old topic - extinction!

[Arkteia]

Every stone that I have bought or seen has some degree of extinction. Well, not every, hence the question.

It is said that extintion is the functinon of the cut, the pavilion is too deep and the light can not get through, hence black spots. But I never read that it is also a function of individual refraction.

Each medium refracts light differently, hence there must be stones more prone to extintion.

In my view, stones that are prone to extinction are sapphires, possibly, darker rubies, spinels, garnets.

Not prone: aquamarines, amethysts, citrines, tourmalines, chrysoberyls. Much depends on color, of course. But stones with higher RI refract light at a sharper angle and it is not directed at the table. Stones with lower refractions direct the light more upwards. Makes sense, because garnets with high RI are often extinct. I wish I could draw it, but I am a bad artist. Anyhow, I hope I made it clear enough.

And if my assumption is right, can adjustments be done when precision cutter is cutting a stone? I mean, automatic correction for the coefficient of refraction. For each type of cut?

 

[zeolite]

http://www.ruby-sapphire.com/brilliance_windows_extinction.htm

 

[RockHugger]

Ii have seen tourm and stones with low ri go extinct. I think it has more to do with tone and cut combined.

 

[Arkteia]

Great article! Richard Hughes rocks, as always. Now I understood it.

 

[Arkteia]

Great article! Richard Hughes rocks, as always. Now I understood it.

 

[chrono]

Crasru,
Believe me when I say I’ve seen tourmalines with extinction too, partly due to the closed C-axis so the ends up being dark/blackish. Tone also definitely seems to play a role in it, in addition to the RI of the material. The link by Zeolite to Richard Hughes’ article is certainly a very good explanation.

 

[Michael_E]

And if my assumption is right, can adjustments be done when precision cutter is cutting a stone? I mean, automatic correction for the coefficient of refraction. For each type of cut?

The RI of a stone only affects the light entering and leaving the stone and so, as far as it relates to extinction, RI a greater effect on the design of the crown of the stone. Making "adjustments" when cutting is only done when there is a problem with the material and in any other case a wise cutter will stay with the cut that they'd planned. That planning should take into account the RI of the material to avoid extinction as much as possible.

The problem comes when one realizes that there are many causes for a stone to show extinction, the greatest of which are outside the control of the cutter. Probably the largest contributor to extinction is the lighting environment that the stone is being viewed in. If you are viewing a stone in a dark environment with a single light source, your stone will look as if it is 60% extinct or more. If you have BIG hair or are viewing the stone at closer than arms length, then you will see larger dark zones because you are seeing the dark reflection of your head in the stone.

Because seeing that dark reflection of you head is one of the biggest contributions to extinction, a good cut design will try to reflect light back to your eyes which is coming from behind you and around the sides of your head. This can be done by increasing the crown height and reducing the table size, (using the RI of the material to bend the light coming from around you head more steeply into the center of the stone), OR by having a relatively shallow crown and a deeper pavilion, (which uses the angles of the pavilion to direct the light coming from around you head back to your eyes). It's a balancing act which, when done right an in a material which is not overly dark, can minimize the extinction in a stone....Provided the viewing environment has adequate lighting.

Dark stones are much more difficult since the increasingly absorb light with every bit of distance that light has to travel through them. Some designs in dark stones show this very dramatically. Designs like marquises and pears have very long light paths towards their tips and so in darker stones can look black at the tips while having very fine color in the center. In a lighter stone the tips may have fine color while the center is pallid and washed out looking. In this case extinction or fine color is all about how much light is absorbed or transmitted as it travels through the stone.

In any case extinction involves many variables and cut is not always the culprit in having an extinct stone, sometimes all you need to do is to hold the stone a little farther away or take it outside on a bright, cloudy day.

 

[Sagebrush]

Michael,

Nice post! Hofer defines extinction as "off axis refraction" not a great deal different from Hughes. Light exiting the crown at an angle from the perpendicular becomes grayer and finally black as the angle increases. In other words, the greater the angle from your eye, the grayer it gets until it reaches black. Thus, if you rotate the stone off axis, you might just get some brilliance.

Extinction's role is not all negative. If all you saw was light, a gem would look like a headlight, extinction is required to carve out scintillation.

Best,

 

[Arkteia]

Thank you, Michael and Richard. Indeed, very nice explanation. Now I think I have a better idea of what to expect and why sapphires of my favorite color always have a certain degree of extinction. It is an interesting topic indeed.

 

[Largosmom]

I really enjoyed this thread too, including the article, which explains the complex topic so well.

Had to chuckle at the thought that people with "Big Hair" will have more problems with gemstone extinction. Having spent my college years and a fair amount of time in the Southern part of the US, I have discovered a higher percentage of ladies with a tendendency to tease more volume out of their locks down here than North of the "Sweet Tea" line.

 

[Arkteia]

Yeah, that is why Dolly Parton prefers diamonds! With sapphires, it would be extinction all over!