Windowing in a sapphire

[corbind]

Hello people,

I've been lurking through this forum for the past few weeks, trying to learn as much as I can as I search for the perfect sapphire. I just received my first sapphire in the mail, from a very reputable precision gem cutter who is well respected on this forum. Before ordering the stone, I asked a bunch of questions to get a solid idea of what I would be getting, as we all know pictures online can be deceiving. One point that I was careful to have clarified prior to placing my order was the topic of windowing. I was assured that the stone did not have any windowing, despite the depth ratio being very low for a sapphire ~58%. I received the stone this week, and it was beautiful in terms of the color, but I am uneasy on the possibility that it my in fact have windowing. Now, I understand that to judge windowing you must look directly into the table/crown... and yes, there does appear to be a point where if the stone is held just right there is no discernible windowing; that said, the absolute slightest change in angle (and I mean slight!) and boom, you have what appears to be a window. I wish I could take pictures, but I won't have a camera for a couple days. So I guess my question is, is this normal? Am I being too picky? Is it normal for windows to appear with the slightest angle change for the optimal viewing angle. The stone will sit in a setting that should improve the closing of windows, but I'm still not sure.

 

[Edward Bristol]

Common problem with precision cutting. The one thing I dislike about it.

Sometimes a nice native belly also has charme.

 

[michael_m]

Thank you Edward! Hard to believe only 1 person has an opinion on this from 120+ views.

 

[gsellis]

Almost every cut gem will show a tilt window. The bigger the table and the more the angle from center, the more it will window. You are describing tilt window. If the table is large and it is cut close to critical angle, it can take as little as 5 degrees before you start seeing a window on side facets. And if you cut a stone deeper than it is wide, you start getting extinction.

 

[Edward Bristol]

It seems to me that some precision cutters take their job more mathematically than artistically.

If you apply only physics and math on the cutting of a gem you may get a "perfect" cut but not art, and the tilt window is one of the side effects.

 

[Pandora II]

Thank you Edward! Hard to believe only 1 person has an opinion on this from 120+ views.

You're not going to get many comments from people if they're not cutters and able to explain the physics behind tilt windows. Also there are no pictures - plus for every member viewing, there are probably 20 lurkers!

If you look directly down on a stone and there is no window, then you can't say that it is windowed per se, but tilt windows can occur at varying degrees of tilt depending on the cut, the RI of the material etc.

The tilt window 'may' become less apparent once the stone is set, depending on the type of setting used.

 

[movie zombie]

a perfect sapphire with absolutely no tilt windowing....and they do exist....is going to break the bank for most people.

not all tilt windows are objectionable. much comes down to your or the wearer's tolerance. i don't care for them at all but would accept a small tilt window for a very fine color stone.

 

[PrecisionGem]

It seems to me that some precision cutters take their job more mathematically than artistically.

If you apply only physics and math on the cutting of a gem you may get a "perfect" cut but not art, and the tilt window is one of the side effects.

Can you explain this Edward, as your logic sounds completely opposite to what I would think. If one can mathematically calculate the angle of the tilt window, how would this not help him to cut a stone an lessen it?

 

[Edward Bristol]

It seems to me that some precision cutters take their job more mathematically than artistically.

If you apply only physics and math on the cutting of a gem you may get a "perfect" cut but not art, and the tilt window is one of the side effects.

Can you explain this Edward, as your logic sounds completely opposite to what I would think. If one can mathematically calculate the angle of the tilt window, how would this not help him to cut a stone an lessen it?

I was waiting for your input!

Not a cutter myself, but as I understand, when one optimizes the angles based on the gem’s refractive index you also come out with an optimized tilt window. Means, that when you have a gem with low index, the tilt window will be worse, like automatically (as result of maths/software). Max brilliancy = max tilt problem?

Is that not so? Happy to learn.

 

[PrecisionGem]

I don't think that is true. I can speak only for my self, but I'm sure it's true with most of the other "precision" type cutters. When creating a new design, one of the things we look for is tilt performance, not just maximum brilliance. Cutting a stone a an exercise in compromises, and so is designing a cut. There are ways to reduce a tilt window, and with the techniques and software available, these can be analyzed and optimized.

 

[mastercutgems]

I am listening


When running a ray-tracing program you figure in these 5 to 15 degree angles and compensate for them; what about a true accurately cut round brilliant; or even one with a zircon style culet; I do not seem to have had problems with a tilt window in my sapphires; especially ones in the medium to medium light colors; and for sure not the lighter colored ones.

