A question on l x w x d ratios.

[Chrometsav]

Hi all,

I have asked about this on a couple of other sites and have not really got a clear answer. I hope someone hear can help.

I am trying to understand when looking at the dimensions of a coloured stone what raises a red flag to the possibility of it being cut too shallow and being windowed?

If I give an example: 8.1mm (L) x 7.5mm (W) x 4.2mm (D)

Is there some sort of ratio whereby I can see that there is a good probability that a stone is cut too shallow and best avoided? E.g should the depth be at least twice the length....That sort of equation!!!!

Many thanks

 

[ma re]

Well there''s no unique answer as many stones have many different optical properties so you need to cut them at different angles to achieve the right effect, and the choice of cut also makes a difference. But I believe that, with many stones, it''s pretty safe to assume that the right depth is around 60% of the stone''s width. However, window is something that becomes easy to spot once you train your eyes a bit, even when judging from photos, so you don''t really need numbers to judge a cut (photo serves as a much better tool). Hope this helps.

P. S. A small window is not a disaster in my opinion, if the stone is very lively and has a rich and desired color (that you''ll enjoy). White diamonds are another thing, but colored stones have a few more aces in the sleeve so are not solely dependant on the cutting

 

[Pandora II]

Not really...

Your eyes are the best tool with coloured stones. Also cut is not king here - colour is, and so many coloured stone collectors will be forgiving on the cutting to get a gem of a premium shade.

No cutter is going to want to loose weight of a valuable stone to fit some kind of ratio or get precision angles if they can produce a beautiful stone without.

 

[Michael_E]

I'd say that 55% is about as low as a colored stone can go and still be attractive, regardless of the cut style or cutter. That's the depth divided by the width. In this case you have 4.2/7.5 = 56% which is right at the bottom edge of the possibility of it's being deep enough. 60% to 65% is better with colored stones. Whether a stone windows or not is not so much dependent on depth as it is cutting style and the angles of the pavilion closest to the culet. I've seen stones with 70+% depth percentages which had significant windowing. These had pavilions which were shaped like a ball having very steep angles at the girdle and very shallow angles at the culet. Depth percentage is only one thing to consider, if you're concerned about cut, try to get a side view. This will give you a pretty good idea if the stone is well shaped or not.

 

[MakingTheGrade]

Thanks Michael for helpful info as always!

Chrometsav, One of the fun thought perhaps frustrating things about colored gems is that their are many more types of cuts and variations in optical properties than you find in the world of diamonds. I''d agree with above posters in saying the eye can probably tell you how the cut is.

And just curious, are you currently in the market for a tsav?

 

[Chrometsav]

Thank you everyone, I understand there are no hard and fast rules but at least the % ratio you have given me helps a little in narrowing it down.

Micheal, is it possible to still have a window even if the stone is not cut too shallow and is this then rectifiable by re-cutting? All hypotheticals I'm just interested!

Making the grade, in regard the tsav, funnily enough I have a very nice one I'd sell or exchange,a genuine reason why that I'd rather not go into here!

Does this site allow people to list rings or stones they wish to part with as I couldn't see an area for it?

 

[MakingTheGrade]

Not really, but some forum members list on ebay or diamondbistro

 

[chrono]

I believe with a depth of 60%, you usually will be fine, however that is not full proof because depth alone doesn’t not mean there is no windowing. It is dependent on all the pavilion angles (usually the last few tiers) that are critical. The short answer is yes, it is possible to still have a window even in a very deep stone. And yes it can be rectified with a recut if you have enough depth to begin with. However, depending on the cost of the material, it may not be worth a recut.

PS is an educational site, therefore, no buying or selling is allowed.

 

[Chrometsav]

Chrono,

Thank you for the help. I have seen a trillion cut Tanzanite I like the look of. The dimensions I was given were as follows.

7.6 x 7.4 x 4.2 mm

Could you please give me your impression on possible windowing etc as it appears a little low depth wise ratio? I was unsure due to the fact it''s a trillion.

Many thanks

 

[chrono]

Trillions are shallower than most shapes so they face up well due to the nature of the cut. I highly recommend a picture from the vendor because we have no information on the angles of the stone and probably never will, thus only a picture will show if the stone has any windows. Pictures might also show any colour zoning, extinction or issues with structural integrity of the stone that might not be mentioned by the vendor. In any case, I would NEVER buy gemstones without a few pictures of the stone.

