Extinction

[Garry H (Cut Nut)]

This is a common topic in the colored stone world.
I just read many of the threads here, inspired by an article Richard Hughes wrote for the australian jewellery valuers mag, and sadly, it seems that few colored stone people understand the cause of extinction- it is the viewers head and body (or camera) obscuring light.

In the Diamond world this was bought to our attention by Bruce Harding, aka Beryl, and I commend his work, which was almost entirely about colored gems, but to date seems mainly have been useful for diamonds.

https://www.pricescope.com/community/threads/the-amazing-work-of-bruce-harding-aka-beryl.151115/
Here is a link to introduce you to Bruce - and i am sure if you invite him, he will come and play here too.

For those of you on this board who love colored gems, you may find this very useful. Bruce's work is very simply explained and although it ends up being very complex, just the first one or two pages will be very instructive.

Meanwhile, a simple experiment:

Take a gem with a lot of extinction in a well lit room (daylight or fluoro tubes - bath rooms with mirrors and lots of light etc) and a sheet of copy paper with a 1/2 inch hole opked in the center.
Look at the gem, then with plenty of light shining on the white paper, peek thru the hole - and you will find a lot of the extinction will have disapeared.

 

[Lady_Disdain]

Thank you Gary - very interesting.

However, the everyday look of the gem is what is more important to me, as a wearer of jewelry. And, everyday, there is neither a black camera nor a white piece of paper in front of the gem. I will have to think about this a while.

 

[LD]

Hi Garry - Please forgive me but I also believe that extinction (in coloured gemstone world) can be caused by the cut and placement of facets rather than just being obscured by the head. Of course head obstruction can occur but so can extinction without that! The darker body colour the gem has, the more obvious extinction will be if it's been cut that way.

Here's an article written by Richard Hughes that explains it much better than I ever could!

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

 

[chrono]

While there is darkness or extinction of a gem that is caused by obstruction or shadowing (which thankfully will go away when the obstruction is removed), there is also a more permanent type of extinction caused by cut, either a poor design or execution of facets. Many gem wearers like to wear their gems and have it look their best without carrying around pieces of paper with holes in them or strapping a super powerful torchlight on their heads.

 

[VapidLapid]

If extinction is caused by the viewer's head then all stones must appear extinct to the viewer.

I am more inclined to think that extinction is a function of some unfortunate pavilion angles working in collusion with some mischievous crown facets to exploit the angles at which light is returned to the viewer such that the light cone expands around the axis of the stone and creates zones of blankness at certain distances. Place the eye in at the distance of the zone and voila, extinctions, One should not have to remove one's head to enjoy one's gems.

 

[Michael_E]

If extinction is caused by the viewer's head then all stones must appear extinct to the viewer.

I think that everyone gets confused by the word "obstruction". Your head isn't blocking light so much the image of your head is being reflected in the gem. The gem can only reflect what's in front of it and the angles on both pavilion and crown determine WHERE each reflection seen in the stone comes from.

With good cutting the reflection of your head is minimized and light is mostly reflected from the areas to the sides of your head. Since the reflection of your head will take up more of the stone the closer you are, you can test whether the "extinction" you are seeing is caused by the reflection of your head by simply moving the stone away from you. If the extinction is reduced or eliminated as the stone is moved away, then you are seeing a reflection of your head. The white paper trick that Garry alludes to has the same effect , but is stronger...great as a method of photographing a gem at it's maximum brightness!

One subtle problem associated with this is that even if a gem is well cut, and light is being reflected from areas to the sides of your head, it can still look extinct. This is because there is little light coming from behind you, which is where the gem is reflecting from. I see so many pictures here about a stone being "extinct" or showing half bright and half light and then people drawing the conclusion that there's a problem with the cut of the stone. The reality is that they are not using good light to view it in. Looking at a gem with darkness behind you or in a room with strong directional light is a very bad way to look at a gem and does NOT mean that the stone is showing extinction. The conclusion? Make sure that you're viewing a gem in decent lighting before you conclude that it's extinct, shows head reflection or is poorly cut.