Now I can see it with a standard emerald cut; but most of the true brilliants in medium to medium light tones with 8 equal mains and the breaks cut at a low enough point on the mains does not seem to show tilt window... Now in ovals where you cut in 4 mains from side to side and cut in more shallow mains on the long ends you will tend to get the blacking out of facets from time to time; but not when equal; like in the super nova oval type design.

Please tell me what shapes you guys are talking about and are they brilliant cuts or step or trap cuts??? Are you seeing through the steps in the traps or steps???

But by all means show me what you are talking about and I am also eager to learn and I do not claim to know it all; or even half...

But I will have to side with Gene as I have cut over a thousand sapphires and not one with a tilt window in a round or similar cut brilliant.

We may be talking about different things as I would assume; yes I know what it spells when you say tilt window that when you tilt the stone in a 10 plus degree angle that you get the window effect and see through the gem and do not get the reflective properties of said gem. Is this correct??? and if so that is what I do not get when I tip a brilliant ; like in a sapphire cut at pavilion main angles of 40 to 41 degrees; add in the breaks at 43 and take them down to 80 percent of the depth to the culet??? Please correct me if I am wrong and I do want to know what you all consider a tilt window. Please humor me

I know there are other factors like a huge table, etc. that can play into light disturbance; but a true window where you can read a newspaper under it ???


Nothing meant to disrespect anyone here.

Most respectfully;

Dana Reynolds, csmg

 

[gsellis]

The critical angle of corundum is 34.4 degrees. With the pavilion culet at 41 degrees, I can get windowing starting at 12 degrees on the star facets in simulation. I can get it through the table at 15 degrees. Even a diamond will do it. I just looked at the forbidden stone (think cube and Zr) in an SRB cut at near ideal (43, 41 and 37, 27 crown). That material has a critical angle of 27 degrees. At about 50 degrees off center, window through the table and I already had one through the star facets.

And if you go too deep, that is where you get a return fall of as the light bounces out of the sides. I attached a sample. On the left is the 15 degrees on corundum through a large table hitting those 41.5 degree facets. On the top right is too deep with the culet mains at 47 degrees. Straight in on the facets goes right out the bottom. Ideal is lower center and viewed at 0 angle.

You can always find a window. If the stone is close to critical angle, and it might be because of rough depth or yield, it will window sooner.



Edit - PS - I did not cut the stone and do not know who did. Just sayin...

 

[mastercutgems]

Thanks George

Is this a window your eye can easily pick up and also does it also have to do with the percentage of table you have in said cut???

I noticed in one view the laser or computer sight was looking through the breaks on the crown facets?? I do not know how your eyes are but it is a lot going on with scintillation, etc. to be looking through the crown facets to pick up a window.

Thank you for showing me on the ray tracing; but to be honest I have not easily seen it on the gems I have cut or from cutters similar to me. As I know the computer like a camera can see things the eye does not easily discern. I guess I was talking about the obvious like a trap cut oval that you literally can see the newspaper below or off to the side. Those things my eyes do not pick up on a standard round brilliant cut at the angles we just talked about. Because you can train the computer to pick up a window between 17.3 and 18.7 degrees but who's eye is looking that precise???

Maybe I just do not have the eyes I used to???

Thank you for sharing this as I too have ran this program and when completed did not see what I consider a true windowing; but like Edward had said maybe I threw out the majority of all the mathematics some years ago when cutting became a job and not a hobby and competition thing I just know my best angles for crowns and pavilions and fudge where I have to without compromising the optical outcome

Always learning

Thank you and Most respectfully;

Dana Reynolds, csmg

 

[innerkitten]

If the stone doesn't wow you it's probably not the one for you. Keep looking and I'm sure you'll find a perfect one. Sometimes it takes time.


P.S. Do you like cabochons? A friend of mine has an original art deco emerald cabochon set in platinum with just small baguettes on the side and it is DTF!

 

[chrono]

A depth of 58% is sort of low of the sapphire but probably might not window; you are most likely seeing a tilt window where a slight tilt allows you to see through the stone (light leakage). The lapidary probably did this for several reasons:
1. Material might have been a little on the dark side, so a slightly shallower stone means a lighter and brighter stone
2. Shape of rough is flattish so he/she tried to conserve the material, going for maximum return.
3. Could be a myriad of other reasons ranging from working around an inclusion(s), trying to cover up colour zoning, or etc.

