What''s the UGF range for classic girdle?

[gladstone]

We all know the ranges for tables, crown and pavillion angles for an ideal diamond. I have not found anywhere the ideal range of upper gridle facets angle for a classic girdle?

Thanks...

 

[Rhino]

Average between 40-43 degrees. Once it starts hitting 45 degrees on the steep end a red flag goes up here as well 39 degrees and under on the shallow end. That's when I want to take a closer look at the stones in question. It is an often neglected measurement that can impact the optics of diamonds that have the most cherry proportion sets.

 

[Cehrabehra]

Date: 10/19/2006 9:52:07 PM
Author: Rhino
Average between 40-43 degrees. Once it starts hitting 45 degrees on the steep end a red flag goes up here as well 39 degrees and under on the shallow end. That''s when I want to take a closer look at the stones in question. It is an often neglected measurement that can impact the optics of diamonds that have the most cherry proportion sets.

more info please!!

 

[pyramid]

This is probably a silly question but...

If the 39 degrees is on the shallower angle and the upper girdle facets would ideally be about 40-43 degrees, why are people told that a crown angle of like 29 degrees is too shallow with regard to chipping. The girdle at the narrowest point between upper girdle facets is below the centre of each upper girdle facet so the crown angle of 29 degrees at the kite facet would be at the widest point on the girdle? I know there is a reason but I just wrongly thought the crown angle being 29 degrees was affecting all along the girdle.

 

[oldminer]

The crown angle is generally a measure of the bezel facets, not the upper girdle facets. The UGF's are steeper than the bezel facets. Don't get them confused.

 

[pyramid]

Yes Oldminer, that is what I was meaning as they are steeper why does the crown angle which comes down to the wider part of the girdle affect chipping of a girdle. Or is it just that the 39 degrees on upper girdle facets (which Rhino said is too shallow) would be too shallow also?

edited to add: I was just thinking and maybe it is just that the 29 degrees even at the widest part of a 'thin' girdle would be too shallow. I should think these things out before I post I think

 

[Rhino]

Date: 10/19/2006 11:20:11 PM
Author: Cehrabehra

Date: 10/19/2006 9:52:07 PM
Author: Rhino
Average between 40-43 degrees. Once it starts hitting 45 degrees on the steep end a red flag goes up here as well 39 degrees and under on the shallow end. That''s when I want to take a closer look at the stones in question. It is an often neglected measurement that can impact the optics of diamonds that have the most cherry proportion sets.

more info please!!

Any more info and I''d have to have you sign a contract.

Jk ... what else would you like to know?

 

[Rhino]

Hi Pyramid,

No question is silly.

Date: 10/20/2006 8:16:10 AM
Author: Pyramid
This is probably a silly question but...

If the 39 degrees is on the shallower angle and the upper girdle facets would ideally be about 40-43 degrees, why are people told that a crown angle of like 29 degrees is too shallow with regard to chipping.

Good question. Interestingly GIA''s study of this subject found that the shallower angles did not contribute to durability problems. Girdle thickness that is too thin is the culprit according to GIA.

The girdle at the narrowest point between upper girdle facets is below the centre of each upper girdle facet so the crown angle of 29 degrees at the kite facet would be at the widest point on the girdle? I know there is a reason but I just wrongly thought the crown angle being 29 degrees was affecting all along the girdle.

It should but this isn''t always the case. In a classic girdle it would. In fact on a classic girdle the girdle thickness at both the crown mains (your point) and also at the halves should be around the same girdle thickness and as Dave pointed out is also steeper angles than the crown mains. In this attached Sarin note the girdle thickness at the bezels and the halves are very close in thickness to each other. 3.0% and 3.1% with the average girdle thickness in the valley''s being 1.4%. Average upper half angles being around 42 degrees. No red flags in this instance.

 

[Rhino]

The girdle graphs from the OGI scanner do a great job of showing girdle variations. This is one of a classic girdle in the bottom right hand corner of this graphic. While it may appear a little lumpy, it is a nice even consistent thickness. Wait till you see how that changes when you alter the upper half angles.

 

[Rhino]

In this stone (with very cherry main proportions) the upper half angles that hit that 45 degree threshold. This stone would and does not make GIA Ex or AGS ideal in the new system of things. Note the relation between the girdle thickness at the mains in relation to the halves now.

 

[Rhino]

Now look at the OGI girdle graph.

