Oval Cuts: Half n half extinction effect?

[MakingTheGrade]

I was wondering what causes the half-bright, half-dark effect that I often see in gems that are oval in cut. For example:

I was hoping some experts might chime in. Is this considered extinction? Some people on PS claim to be turned off by this effect, yet I see this effect in photos of gems on sale often enough, is it just hard to avoid in ovals?

Thanks!

 

[Michael_E]

Date: 5/31/2009 6:51:08 PM
Author:MakingTheGrade
I was wondering what causes the half-bright, half-dark effect that I often see in gems that are oval in cut.... Is this considered extinction? Some people on PS claim to be turned off by this effect, yet I see this effect in photos of gems on sale often enough, is it just hard to avoid in ovals?

I wouldn''t call it extinction, since the darkest color looks to me like the reflection of the photographer. The lightest color on the right looks like light from a window or something. This effect is caused by the gem having a pavilion which is cut in what''s known as a "keel cut", where the pavilion of the gem is shaped somewhat like the bottom of a boat. In these cuts the light reflecting from one side is coming from the opposite side of the stone, so the bright area on the right is reflecting from the left, and vice versa. If the photographer had known this they could have avoided the effect by having even light on both sides of the stone during the taking of the photograph.

This doesn''t happen often on brilliant cuts and never on Barion cuts, (a deeper form of brilliant cutting).

 

[MakingTheGrade]

Thanks Michael!
So in real life, will it sometimes take on that effect when the lighting is non evenly distributed across the face of the gem?

 

[chrono]

Michael,
I''ve seen this 1/2 and 1/2 effect in person on a variety of gemstones. Sometimes it is outdoors and sometimes indoors. I''ve seen it in tourmalines, garnets and spinels. Your explaination is fine for photographing but why am I also seeing it in person where the light distribution is even? I''ve also seen it in modified barions.

 

[colormyworld]

Date: 5/31/2009 9:40:04 PM
Author: Chrono
Michael,
I''ve seen this 1/2 and 1/2 effect in person on a variety of gemstones. Sometimes it is outdoors and sometimes indoors. I''ve seen it in tourmalines, garnets and spinels. Your explaination is fine for photographing but why am I also seeing it in person where the light distribution is even? I''ve also seen it in modified barions.

Chrono, Out side in direct sunlight is a "single point" light source is the reason for the observed effect.

 

[chrono]

Date: 5/31/2009 10:02:21 PM
Author: colormyworld

Date: 5/31/2009 9:40:04 PM
Author: Chrono
Michael,
I''ve seen this 1/2 and 1/2 effect in person on a variety of gemstones. Sometimes it is outdoors and sometimes indoors. I''ve seen it in tourmalines, garnets and spinels. Your explaination is fine for photographing but why am I also seeing it in person where the light distribution is even? I''ve also seen it in modified barions.

Chrono, Out side in direct sunlight is a ''single point'' light source is the reason for the observed effect.

So direct lighting outdoors (but not strong sunlight, but not cloudy either) is considered a single point even though the light is even outdoors? I would have thought otherwise since light waves travel spread out evenly, as I recall in physics.

 

[T L]

Date: 5/31/2009 9:40:04 PM
Author: Chrono
Michael,
I''ve seen this 1/2 and 1/2 effect in person on a variety of gemstones. Sometimes it is outdoors and sometimes indoors. I''ve seen it in tourmalines, garnets and spinels. Your explaination is fine for photographing but why am I also seeing it in person where the light distribution is even? I''ve also seen it in modified barions.

Me too

My ugly half extinct barion cut tourmaline.

 

[colormyworld]

I will not argue physics with you but when the light is not difused like on a cloudy day where the the whole sky is emmiting more or less equal amounts of light compared to a cloudless day where the sun is by far the brightest point in the sky. Of course the only light that does not spread out in a cone is laser light and I don''t think anyone is talking about that kind of light.

 

[Michael_E]

Date: 5/31/2009 10:16:24 PM
Author: tourmaline_lover

Date: 5/31/2009 9:40:04 PM

Author: Chrono

Michael,

I''ve seen this 1/2 and 1/2 effect in person on a variety of gemstones. Sometimes it is outdoors and sometimes indoors. I''ve seen it in tourmalines, garnets and spinels. Your explaination is fine for photographing but why am I also seeing it in person where the light distribution is even? I''ve also seen it in modified barions.

Me too

My ugly half extinct barion cut tourmaline.