 

[PrecisionGem]

I'm with the rest of you PS's on this one. Extinction is a function of the cut, and also the material. There are stones that have almost no extinction, and others that all almost all extinction in the same lighting. It just cant be a head shadow. Try like you will with a cut, and a dark stone will still be a dark stone. There are things in the cut that can improve it, but not enough to turn it into a stellar piece.

 

[chrono]

Michael,
While your suggestion to view a stone that shows the half and half extinction under a different lighting to minimize this effect is a good one, it still doesn’t resolve the fact that it continues to bother many owners of these stones who frankly do not want to see this characteristic at all under any lighting circumstances. Perhaps the place where they wear such a gem most frequently has this darkness behind them or has strong directional lighting and is unavoidable. I choose to return such stones because I have seen with my own eyes that some stones will show this half and half extinction in such lighting but also some stones do not, and those that don’t are obviously keepers.

 

[LD]

I'm with the rest of you PS's on this one. Extinction is a function of the cut, and also the material. There are stones that have almost no extinction, and others that all almost all extinction in the same lighting. It just cant be a head shadow. Try like you will with a cut, and a dark stone will still be a dark stone. There are things in the cut that can improve it, but not enough to turn it into a stellar piece.

Absolutely agree! If you also add in obstruction of your head whilst looking at such a stone you're completely onto a loser!

 

[movie zombie]

cannot abide extinction. i understand the "head obstruction" explanation......and continue to be critical of a poorly cut stone that despite all efforts at lighting and making sure there is no "head obstruction" are extinct.

while some stones are just over saturated or dark by nature in larger sizes, i also believe that some cutters are maximizing the size of a stone and unwilling to sacrifice a lot of rough in order to eliminate extinction.

i also believe that many consumers don't know the difference or even care. the price of color stones goes up as the quality of cutting goes up and i'm not sure that buyers are willing to pay for better than average stones. of course, quality of the rough in the first place is also a limiting factor........but again, there is something for every budget. which is a good thing.


MoZo

ps and the reality is that the stone despite extinction may still be a bargain. one cannot expect the perfect stone for basement bargain prices. one can hope, one can search, and it can happen.....but rarely. the best one can hope for is to get the best buy for whatever the $ being spent. and while i am very Very VERY critical re stones posted here at P* re extinction [and windowing as well], i realize that in some cases people may have passed on stones that would perform adequately for the $ being spent.

 

[lelser]

When we redid our kitchen, we weren't happy with the finish on the custom cabinets. The kitchen contractor tried to convince us that we needed to light the room differently to mask the problem. Not the right answer and he took the doors back to refinish them.

"Head shadow" and extinction are not the same thing. You can get head shadow on a perfectly well cut stone. A stone that's got extinction will have problems whether there's an obstruction or not.

Lisa
www.lisaelser.com

 

[beryl]

. Read "Faceting Limits", Fall 1975, in "Gems & Gemology", now available online in GIA's archives. It has been mostly reproduced at Russian website http://www.gemology.ru/cut/index.htm . I never used the term 'extinction' and tried to avoid the term 'head shadow'; in fact, I posted an illustrated item in 'Diamond Talk' showing that head obstruction and head shadow are not the same.
. I have just read an article by Hughes sent to me by Garry Holloway. It does not explain 'extinction' well, and I am amazed that it does not list the 1975 GIA article in its references. Vargas' 2nd edition, circa 1976, discusses it at some length.
. John Sinkankas was, of course, unhappy that I suggested errors in his recommendations; in fact, it was by cutting a garnet at 40/40, per his suggestion, that I 'discovered' the problem of viewer head obstruction. With the light behind me, the stone got dark when it came closer to my face, as one writer has mentioned here. (AGS says the right word is 'obscuration'.)
. The most amazing result of that study was that, when I compared my findings to the cuts deemed 'best' by centuries of trial-and-error, they all fell on the edges of the 10° head obstruction range, which Garry points-out was due to the pleasing light-dark contrast that resulted (note: 'brilliance' and 'brightness' are different things). This was best summarized in a talk I gave to a gemology club in Antwerp, May 2005. Copy available on request.
. This subject was also discussed in an article "Optimizing Faceting for Beauty" by Anton Vasiliev, originally in Russian Gemological Bulletin 2002, 5(2) and then in British "Journal of Gemmology", January 2004. Anton is an optics physicist and has a colored-stone-cutting business in Nepal and Armenia. He is also the architect of "Facet Designer" which I can send you to study optical effects of various cuts and lighting.

