Refractive Index

[kenny]

I enjoyed this write up on RI and thought I'd share it.

Feel free to skip over the math and just you'll still find plenty of good info in the text.

http://www.usfacetersguild.org/articles/bob_keller/refractive_index/

 

[chictomato]

Wow tks for sharing this detailed write up!

 

[kenny]

Question for our experts: Is the following statement correct?. . .

Because they have the highest RI, diamonds can be cut more shallow than other gems.

(Now, obviously any gem can be cut to any dimensions, but I'm talking about for good light performance like avoiding windowing.)

 

[PrecisionGem]

Kenny, the answer is yes.

The higher the RI, the lower the angle required to reflect the light, rather than let the light pass through the stone creating a "window" effect. In colored stones, most cutter are working with many different types of stones and each type will have a somewhat different refractive index. This means that if one were to cut a stone from Quartz, the angles used would be different than from a garnet. We typically tweak the angles on stones all the time depending on the material.

It's much easier to get a flashy stone from a high refractive index material than a low. The funny thing is, in the diamond business much discussion is made about depth % and angles, when they are way more forgiving than in a colored stone. I have never cut a diamond, but I have cut CZ who's RI is very high, close to that of a diamond, and it's difficult for the cut not to look good. Even a bad cutting job looks good. Quartz on the other hand is much trickier to get a pretty stone.

 

[kenny]

Kenny, the answer is yes.

The higher the RI, the lower the angle required to reflect the light, rather than let the light pass through the stone creating a "window" effect. In colored stones, most cutter are working with many different types of stones and each type will have a somewhat different refractive index. This means that if one were to cut a stone from Quartz, the angles used would be different than from a garnet. We typically tweak the angles on stones all the time depending on the material.

It's much easier to get a flashy stone from a high refractive index material than a low. The funny thing is, in the diamond business much discussion is made about depth % and angles, when they are way more forgiving than in a colored stone. I have never cut a diamond, but I have cut CZ who's RI is very high, close to that of a diamond, and it's difficult for the cut not to look good. Even a bad cutting job looks good. Quartz on the other hand is much trickier to get a pretty stone.

Thanks, next question.

Can you cut a non-diamond to have light performance that is the same as a diamond by making it the right depth to compensate for the difference in RI? - or are there other properties besides RI that prevent this?

IOW can a non-diamond have the same light performance (face up) as a diamond?

 

[gsellis]

It's much easier to get a flashy stone from a high refractive index material than a low. The funny thing is, in the diamond business much discussion is made about depth % and angles, when they are way more forgiving than in a colored stone. I have never cut a diamond, but I have cut CZ who's RI is very high, close to that of a diamond, and it's difficult for the cut not to look good. Even a bad cutting job looks good. Quartz on the other hand is much trickier to get a pretty stone.

Thanks, next question.

Can you cut a non-diamond to have light performance that is the same as a diamond by making it the right depth to compensate for the difference in RI? - or are there other properties besides RI that prevent this?

IOW can a non-diamond have the same light performance (face up) as a diamond?

For your question Kenny, RI has more to do with critical angle and optimal cut design. The above mentioned material can be cut with pavilion angles below 28 degrees (diamond has an even higher RI). But, the optimal angle for >90% return (up to 94) is similar to the best angles for quartz with a critical angle (before it windows) at 41 degrees and crown mains at 36. You get 97% at 40.5 and crown mains at 14 degrees. Quartz is tops at 41.5 and 21 degrees for 96% return. That is based on an optimal design. Various cuts even with optimal angles can lower performance. An example is Jeff Graham's Memory. It is a great cut to increase the color of a light stone as it traps the light in it. But normal rough can be too dark. And the angles can be at the best angles for the material...

What really makes diamonds pop is the high dispersion (dicussed later in your link). Here is another table from Bob that you can compare dispersion, RI, and birefringence which you can put together. Think of zircon and sphene and you can see that they have pretty good dispersion and birefringence.

http://www.rockhounds.com/rockshop/gem_ ... rties.html

I usually go to the table when I am adopting a cut from one material to another. If you have a cut made for quartz, using it on opal will result in a window without changing the pavilion angles.