 

[PrecisionGem]

Using a depth measurement to try to figure out if a stone will be windowed is really pretty useless. First you would have to know what percentage of that depth is in the pavilion, and what percentage is in the crown, and how thick the girdle is. Even then, it''s still pretty useless information. You could cut a typical round stone with a step pavilion, with a very deep pavilion using these angels.

Tier 1 - 60 degrees
Tier 2 - 48 degrees
Tier 3 - 38 degrees
Tier 4 - 32 degrees

The pavilion could be deep, but tiers 3 and 4 will window since they are cut below the critical angle.

The best way to see if a stone will window is to just look at it and see if it does. The best judge of any stone are you own eyes. Always insist on a picture that is straight on. When stones are photographed at an angle to the table, it''s difficult to discern a window from a tilt window. Almost any cut that has a table when tilted will eventually window, and this is called a "tilt window".

 

[Michael_E]

Date: 9/7/2009 10:08:10 PM
Author: Chrometsav
Micheal, is it possible to still have a window even if the stone is not cut too shallow and is this then rectifiable by re-cutting?

Yes, if the stone has a high w/d ratio then it can generally be re-cut to rectify windows caused by angles which are too low around the culet...without changing the face up outline of the stone. This is often a good way to get a good deal on medium value stones, as deep and windowed stones are often less expensive and make good candidates for re-cutting. This will not happen with higher valued stones, since depth and windowing are much less of a factor in their pricing.

 

[LD]

Date: 9/15/2009 9:48:48 AM
Author: PrecisionGem
Using a depth measurement to try to figure out if a stone will be windowed is really pretty useless. First you would have to know what percentage of that depth is in the pavilion, and what percentage is in the crown, and how thick the girdle is. Even then, it''s still pretty useless information. You could cut a typical round stone with a step pavilion, with a very deep pavilion using these angels.

Tier 1 - 60 degrees
Tier 2 - 48 degrees
Tier 3 - 38 degrees
Tier 4 - 32 degrees

The pavilion could be deep, but tiers 3 and 4 will window since they are cut below the critical angle.

The best way to see if a stone will window is to just look at it and see if it does. The best judge of any stone are you own eyes. Always insist on a picture that is straight on. When stones are photographed at an angle to the table, it''s difficult to discern a window from a tilt window. Almost any cut that has a table when tilted will eventually window, and this is called a ''tilt window''.

This is the best advice anybody buying coloured gemstones could have. What are your eyes telling you? Forget measurements!

 

[Chrometsav]

Thank you once again for the replies. I have grasped the big window examples most websites show when talking about this subject. It is the smaller window I think usually described by sellers as ''slight window'' that I fall down on.

When talking about a ''tilt'' window, does this mean looking through the stone via the table if the stone is tilted means windowing here is normal?

E.g. if I use the common example of a page of writing and look through the table with the stone tilted, is it normal to be able to see the writing?

Thanks again all who have contributed!

 

[chrono]

Date: 9/15/2009 9:03:29 PM
Author: Chrometsav
Thank you once again for the replies. I have grasped the big window examples most websites show when talking about this subject. It is the smaller window I think usually described by sellers as 'slight window' that I fall down on.

When talking about a 'tilt' window, does this mean looking through the stone via the table if the stone is tilted means windowing here is normal?

E.g. if I use the common example of a page of writing and look through the table with the stone tilted, is it normal to be able to see the writing?

Thanks again all who have contributed!

Yes, that is correct. If the colour is very good to exceptional, I usually forgive a very small window. In coloured stones, colour always comes first.

 

[haagen_dazs]

since cut is not as important and colour is very important in these coloured gem stones.

how do the cutters cut their rough?

i suppose sophisticated computer software analysis is not really required.

do the cutters kind of cut a few major facets, look at the stone, think about it, cut some more here and there, look and stare, ponder on what to cut next and just whack off more sides to create more facets to keep the colour in? (this is in reference to non precision cut gem stones)

 

[chrono]

The software is only as good as the user. The first thing the cutter does, is to "read" the rough. He has to study the location and type of inclusions (cut around it or remove it), study how to remove any zoning, how to maximize yield which also determines the best shape/outline to cut, how to play up the colour (which axis to cut from), and etc. This will be true for all cutters (native and precision).