Well you''ve got me there. I guess that I just make a point of viewing gems in more evenly lit surroundings...I usually put them under one of those fluorescent ring lights like you''d find in a doctors office. It lights everything in a diffuse circle so I don''t see that effect very much. On the other hand, tourmaline lover, maybe you could look on the bright side and call your tourmaline a "bi-color" and call it good !

 

[colormyworld]

Date: 5/31/2009 10:16:24 PM
Author: tourmaline_lover

Date: 5/31/2009 9:40:04 PM
Author: Chrono
Michael,
I''ve seen this 1/2 and 1/2 effect in person on a variety of gemstones. Sometimes it is outdoors and sometimes indoors. I''ve seen it in tourmalines, garnets and spinels. Your explaination is fine for photographing but why am I also seeing it in person where the light distribution is even? I''ve also seen it in modified barions.

Me too

My ugly half extinct barion cut tourmaline.

Hey TL if you do not like that stone I will be glad to put it away where you will never have to see it again. It looks like it is just "ugly" enough to make it into my collection.

 

[T L]

It always looks like this, in all light sources, unless I'm viewing it directly head on. I detest the cutting and polish on this stone. The color isn't that incredibly horrid, but the cut is really not suited to showing it off for it's best potential. It's an Afghan blue/green with an obvious grey mask.

One stone I really really regret keeping. I just wanted to post this since the OP was talking about this half extinct effect, and this is a prime example.

 

[T L]

Date: 5/31/2009 10:43:17 PM
Author: colormyworld
Hey TL if you do not like that stone I will be glad to put it away where you will never have to see it again. It looks like it is just ''ugly'' enough to make it into my collection.

I see the aliens have taken over your body again CMW.

 

[PrecisionGem]

That type of modified emerald cut will always look like that, with one half lighting up at a time. It''s late, but if I remember, tomorrow I''ll post a cutting a diagram that will explain why.

 

[chrono]

Date: 5/31/2009 11:01:10 PM
Author: PrecisionGem
That type of modified emerald cut will always look like that, with one half lighting up at a time. It''s late, but if I remember, tomorrow I''ll post a cutting a diagram that will explain why.

Gene,
If that''s the case, why do cutters even bother cutting it like that? I don''t think it looks attractive being half lit and half dark.

CMW,
Thanks for the explaination. It makes sense that the sun, being the brightest object in the sky on a cloudless day, would be considered the single point when you put it that way.

 

[MakingTheGrade]

Date: 5/31/2009 11:06:14 PM
Author: Chrono
Gene,

If that''s the case, why do cutters even bother cutting it like that? I don''t think it looks attractive being half lit and half dark.

I kind of wondered that too...is it just really hard to cut ovals so that the effect isn''t there? One would think if such a design existed where this effect wasn''t seen, all the cutters would adopt that design instead?

 

[colormyworld]

Just to expand on that a little. A single point as a light source will cause more extinction to be seen. For this reason and direct sunlight being such a strong "harsh" light is the reason that most say it is not the best for viewing faceted stones in.

 

[T L]

Date: 5/31/2009 11:06:14 PM
Author: Chrono

Date: 5/31/2009 11:01:10 PM
Author: PrecisionGem
That type of modified emerald cut will always look like that, with one half lighting up at a time. It''s late, but if I remember, tomorrow I''ll post a cutting a diagram that will explain why.

Gene,
If that''s the case, why do cutters even bother cutting it like that? I don''t think it looks attractive being half lit and half dark.

CMW,
Thanks for the explaination. It makes sense that the sun, being the brightest object in the sky on a cloudless day, would be considered the single point when you put it that way.

Yes, tell me about it.

What is the point of cutting something that ends up looking like a half dark stone.

 

[sleepyspinel]

Maximizing yield for a given shape of rough would be my guess.

 

[chrono]

Date: 6/1/2009 12:32:13 AM
Author: sleepyspinel
Maximizing yield for a given shape of rough would be my guess.

But what’s the point of maximizing yield if it doesn’t look attractive and the price has to be lowered accordingly?

 

[MakingTheGrade]

Date: 6/1/2009 8:08:33 AM
Author: Chrono

Date: 6/1/2009 12:32:13 AM

Author: sleepyspinel

Maximizing yield for a given shape of rough would be my guess.

But what’s the point of maximizing yield if it doesn’t look attractive and the price has to be lowered accordingly?

That was what I was wondering, so I thought maybe it''s just so hard to avoid in oval cuts... Although, in terms of attractiveness, I don''t think it bothers me personally. So maybe if enough people don''t mind, there''s no incentive to change it?

