vendors and appraisers: shallow crowns and thin girdles

[strmrdr]

After John''s presentation last night I did some searching.
What I found was why consumers were recommending against thin girdles at times.

Garry is on the record saying that shallow crowned diamonds under around 33 to 33.5 degrees need a thicker girdle for durability reasons.
Dave A. is on the record that shallow crowns are a durability issue.

We need to get to the bottom of this so the best advice can be given here.

Is a thin girdle an issue with shallow crowns?

 

[Ellen]

Glad you asked this, I''ve been wondering the same. And another question, as a few examples have come up recently.

Where is the cut off for a shallow crown, the absolute lowest one should consider?

 

[Lorelei]

Date: 1/26/2007 6:33:51 AM
Author: Ellen
Glad you asked this, I''ve been wondering the same. And another question, as a few examples have come up recently.

Where is the cut off for a shallow crown, the absolute lowest one should consider?

Ditto - good point Storm.

 

[diagem]

Date: 1/26/2007 6:24:44 AM
Author:strmrdr
After John''s presentation last night I did some searching.
What I found was why consumers were recommending against thin girdles at times.

Garry is on the record saying that shallow crowned diamonds under around 33 to 33.5 degrees need a thicker girdle for durability reasons.
Dave A. is on the record that shallow crowns are a durability issue.

We need to get to the bottom of this so the best advice can be given here.

Is a thin girdle an issue with shallow crowns?

Common sense...

The thinner the girdle plane + the shallower the crown angle = the weaker the girdle.

But that doesnt mean that a thin girdle with an extra deep crown angle or pavillion angle..., let say 40+ deg. doesnt ship!
As perfect examples see the Old-Mine Cuts and Old Euro''s. i have not seen yet an Antique Diamond that does not possess what I call "Antiquity evidence" on either the girdles or facet junctions.

Diamond is the hardest substance but it is as fragile as it looks and feels...

The only "partly" way to delay chips on a girdle is by having the diamond set by a "proffesional" setter/Jeweler.
You cannot avoid the wear and tear process.

The good side to it is: some jewelery seekers think the wear and tare (to a limit) is quite appealing....

Hope my answer makes sense...

 

[Garry H (Cut Nut)]

Date: 1/26/2007 6:24:44 AM
Author:strmrdr

Garry is on the record saying that shallow crowned diamonds under around 33 to 33.5 degrees need a thicker girdle for durability reasons.

Is a thin girdle an issue with shallow crowns?

Yes, and it is a sliding scale - not a thin = this and there is no single cut off.

The desk top version of HCA has all my formulae in it (with a lot of Leonids help).

I consider the total of crown angle and pavilion angle - so if the stone has a good cut at say 30 degrees - then the pavilion will be deeper than 41 degrees and the total is 71 degrees.

I do not have the formulae to hand - but such a stone would begin being penalized after the girdle is less than 3% or slightly thick, and a warning appears soon after as the girdle gets thinner.

The online version does some of this also - but your input is the total depth because we can not trust consumers to enter the correct girdle thicknes (valley vs mains etc).

This is based on my experiance of having seen many chipped diamonds.

I have no problem with a thick girdled 25 degree crown angled stone for durability.

 

[oldminer]

Thick girdles help prevent chipping, but make the diamond less well cut. Shallow crown angles make a stone less durable, but this certainly is counteracted, to verying extents, by thick girdles. The problem I have is that thickening of the girdle is a poor cutting choice for maximizing beauty and spread, and a shallow crown angle on the majority of cutting styles it appears on hurts durability. This is the reality of available stones, not hypothetical stones.

Its difficult to imagine anyone, except Garry, wanting to bother with shallow crown, thick girdle diamonds and calling them "special" in some way that the entire balance of the trade would look at and say, What, are you joking?". I know Garry can prove that some of these strangely configured diamonds have huge beauty potential, Especially ones with normal girdle thickness and shallow crown angles. They could be excellent for earrings or pendants, but I would question their durability in a ring.

In the end, I continue to see the Best diamonds cut to configurations we all accept as normal. Unusual cutting styles may provide some limited alternatives, but there probably will be some trade offs that need to be compromised upon or explained.

 

[strmrdr]

The problem is Dave that at least 4 of these stones have came up recently so someone is cutting them.
Near H&A to H&A 56-58 tables 32.5-33.5 crowns and 40.6-40.8 pavilions.
If we are going to be seeing more of these we need to know how to properly advise the people looking at them.
So far its been a case by case basis and seeking Garry''s advise for the most part which is good but not everyone considering such a stone is going to post here.
The lurkers far outnumber the posters here.
Unfortunately it is not as easy as it seems to come up with good advise on these.
You could be right and rejecting them may be the answer but i''m not sure that''s good advise if there is no real problem with the stone.