 

[diagem]

. The most amazing result of that study was that, when I compared my findings to the cuts deemed 'best' by centuries of trial-and-error, they all fell on the edges of the 10° head obstruction range, which Garry points-out was due to the pleasing light-dark contrast that resulted (note: 'brilliance' and 'brightness' are different things). This was best summarized in a talk I gave to a gemology club in Antwerp, May 2005. Copy available on request.

Interesting, I guess genuine T&E is still a must
I would love to read a copy, do I need to put a special request?

 

[Garry H (Cut Nut)]

When we redid our kitchen, we weren't happy with the finish on the custom cabinets. The kitchen contractor tried to convince us that we needed to light the room differently to mask the problem. Not the right answer and he took the doors back to refinish them.

"Head shadow" and extinction are not the same thing. You can get head shadow on a perfectly well cut stone. A stone that's got extinction will have problems whether there's an obstruction or not.

Lisa
www.lisaelser.com

Thanks Lisa,
I am sorry for mixing up terms - but I hope people got the message.

Folks extinction definitely occurs because of longer ray paths in darker material - a less common factor with diamonds.

But that is a smaller cause than those defined by Bruce / Beryl in his brilliant article that he linked to. Thanks for joining Bruce.
I made 3 DiamCalc video's with emerald green sapphire (do not have a ruby or blue spectra) to help show this effect. Since we do not have our video system yet I asked Andrey for permission to link to You tube.
1 - dark behind stone (all in my store lighting) http://www.youtube.com/watch?v=c-V7XQR1FtU
2 - with graph paper behind http://www.youtube.com/watch?v=8w0Uz5CAjOw&feature=related
3 - as above but with the observer's head removed. http://www.youtube.com/watch?v=ciH10FXr-xg&feature=related

Of course it would be better if all 3 played together, but I have no idea how to do that.

However you can open and play 3 windows at once.

 

[LD]

Garry - I apologise if I'm repeating myself but this thread might be very confusing for newbies to coloured gemstones! Extinction and obstruction (aka head shadow) are two different animals with coloured gemstones and can occur separately as well as together. However, they are most definitely not the same thing. Extinction can occur simply because of the cut of the gemstone and nothing to do with obstruction. Nothing will make extinction in a gemstone disappear BUT in some lighting conditions the extinction will be far less noticeable than in others. However, if you're photographing a gem and your head casts a shadow on a gem, once your head is removed(!), the problem should go away!

Easy answer to obstruction (head shadow) is to remove your head when photographing gemstones

Not so easy answer to extinction is to possibly have the gem re-cut but this opens up an entirely new thread on the pros and cons!

 

[Garry H (Cut Nut)]

Garry - I apologise if I'm repeating myself but this thread might be very confusing for newbies to coloured gemstones! Extinction and obstruction (aka head shadow) are two different animals with coloured gemstones and can occur separately as well as together. However, they are most definitely not the same thing. Extinction can occur simply because of the cut of the gemstone and nothing to do with obstruction. Nothing will make extinction in a gemstone disappear BUT in some lighting conditions the extinction will be far less noticeable than in others. However, if you're photographing a gem and your head casts a shadow on a gem, once your head is removed(!), the problem should go away!

Easy answer to obstruction (head shadow) is to remove your head when photographing gemstones

Not so easy answer to extinction is to possibly have the gem re-cut but this opens up an entirely new thread on the pros and cons!

Please read Bruce's article

 

[LD]

I have! I'm clearly thick because the article refers to diamonds (although I appreciate Bruce has mentioned a garnet in his post here) and the importance of cut etc. So that does bear out what has been said in this thread that (forget the math/critical angles for a minute) that cut is important and can be the cause of extinction - but not obstruction.

I must be missing a few points along the way (something to do with removing my head when taking photos I think) and as it's after midnight here I'm going to bed to ponder!