 

[PrecisionGem]

The gem trade has been able the thrive on that fact that for the most part it''s dealing with an un-informed customer base. Most jewelery customers, and even jewelers for that matter know very little about colored stones. Sure they have all heard about the 4 C''s, but thats in reference to diamonds. Colored stones are a whole different beast. Most people have never seen a well cut stone, or even knew such a thing existed.

They know: Sapphires are blue, rubies are red, emeralds are green, and A DIAMOND IS FOREVER! So I really need a diamond, because then he will love me for ever.

The beautiful thing about forums such as Pricescope, is the customer is now getting knowledge and information, and with that, is wanted something better than what''s being offered.

As far as the extinction in the oval, this is what''s called the bow tie effect. Here''s a link to a nice article about this, it''s geared toward cutters, but the pictures make it clear what''s going on.

http://www.rockhounds.com/rockshop/gem_designs/bow_tie_blues/

 

[chrono]

Gene,
It’s interesting that you call the ½ and ½ extinction on the oval a bowtie. I thought a bowtie is when the extinction looks literally like a bowtie. So this effect is also called a bowtie in rectangular stones too?

 

[T L]

Date: 6/1/2009 12:32:13 AM
Author: sleepyspinel
Maximizing yield for a given shape of rough would be my guess.

Correct, that''s why he told me he cut it this way. Still a putrid cut though.

 

[PrecisionGem]

Sorry, I didn''t mean to imply the TL, modified emerald cut had a bow tie. The bow tie was in reference to the first oval posted at the top of the thread.

 

[T L]

Date: 6/1/2009 8:49:57 PM
Author: PrecisionGem
Sorry, I didn''t mean to imply the TL, modified emerald cut had a bow tie. The bow tie was in reference to the first oval posted at the top of the thread.

I knew you weren''t talking about my stone Gene. I was just replying to SleepySpinel. Besides, if you did insult it, I wouldn''t blame you - LOL!!

 

[icekid]

Date: 6/1/2009 8:54:04 PM
Author: tourmaline_lover

Date: 6/1/2009 8:49:57 PM
Author: PrecisionGem
Sorry, I didn''t mean to imply the TL, modified emerald cut had a bow tie. The bow tie was in reference to the first oval posted at the top of the thread.

I knew you weren''t talking about my stone Gene. I was just replying to SleepySpinel. Besides, if you did insult it, I wouldn''t blame you - LOL!!

You know TL, even tourmalines with extinction/crappy cuts have feelings too

 

[T L]

Date: 6/1/2009 9:08:56 PM
Author: icekid

You know TL, even tourmalines with extinction/crappy cuts have feelings too

Not when they''re grey and cold!!

 

[Sagebrush]

All,

A good deal of the problem you are discussing has to do with angle of view. Extinction is off axis refraction. The light is going somewhere but not where you are. The further off axis the darker it is, gray to black (the absence of light).

Any elongated cut can exhibit this phenomenon. That is why measuring brilliance in a stone involves looking at the long axis of the stone at about 45 degrees from the perpendicular using an overhead light source. At this angle you will normally see light refracting from the bottom half of the stone. Grading an oval for example at this angle, you measure the refraction in the bottom half the stone. Say half is brilliant, 50% of half or 25% of the whole of the stone. Then turn it 180 degrees and measure the bottom half again, if it is also 1/2 brilliant you add the two together, 25% + 25% = 50% overall brilliance.

Why can''t you just put the stone face up perpendicular to the light source? Because, when you look at it you must put your eyes perpendicular to the light source which puts your head between the stone and the light source and your head then casts a shadow.

In sunlight you can use another method. Turn your back to the sun and view the stone perpendicular to your angle of vision. The light diffuses around your body uniformly lighting the stone. At this angle you can judge the overall brilliance of the entire stone, no math required.

 

[chrono]

Richard,
That''s all fine and dandy but it would be rather inconvenient to have to angle my ring all the time to get rid of this 1/2 and 1/2 effect.

 

[T L]

Date: 6/2/2009 9:58:16 PM
Author: Chrono
Richard,
That's all fine and dandy but it would be rather inconvenient to have to angle my ring all the time to get rid of this 1/2 and 1/2 effect.

I know, it's incredibly hilarious. The gem cutter of my stone above told me that there's no half extinction if my line of sight is exactly perpendicular to the table. I mean how ridiculous is it to expect someone to always look at their stone that way? Good grief!