window when stone is tilted?

[hoofbeats95]

Is it common for a window to appear when a stone is tilted? The tourmaline I just got from Barry does this. I didn''t think too much of it because I figure he is a great cutter and it''s ok. However, in the past couple das I''ve read a couple posts that comented on this. The topic was a bit OT for me to ask for more info. So I thought I would post a new topic.

 

[LD]

You often see this but I don''t believe it''s actually a window. A window should only be judged from top down (i.e. looking straight down into the stone). If you tilt and look through the top and out of one of the sides you''ll often see a less saturated area and that can be due to the placement of facets.

 

[T L]

Stones with lower RI''s like beryls and tourmalines will have a "tilt window." You don''t see this as much on well cut garnets and diamonds because they have a higher RI. Some cutters have advanced facet patterns to even try to minimize tilt windows (Gene''s "star cut" is an example). However, what you''re seeing is normal, even in a decent cut stone.

 

[chrono]

TL explained it very well and clearly. I am in agreement that it depends on the RI of the stone and partially depends on the shape and cut of the gemstone. This tilt window is normal and acceptable.

 

[hoofbeats95]

I figured it was normal. I also realized that it''s really only a window when it appears looking from the top down. But since I read about it in a few threads I wanted to ask. Thanks for the quick replies! I''m off to see what info I can learn about RI. I know the terms, but still have a hard time understanding and applying. I read about stones when I have a spare moment here and there. That doesn''t allow me to really commit to memory as I would like. But I figure some people take smoke breaks at work. I take a PS break!

 

[colormyworld]

I guess I will weigh in on this subject as well.

Of course a gems RI does effect how easy a tilt window will become visable. There are other factors that also cause a tilt window to show. All stones will show a tilt window if tilted enough. Diamonds, garnets and other gems with high RIs are not immune even in a round cut shape. While RI is the major factor. Pavillion angles and crown angles also play a major role. A stone cut to within a hairs breath of its critical angle will show a tilt window quicker than a stone cut with a deeper pavillion. Heck those stones that many around here poo poo because of "native" cutting with a "belly" will often show less tilt window because of this belly. Exacting meet points have nothing to do with whether a stone shows a tilt window. Some seem to think that it is a waste of money to pay for gemstone that does not face up as large as it could if it were cut to critical angles but I submit that it is a trade off in that the stone cut a little deeper or has a slight belly will not show a tilt window as much as a stone cut to its critical angle.

 

[cushioncutnut]

Learn something new everyday! Thanks for that info CMW. I happen to love my rather deeper cut stone!

 

[PrecisionGem]

The "critical angle" is the angel on the pavilion at which a ray of light perpendicular to the table entering the stone, and hitting the pavilion facet will reflect and not pass through. Cutting below this angle will window, as the ray of light does not reflect back.

Now, if that angle for a certain stone is 38 degrees, and the pavilion was cut at 39, looking straight down into the pavilion, the stone will not window. However, if you tilt the stone 1.5 degrees, now you have effectively changed the angel of the pavilion in respect to your eye to 37.5 and you will see a tilt window. It would seem that the answer would be to cut the stone with a pavilion at maybe 45 degrees, and then you would have to tilt the stone very far to create the tilt window. The problem with this is that you will create extinction in the stone.
So what a clever cutter will do, is play with the crown of the stone. Bend the light on the crown before it gets to the pavilion. Crowns with smaller tables such as checker boards or other fancy cuts can virtually eliminate a tilt window, and extinction.

 

[colormyworld]

Gene does extinction play any role in the scintilliation a stone displays?

 

[PrecisionGem]

To get scintillation, you some difference in light and dark facets, but you don''t want a stone with actual extinction. If all the facets reflected light exactly the same way, you would have a bright stone, but no scintillation. The problem often is that the stones are cut deep, with with a very flat crown. This kills the dispersion. You can create more dispersion with a higher crown. It''s all a balancing act, and there are trade offs and gains for most everything. But a well cut stone, will ALWAYS out perform a poorly cut stone.