 

[Garry H (Cut Nut)]

I have! I'm clearly thick because the article refers to diamonds (although I appreciate Bruce has mentioned a garnet in his post here) and the importance of cut etc. So that does bear out what has been said in this thread that (forget the math/critical angles for a minute) that cut is important and can be the cause of extinction - but not obstruction.

I must be missing a few points along the way (something to do with removing my head when taking photos I think) and as it's after midnight here I'm going to bed to ponder!

Wrong, the article refers to all gems and I think the RI in the example used thoughout is for 1.6.
(I am sad that some Gem fans seem to have a 'thing' about diamonds - the rules are exactly the same. There has been more trial and error and more accurate tools and data used for diamonds. But Bruce's work is valid for all materials. And it is so wonderfully simple. Now how much contrast you choose to have (contrast in diamonds more or less =extinction) is up to you - but I would suggest that it should be releated to the tone and size of the material - and you guys could easily have that discussion and add some real value)

Bruce did not give the direct link - it is here
http://www.gemology.ru/cut/english/faceting/
There are several short pages and the all but last pages gives data for several gem stones http://www.gemology.ru/cut/english/faceting/f5.htm#chartformaterials

If you follow those cutting rules then extinction is only a problem from longer ray paths in very dark material.

 

[beryl]

. Unfortunately, the Russian reproduction of "Faceting Limits" was posted under the umbrella of 'diamonds', which has caused confusion to many. The charts formed in the text, to illustrate the various points, were for RI=1.67 (peridot & spodumene) because it fit nicely but the final charts included RI 1.55, 1.58, 1.62, 1.72, 1.76 amd 1.85. Diamond was added last only because it seemed fair to do so (I had no interest). In the GIA article you can see all 8 charts on one page, as in my Antwerp presentation, whereas in the Russian one you must click on each little grid icon individually. The Russians also did not include the formulas but GIA made a typo there and most folks don't care anyway (btw: GIA reversed Fig.1 & 2, which illustrate head obstruction).
. Garry, since we cannot swap e-mail addresses here, I suggest that this is the place for the Antwerp presentation I retracted from PriceScope 3 years ago because it did not belong in the diamond area. Is there such an area for colored-stones only?
. DiaGem: wait for this or get my e-mail address from Garry or Andrey. Your comment about trial-and-error is a bit incorrect; the T&E results of centuries were correct and "Faceting Limits" agrees with them, so now we can get in the ballpark (on common designs) with this new knowledge and limit our T&E to tweaking them. As far as fancy designs, such as several of you use for logos here, it's mostly luck - there is no T&E. Some new designs with symmetry can be tested on Anton Vasiliev's software "Facet Designer". I found a nice 5-sided one this way. Just like engineering: the formulas we have today are the result of T&E; now we use them to do things right the first time.
. While the factors studied in FL are common to colored-stones and diamonds, the two are vastly different. In diamonds light return is major so the cuts are of few types such as the round brilliant; and the concepts of FL are most significant. In colored-stones, color and pretty patterns are more important and FL is only a general guideline. In cutting, also, the problems are very different: hardness & grain in diamond vs softness, birefringence, and pleochroism in colored-stones. Did you ever wonder why you rarely see round tourmalines or emeralds? I find that the people on each side of the fence know little about the other side, but the behavior of light is almost the same.

 

[diagem]

. DiaGem: wait for this or get my e-mail address from Garry or Andrey. Your comment about trial-and-error is a bit incorrect; the T&E results of centuries were correct and "Faceting Limits" agrees with them, so now we can get in the ballpark (on common designs) with this new knowledge and limit our T&E to tweaking them. As far as fancy designs, such as several of you use for logos here, it's mostly luck - there is no T&E. Some new designs with symmetry can be tested on Anton Vasiliev's software "Facet Designer". I found a nice 5-sided one this way. Just like engineering: the formulas we have today are the result of T&E; now we use them to do things right the first time.
. While the factors studied in FL are common to colored-stones and diamonds, the two are vastly different. In diamonds light return is major so the cuts are of few types such as the round brilliant; and the concepts of FL are most significant. In colored-stones, color and pretty patterns are more important and FL is only a general guideline. In cutting, also, the problems are very different: hardness & grain in diamond vs softness, birefringence, and pleochroism in colored-stones. Did you ever wonder why you rarely see round tourmalines or emeralds? I find that the people on each side of the fence know little about the other side, but the behavior of light is almost the same.