Not to pick on you but im trying to get a handle on this and when our experts disagree it becomes hard for the consumers here to know what to do.

How do you feel we should handle this?
Do you feel outright rejection is the right answer? If so could you expand on why or why not.

Garry if you could answer the same questions it would help also.
As well as any other expert who would be willing too help out.

 

[Ellen]

Ditto to everything strm said.

 

[oldminer]

It is the general contention that crown angles below 30 degrees have durability problems. A 32.5 degree crown is therefore not what I was discussing. If one can get high performance without treading on the durability issue, then I say go for it. If one can avoid thick or even thicker girdles that is also a good thing. To trade off a standard well cut stone, for an unusually cut shallow crown or thick girdle is not nearly as good a thing to do. Shallow girdles by GIA definition are below 30 degrees and that''s how I use the term.

I would not be surprised to find a 32.5 degree crown along with a thin girdle might be more prone to chipping than a steeper crown angle stone with the same girdle, but I have no factual data to back up my speculation.

 

[diagem]

Date: 1/26/2007 9:22:30 AM
Author: oldminer
It is the general contention that crown angles below 30 degrees have durability problems. A 32.5 degree crown is therefore not what I was discussing. If one can get high performance without treading on the durability issue, then I say go for it. If one can avoid thick or even thicker girdles that is also a good thing. To trade off a standard well cut stone, for an unusually cut shallow crown or thick girdle is not nearly as good a thing to do. Shallow girdles by GIA definition are below 30 degrees and that''s how I use the term.

I would not be surprised to find a 32.5 degree crown along with a thin girdle might be more prone to chipping than a steeper crown angle stone with the same girdle, but I have no factual data to back up my speculation.

Makes total sense!

I dont think there is any scientific formula to this question...
Shallow crowns below 30 deg. by GIA definition is aimed towards the face-up appearance and not durability. (So i think and hope.)

A diamond can chip,break (split on the natural plane) just by falling on a hard surface..., and not just on hitting the girdle plane, but hitting anywhere, that was proven unfortunately numerous times.

 

[strmrdr]

Date: 1/26/2007 9:22:30 AM
Author: oldminer
It is the general contention that crown angles below 30 degrees have durability problems. A 32.5 degree crown is therefore not what I was discussing. If one can get high performance without treading on the durability issue, then I say go for it. If one can avoid thick or even thicker girdles that is also a good thing. To trade off a standard well cut stone, for an unusually cut shallow crown or thick girdle is not nearly as good a thing to do. Shallow girdles by GIA definition are below 30 degrees and that''s how I use the term.

I would not be surprised to find a 32.5 degree crown along with a thin girdle might be more prone to chipping than a steeper crown angle stone with the same girdle, but I have no factual data to back up my speculation.

Thanks for the clarification Dave.

 

[oldminer]

The comment GIA puts, or used to put, on their reports about shallow crown angles read: Crown angle below 30 degrees". The reason this was there was to highlight the durability issue involved with such stones. It had nothing to do with appearance. Most shallow crown rounds are shallow in total depth as well. Princess cuts often have very shallow crown angles with very deep/steep pavilions. We do see breakage of pointed corners on princess cuts and I am not at all sure if the occurence of point chipping is related to crown angle or only to point sharpness.

 

[diagem]

Date: 1/26/2007 10:03:58 AM
Author: oldminer
The comment GIA puts, or used to put, on their reports about shallow crown angles read: Crown angle below 30 degrees''. The reason this was there was to highlight the durability issue involved with such stones. It had nothing to do with appearance. Most shallow crown rounds are shallow in total depth as well. Princess cuts often have very shallow crown angles with very deep/steep pavilions. We do see breakage of pointed corners on princess cuts and I am not at all sure if the occurence of point chipping is related to crown angle or only to point sharpness.

Doesnt make sense to me personaly but i could be.... i never checked...

In regards to Princess cuts i would fully assume that its pointed corners are being chipped due to "point sharpness", the first girdle step facet on the princess crown is usually a deep angle (but of small size.) I dont think i ever saw a comment of this sorth on a princess GIA report.

 

[diagem]

By the way David, are the GIA comments on "over 40 degrees " too for durability reasons??

 

[oldminer]

No, the comment used to be on round diamonds with crown angles in excess of 38 degrees. It may no longer be there. It had nothing to do with durability. It had to do with the stone being a lumpy make (probably too deep and not a good spread for the weight). Steep crown angles have a similar effect on spread as thick girdles have. The spread becomes less beneficial.

