. I wonder what the reason is...

 

[stebbo]

Date: 8/18/2007 3:07:12 PM
Author: JohnQuixote

I believe they do take it into consideration Stebbo.Traditionally, 30 degrees was the ‘flagged’ angle but many of us here exercise caution before that! In their new system the GIA won’t give EX to any diamond with a crown angle < 31.5 degrees or any girdle ranging to VTN.The AGS judges girdle durability as part of overall configuration (graded under ‘proportions factors’) and a dangerous situation won’t earn 0.32.2 is their cutoff for ideal light performance and that’s with 60-61% tables.They’re stricter with smaller table sizes.

I was more concerned with the Girdle Thickness grade itself--whether it takes enough into account to be meaningfully listed separately on a report, especially as the 'chipability' is taken into account in cut grades or mentioned in the comments. As far as looking at the grade to reveal weight retention, well the GIA's doesn't capture deceitful painting (again cut grade and comments gets that, even if they can wrongfully punish non-deceitful painting). The AGS's girdle thickness punishes both deceitful and non-deceitful painting.

I’m hear what you’re saying about larger diamonds, but there is more mass to them...I’m not a lab guy, but requiring proportional greater thickness along a larger diamond’s vulnerable edge makes sense to me; like a larger boat should have a thicker hull than a smaller boat. What do you think?

The way I see it John is that the diamond's mass would only come into it if the diamond was dropped, where its own momentum would be responsible for the damage (like with the boat). But in the normal case where a diamond is mounted securely on a much heavier ring, which is semi-rigidly moving with a hand, arm and body, the diamonds mass becomes irrelevant. As far as I could imagine, even during setting it's the prongs that will cause chipping, not the diamonds mass.

Having said that, possibly a bigger diamond might internally dampen a shock wave better? (I'm not saying well, just better).

 

[Garry H (Cut Nut)]

In addiution to what has been said, and as possible corrections -(have not read everything as I am skiing at present).

GIA previousluy assessed at at girdle valleys (thin points). They claim this is done by human estimation and not measuerd.

AGS previously measured at the thin points.

Now both labs use the thick points, but AGS measures the thinnest part too when it it too thin and a risk. But AGA uses now the same % system for all shapes - eg princess etc. And a .5ct thin is too thin in my book, where as a 10ct slightly thick is too thick.

GIA''s sysystem seems best to me, but if it is really done by human graders then could not be as consistant - but extra facets can easily trick their Sarin scanners- but not a good grader.

My own approach is to adjust chipping risk for girdle thickness and crown angle concurrently.