You can see how the girdle is thicker at the bezels/mains than at the halves and the graph is notably different than the classic girdle. The OGI actually generates 2 other girdle reports that show even more detail.

 

[Rhino]

On the flip side here''s where the upper halves go towards the shallow end. In this stone 36 degrees. The girdle thickness at the halves and bezels are reversed from the prior stone as they are now thicker at the halves and narrower at the mains. This stone does not make GIA Ex but does make AGS Ideal. Other experts in the field have noted the visual change in appearance in stones like this.

 

[Rhino]

The OGI girdle graph. These differences can also be seen in other technologies such as ASET. Hope that helps.

 

[Rhino]

Date: 10/20/2006 8:52:29 AM
Author: Pyramid
Yes Oldminer, that is what I was meaning as they are steeper why does the crown angle which comes down to the wider part of the girdle affect chipping of a girdle. Or is it just that the 39 degrees on upper girdle facets (which Rhino said is too shallow) would be too shallow also?

edited to add: I was just thinking and maybe it is just that the 29 degrees even at the widest part of a ''thin'' girdle would be too shallow. I should think these things out before I post I think

Don''t feel bad Pyramid. It''s always ok to ask questions. Questions encourage research and learning is always a good thing. Shallow angles on the crown mains or the halves don''t necessarily mean durability issues that I know of. It has been suggested that crown angles under 30 degrees is detriment to chipping but honestly I''ve never tried to break a stone with angles under 30 degrees. At least not yet anyway.

Kind regards,

 

[pyramid]

Thank you for that Rhino, it is interesting to see the differences. I see from the titles at the foot of your pages that these are examples of painting and digging. I was just wondering if it was something to do with the rough but then wondered about painting digging and I see that is what you have titled them.

 

[jasper]

Date: 10/19/2006 8:15:08 PM
Author: gladstone
We all know the ranges for tables, crown and pavillion angles for an ideal diamond. I have not found anywhere the ideal range of [upper girdle facet angles] for a classic girdle?.

Date: 10/19/2006 9:52:07 PM
Author: Rhino
Average between 40-43 degrees. Once it starts hitting 45 degrees on the steep end a red flag goes up here as well 39 degrees and under on the shallow end. That's when I want to take a closer look at the stones in question. It is an often neglected measurement that can impact the optics of diamonds that have the most cherry proportion sets.

Gladstone, Jonathan, and Dave,

Thank you for discussing this topic. I am curious about it, since it addresses one of the weaker points in Tolkowsky's Diamond Design.

The ideal upper girdle facet angle depends on several things:
1) Whether either girdle end of the upper girdle facet is higher or lower than the other. I assume that they are theoretically the same height on a "classic" girdle -- but girdle graphs like the ones Rhino has posted leave room for doubt.
2) The crown angle.
3) The star : upper girdle ratio.
4) The table ratio.
5) Whether the kite facet has four sides or six sides.
As Rhino often points out, there is considerable variation in these measurements from one facet to another of the same diamond -- not to mention the inevitable measurement errors. Also, another way of looking at it is that the star : upper girdle ratio depends on the upper girdle facet angle, and the rest of these things.

Tolkowsky gives a value for the upper girdle facet angle of 42 degrees (for a crown angle of 34.5 degrees), which is consistent with Rhino's experience.

Tolkowsky gives a value for the star facet angle of 15 degrees (for a crown angle of 34.5 degrees), which seems odd to me (and others).

But as Thomas Kabele (and others) have pointed out, Tolkowsky does not explain how he came up with the star facet and upper girdle facet angles -- and they seem inconsistent with some other assumptions about the "classic girdle".

My calculations suggest that with:
1) Both girdle ends of the upper girdle facet at the same height
2) a crown angle of 34.5 degrees
3) a star : upper girdle ratio of about 50 : 50
4) a table ratio in the mid 50 percent range
5) a 4-sided kite facet
That the star facet angle is about 19 to 20 degrees,
and the upper girdle facet is about 39 degrees.

http://www.folds.net/diamond_design/index.html#faceting
http://www.folds.net/diamond_design/index.html#ideal

The fact that my calculated upper girdle facet angle does not match Rhino's observed values suggests that there is either something wrong with my assumptions, or with our assumptions about how the measurements are done.

-- Jasper

Edited to fix crown angle: 34.5 degrees, not 40.75 degrees

 

[jasper]

Oops. I have fixed the crown angle in my previous post.

-- Jasper