 

[pyramid]

/

 

[JohnQuixote]

Date: 1/26/2007 6:24:44 AM
Author:strmrdr
After John's presentation last night I did some searching.
What I found was why consumers were recommending against thin girdles at times.

Garry is on the record saying that shallow crowned diamonds under around 33 to 33.5 degrees need a thicker girdle for durability reasons.
Dave A. is on the record that shallow crowns are a durability issue.

We need to get to the bottom of this so the best advice can be given here.

Is a thin girdle an issue with shallow crowns?

I don’t think there’s any disagreement Strm (except in defining shallow).

At the top of the session I said my comments pertain to rounds with a top cut grade.You’re in good hands with the major labs:The GIA won’t give EX to any diamond with a crown angle
Dave, Garry & DG covered the bases.30 degrees has been the traditional ‘flagged’ angle but many of us exercise caution before that.

Remember also that girdle thickness is a range.If a CA/girdle combo concerns you take a closer look.A girdle graded “thin” may be 1 micron away from “medium.”It’s a sliding scale, like many things in this biz.

 

[JohnQuixote]

For anyone interested, here''s GIA definitions:

Slightly shallow crown = at or near 31.5
Moderately shallow crown = at or near 26.5
Shallow crown = at or near 22.0
Very shallow crown = at or near 20.0
Extremely shallow crown < 20.0

 

[Ellen]

Date: 1/27/2007 4:24:30 PM
Author: JohnQuixote



I don’t think there’s any disagreement Strm (except in defining shallow).

At the top of the session I said my comments pertain to rounds with a top cut grade.You’re in good hands with the major labs: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 its overall configuration assessment and a dangerous situation won’t earn 0.I believe 32.2 is their cutoff for ideal light performance anyway.

Dave, Garry & DG covered the bases.30 degrees has been the traditional ‘flagged’ angle but many of us exercise caution before that.

Remember also that girdle thickness is a range.If a CA/girdle combo concerns you take a closer look.A girdle graded “thin” may be 1 micron away from “medium.”It’s a sliding scale, like many things in this biz.

This is what I''m wanting to know. Exactly, or close to, WHERE do you start cautioning a poster?

 

[kcoursolle]

Date: 1/27/2007 4:36:50 PM
Author: Ellen

Date: 1/27/2007 4:24:30 PM
Author: JohnQuixote




I don’t think there’s any disagreement Strm (except in defining shallow).

At the top of the session I said my comments pertain to rounds with a top cut grade.You’re in good hands with the major labs: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 its overall configuration assessment and a dangerous situation won’t earn 0.I believe 32.2 is their cutoff for ideal light performance anyway.

Dave, Garry & DG covered the bases.30 degrees has been the traditional ‘flagged’ angle but many of us exercise caution before that.

Remember also that girdle thickness is a range.If a CA/girdle combo concerns you take a closer look.A girdle graded “thin” may be 1 micron away from “medium.”It’s a sliding scale, like many things in this biz.

This is what I''m wanting to know. Exactly, or close to, WHERE do you start cautioning a poster?

I''m wondering the same thing ellen, GIA seems to be less strict...but I''ve been warning psers when it''s more shallow than 32.5...maybe i''m being too strict.

 

[JohnQuixote]

Date: 1/27/2007 4:36:50 PM
Author: Ellen

Date: 1/27/2007 4:24:30 PM
Author: JohnQuixote




I don’t think there’s any disagreement Strm (except in defining shallow).

At the top of the session I said my comments pertain to rounds with a top cut grade.You’re in good hands with the major labs: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 its overall configuration assessment and a dangerous situation won’t earn 0.I believe 32.2 is their cutoff for ideal light performance anyway.

Dave, Garry & DG covered the bases.30 degrees has been the traditional ‘flagged’ angle but many of us exercise caution before that.

Remember also that girdle thickness is a range.If a CA/girdle combo concerns you take a closer look.A girdle graded “thin” may be 1 micron away from “medium.”It’s a sliding scale, like many things in this biz.

This is what I''m wanting to know. Exactly, or close to, WHERE do you start cautioning a poster?

I think that''s the issue Ellen... I would need more info, preferably the diamond in-hand, before promoting or condemning it. For example, CA X with girdle Y% may be of concern, but CA X with girdle Y+0.4% might be just fine (where Y and Y+0.4% fall into the same thickness). I''d also like to know how much variance there is in the CA? What are the symmetry variations (is it wavy girdle for instance)? Is there bearding? Is this a diamond with feathers? etc etc.

