help- girdle % on small stones

[mm]

I am considering buying some small round brilliants (approx 1/3 ct each) to make a triple diamond ring (1 ct tw).

Most of the diamonds in this size range on the known-HCA search have girdles between 0.7% and 2.5- which rate between thin and slightly thick on AGS rules- all apparently ok for durability.

However, because the size of these diamonds is quite small, these exact same girdles are rated by GIA as extremely thin, very thin and thin (assuming a 4.4mm diameter- 0.7% girdle thickness is only 0.03mm).

So I assume this meant they are actually NOT safe from chipping when considering this sized stone.

What is the minimum mm thickness of girdle that I should look for in stones of between 0.3 and 0.4ct?

cheers
Matt

 

[valeria101]

0.007 x 4.4mm = 0.0308mm
0.007 x 6.5mm = 0.0455mm

AGS's "thin" is safe for a 1 carat stone ( ~ 6.5mm diameter) and this would be 0.045mm - the same would be 0.1% for your stones which is still marked "thin" by AGS. Probably choosing "medium" or "thin-medium" thickness for the girdle would answer your worries ...

 

[Garry H (Cut Nut)]

MM you are correct - well done!

Martey Haske has written this GIA approach up rather well.
I prefer GIA to AGS on this issue.
www.gis.net/~adamas/cut.html

 

[mm]

Thanks cut-nut, but it's hard to find GIA stones with know HCA on pricescope- I guess I will have to get a sarin from the suppliers- which can take a while. Oh well!

Valeria- i think you calculation is wrong- a girdle 7% (0.07) thick would rate as extremely thick on AGS, not thin.

So the thickness of an AGS "thin" girdle (say 0.7% or 0.007) even for a 6.5mm stone would only be 0.0455mm. This would still rate as 'extremely thin' for a GIA rating.

Given that, for a 6.5mm 1 carat stone you would have to have a "slightly thick" AGS rating of 1.85% to just scrape into a barely safe "thin" rating for GIA.

So, is AGS "thin" really safe for a 1 carat stone? If so, does this mean that GIA "extremely thin" is safe for a 1 carat stone?

cheers
mm

 

[valeria101]

Oh boy

did I leave my arithmetic home in that post ... But maybe I can make good

Your Q is quite intriguing

The respective article sounds very instructive, since they find that the ranges used to qualify girdle thickness actually determine a "CONSTANT level in millimeters for each of the boundaries, independent of base diameter". This does sound close to your concern... I suppose.

It seems that to get a rather "thin" 0.1mm of a girdle in one of these stones, you would need the girdle percentage to get as high as 2%-2.2%. definitely not imposible to find.

However, I am wandering how far ca this logic be pushed. For example, the same dillema would be somewhat less frustrating to someone seeking a 0.6 carat and completely puzzling for someone looking for 0.05! It does not appear that the diamond trade is extremely concerned to rationalize all of it's rules (also PS exists and so does the article we are citing, obviously there is no complete theory yet

)... but here there might be more to this story.

Two arguments come to mind:
#1: stones of different sizes get different uses (ex: the 0.05 will likely be very protected once set, and 'casualties' are acceptable during the setting itself given the low cost of each tid bit)
#2: the small stone not only have thinner girdles on average, but also shorter girdles - so they are not only less exposed but also much better protected by the setting (prongs cover much more of them and the thickness of the prongs protectd the remaining exposed girdle between them).

I am almost sure no one modeled this to get quantitative results, but the idea sounded reasonable enough for me to post.

 

[mm]

Thanks valeria, i had trouble launching that link from my computer.

Using the sarin numbers to get an actual mm measure of the thickness sounds like the way to go.

However, it is a bit scary to think that most people are using % girdle measurements without consdiering what that actually means for stone safety at the smaller end of the size range (even at 1 carat).

cheers
mm

 

[valeria101]

This is what I meant...

I am not sure all folks are that blind, but unlike any other aspect of cut, this issue seems to have been deffered to "trust the Lab" - not the usual PS attitude. I am pretty sure someone considered all this, somewhere, otherwise the respective ranges would not have been chosen as Martin Haske demonstrates.

 

[mm]

Thanks Valeria, I see what you mean- I guess I am also worried about the fragility of the girdle when being set in the first place, as well as general wear and tear.

But that is a really good example of how the smaller stones are less exposed.

Mind you, its not much comfort for the people with AGS thin- GIA extremely thing 1ct girdles!

cheers
mm

 

[valeria101]

----------------
On 5/11/2004 12:41:14 AM mm wrote:



I am also worried about the fragility of the girdle when being set in the first place,

----------------

I hope this is not one message too many (last, promise!) but I'd better say it...

Are you going to take the rink of setting the stone or is the jeweler? As far as I understand it, the first and foremost use of those words ("very thin", "medium" what not) is not to insure diamonds remain forever whole, but to insure that they are recognized as more or less safe. So that there is no excuse for some of the stones (labeled safe) to get damaged. One thing to chat with the jeweler about... I guess.
On the other hand, it is much easier to let the seller of the stones do the setting (if feasible) - so that no ambiguity about who chips what arises.

This is one reason why buying melee is by no means a great deal... for example – it is known that they are not all going to survive the setting chore.

The respective "medium" girdle for 1/3carat stones (~ 0.1mm, right ?) is supposed to be safe for tension setting - and there is no worse a stone can endure... This would be a question for “Boonerings” to settle, I suppose.


There is an endless dispute here about "very thin girdles" and so forth - so this is why I am mostly writing, not just to swamp your thread

 

[mm]

I haven't sorted out who would take the risk in the setting- I am looking at getting a friend to do the ring. In those circumstances I guess I would take the risk- so that's why i'm trying to leant owards a 'safer' girdle.

Thanks for your advice, it's been very helpful.

cheers
mm