Depending on the material, low RI material tend to show window tilt far more easily than gemstones with high RI. The "busier" the design, also the less likely to show window tilt because of all the many various facets bouncing light back to the eye. If a window tilt really bothers you (and it's okay to be bothered by it, everybody is different) while still within the return period, by all means return the stone. There's no reason to keep a stone you don't truly love if the opportunity is still open.

 

[bmg]

Hello people,

I've been lurking through this forum for the past few weeks, trying to learn as much as I can as I search for the perfect sapphire. I just received my first sapphire in the mail, from a very reputable precision gem cutter who is well respected on this forum. Before ordering the stone, I asked a bunch of questions to get a solid idea of what I would be getting, as we all know pictures online can be deceiving. One point that I was careful to have clarified prior to placing my order was the topic of windowing. I was assured that the stone did not have any windowing, despite the depth ratio being very low for a sapphire ~58%. I received the stone this week, and it was beautiful in terms of the color, but I am uneasy on the possibility that it my in fact have windowing. Now, I understand that to judge windowing you must look directly into the table/crown... and yes, there does appear to be a point where if the stone is held just right there is no discernible windowing; that said, the absolute slightest change in angle (and I mean slight!) and boom, you have what appears to be a window. I wish I could take pictures, but I won't have a camera for a couple days. So I guess my question is, is this normal? Am I being too picky? Is it normal for windows to appear with the slightest angle change for the optimal viewing angle. The stone will sit in a setting that should improve the closing of windows, but I'm still not sure.

Every well cut corundum(sapphire) should have a tilt window usually at 10 - 20 degrees (for a RB design) and often less for other designs. You just can't work with 1.76 RI material and get nice color and brilliance without it. That should be okay for everyone, you look at your stone faceup most of the time and rarely more than 15 degrees from the side.

Poorly cut stones with large tables and shallow pavilions may show a tilt window with less tilt, these are often found in native cut stones where a greater emphasis was placed on conserving weight.

I don't know what design was done but 58% depth is unusual for sapphire and usually indicates that other factors were deemed more important to the cutter than a potential tilt window:

1) Dark material that needed to be made lighter
2) Conserving weight as much as possible in a shallow piece of rough
3) Avoiding inclusions

I am sure their are other reasons for cutting the stone shallow the precision cutters can comment further.

In general though the precision cutters I have spoken to in this forum emphasize overall beauty (color, saturation, brilliance) over weight saving in their designs. This has a greater emphasis for them than your average native cutter who will probably be focussing on mostly color and weight.

Overall though if you don't like the stone return it, if you don't like the cutter use someonelse regardless of how reputable they are.
Just my 2 cents hope it helps.

 

[Michael_E]

Now, I understand that to judge windowing you must look directly into the table/crown... and yes, there does appear to be a point where if the stone is held just right there is no discernible windowing; that said, the absolute slightest change in angle (and I mean slight!) and boom, you have what appears to be a windowing...So I guess my question is, is this normal? Am I being too picky? Is it normal for windows to appear with the slightest angle change for the optimal viewing angle.

What you are seeing does not sound like a window to me. It sounds like you are looking at a reflection of your head and shoulders. In addition to the possibility of a window or even a tilt window, a cutter needs to be concerned about how the reflection of the viewers head will be seen at different distances from the stone. This reflection can appear as a dark spot that occurs with a very slight tilt of the stone and can sort of mimic a tilt window to someone who is not aware of what they're looking at. Hold your stone over a brightly lit piece of white paper and tilt it. Do you see a dark spot, (your reflection), or a light area, (a tilt window is light colored when the stone is back lit...a big trick of sellers on E-bay).

With regard to why "native" cutters cut the way they do...it is also because of the benefit of multiple tiers of facets commonly used on the pavilions of their stones. This is because only certain tiers of facets which oppose each other at just the right angles will reflect the viewers head and since their are more and smaller facets in those tiers, the dark reflected areas are smaller and of dissimilar sizes, fooling the eyes, (and the brain when it blends the view of the stone from both eyes). This also has the added benefit of hiding small inclusions more effectively than those in precision cut stones, (where any unevenness is easier to spot).

 

[Edward Bristol]

It is a complicated topic and I understand only half.

I adore a precision cuts and would love to see all gems cut with devotion and perfection plus artistic charm.

But I do have a couple of Christmas wishes on that account:

- More facets
- Less tilt window
- Better rough, especially in the cheaper varieties
- More common shapes (perfect classics rather than more fancy shapes)
- Corundum and other high-end gems (smaller if necessary)