I understand the desire for decisive answers but diamonds aren''t cast identically. Just like SI clarity calls, further hands-on analysis is necessary for individual stones in such situations.

 

[JohnQuixote]

Date: 1/27/2007 4:44:05 PM
Author: kcoursolle

I'm wondering the same thing ellen, GIA seems to be less strict...but I've been warning psers when it's more shallow than 32.5...maybe i'm being too strict.

I don't think a storm warning (no pun intended)

is unreasonable K. "Bears further inspection" is fine at that mark, particularly since it's in that range the labs interpret as the borderline for best performance.

 

[strmrdr]

To close this thread out is it acceptable to everyone that with crown angles under 32.5 with thin girdles that the right response would be to recomend they talk to the vendor about durability issues and or post the pavilion angle and the girdle range and if there are any naturals on the girdle if the vendors response is lacking.
With an appraisal recomeded if the consumer is still concerned.

 

[Garry H (Cut Nut)]

Silly me - i have described the basics of HCA''s girdle calculations here http://diamond-cut.com.au/17_girdle.htm
It was 7 years ago - and uses the girdle thickness at the thin part.
I could review this today I think.

Meanwhile Leonid has put this onto his rather long list of things to do (he has a lot of development work for the communication program) - to open the back of HCA and see what our rules are with the idea of making a useful table for the tutorial.

The simple rule you posted is OK Storm - but we can do better I think

 

[Garry H (Cut Nut)]

Date: 1/27/2007 4:33:15 PM
Author: JohnQuixote
For anyone interested, here''s GIA definitions:

Slightly shallow crown = at or near 31.5
Moderately shallow crown = at or near 26.5
Shallow crown = at or near 22.0
Very shallow crown = at or near 20.0
Extremely shallow crown quote]
Actually john it seems GIA and moi are in agreement for once

Here is my promised graph that underlies the desk top HCA version.

the lower axis uses the total crown and pavilion angle because if a diamond has a very deep 43 degree pavilion angle and say a 30 degree crown angle then it is not as likely to chip as if the stone is 30 with 39 degrees.
For pscopers convenience I have added a line with crown angle for 41 degree pavilion angle.

Within each of these bands the following warnings are given:

0.0 - 0.5 Warning – The girdle of this diamond may chip if worn in an exposed ring setting. Warn jewelers to handle with care.
0.5 - 1.0 Warning – The girdle of this diamond is likely to chip if worn in an exposed ring setting. Warn jewelers to handle with care.
1.0 - 1.5 Warning – The girdle of this diamond is very likely to chip if worn in an exposed ring setting. Warn jewelers to handle with care.
1.5 - 2.0 Warning – The girdle of this diamond is extremely likely to chip if worn in an exposed ring setting. Warn jewelers to handle with care.
> 2.0 Warning - Reject this diamond because of an extreme risk of chipping.

Some of you will say "we dont like diamonds with those proportions" (in a whiney wingey voice).

And I say to you "I designed a cut grading system that grades almost all diamonds with a demacratic process". But essentially when these penalties are added to other light performance and possibly spread penalties - we do get a commonsense grade.

Would it be helpful if this was placed in the Tutorial or somewhere?
If so any computer wizzes who can do it better an i can?

(BTW John P - do you need a reminder for something else?)

 

[JulieN]

more kinks for the angles>31 and girdles<thin ?

 

[Ellen]

Date: 1/27/2007 5:44:03 PM
Author: JohnQuixote

I think that''s the issue Ellen... I would need more info, preferably the diamond in-hand, before promoting or condemning it. For example, CA X with girdle Y% may be of concern, but CA X with girdle Y+0.4% might be just fine (where Y and Y+0.4% fall into the same thickness). I''d also like to know how much variance there is in the CA? What are the symmetry variations (is it wavy girdle for instance)? Is there bearding? Is this a diamond with feathers? etc etc.

I understand the desire for decisive answers but diamonds aren''t cast identically. Just like SI clarity calls, further hands-on analysis is necessary for individual stones in such situations.

John, I missed this. Thanks for responding, it helps.

 

[Eva17]

? regarding the statement: crown angles are greater than 40 deg. I see this on lots of cushions. is this a precaution on that particular cut?

Thanks!

 

[JohnQuixote]

It's a fairly steep crown Eva so no worries in relation to the shallow discussion.

Date: 1/29/2007 12:31:20 AM
Author: Garry H (Cut Nut)

(BTW John P - do you need a reminder for something else?)

Jiminy Cricket. I forgot to send it. 'Tis on my flash drive; will send tomorrow.