\

I agree we got our cut basis from Centuries of trial & error, so obviously today the T&E is cut down to what you called tweaking but it is still a must and an integral part of the designing of light behaviour in Gems. Off course I speak as a cutter for Diamonds (and not colored Gems) but also believe the behaviour of light is the same just calculated differently in different mediums.

I am confused though about your remark:

As far as fancy designs, such as several of you use for logos here, it's mostly luck - there is no T&E.

care to elaborate a bit more on that? I would just like to add..., not everything do-able virtually and/or in theory is do-able in practice, but I must admit I dont have any specific experience with Vasiliev's software "Facet Designer".

Any reason I notice plenty of Zambian rounds vs. other sourced Emeralds?

Thank you, I will ask Garry for your email address.

 

[beryl]

Dia Gem:
. I mean that those fancy shapes, used for logos because of their uniqueness, are probably one-of-a-kind = no trial-and-error; this may be an incorrect assumption. You ask good questions; thank you.
. I am unfamiliar with the round Zambian emeralds you mention (I have been commercially inactive for many years). I would like to see the finished stones and the corresponding rough; the gem axis must be the C-axis of the crystal and the color pale. I cut a round tourmaline once like this - almost white and worthless from the side, but a gorgeous copper-pink endwise; it was amoung the scraps scooped-out of the bottom of a box in Brazil (circa 197!
. Conversely, how often do you see a rectangular natural sapphire?
. I look forward to sharing the Antwerp paper with you - my best work, very brief - the cream that came to the top.

 

[diagem]

Dia Gem:
. I mean that those fancy shapes, used for logos because of their uniqueness, are probably one-of-a-kind = no trial-and-error; this may be an incorrect assumption. You ask good questions; thank you.

I would say its a general statement if you are talking about special designed fancy shaped brands, some of those go through extensive trial & error prior to being released to be marketed to consumers..., on the other hand a one-of-a-kind would probably not gone through any T&E.

I am unfamiliar with the round Zambian emeralds you mention (I have been commercially inactive for many years). I would like to see the finished stones and the corresponding rough; the gem axis must be the C-axis of the crystal and the color pale. I cut a round tourmaline once like this - almost white and worthless from the side, but a gorgeous copper-pink endwise; it was amoung the scraps scooped-out of the bottom of a box in Brazil (circa 197!
. Conversely, how often do you see a rectangular natural sapphire?
. I look forward to sharing the Antwerp paper with you - my best work, very brief - the cream that came to the top.

You can see them easily marketed by Tiffany's et al, they are definitely not of pale color saturation, likewise I too am looking forward to reading the Antwerp paper, thanks for the offering

 

[beryl]

DiaGem:
. Are these 'Zambian' emeralds the same as the ones I knew of in the 70's as 'Sandawana'. An emerald dealer visited my shop while I was looking at one and saw immediately that it was not Colombian; I vaguely recall that they were different but can't remember how. I will check Tiffany's and/or some local jeweler friends, but this will not show me the rough.
. I just remembered: I have an article by Anton Vasiliev discussing the differences in emeralds from different locations; hopefully I may have already translated most of it. I recall that he discussed the varying percentages of vanadium, etc. Emeralds are very important in Russia because of the Ural sources, and those are different too. I think I have not seen Ural emeralds. Whereas Guebelin distinguished Colombian emeralds by their inclusions, Anton can probably tell you where in the world an emerald came from (he uses spectroanalysis in addiion to other criteria). I may also check with John Koivula at GIA (he worked with Guebelin).
. My forum name 'beryl' came from my diggng of it in Massachusetts and New Hampshire mines, reworked in the 50's & 60's for beryllium for the space effort (milky green or brown shades, mostly not gem quality). 30 years ago my son and I uncovered the Reynolds mine mentioned by Bauer in the 1800's as the finest aqua in the world (before Maxixe). It is less than 20 miles from here; the color and clarity are great but the stones were shattered by dynamite when last operated in 1917. Today's ecology laws prevent us from working the mine effectively (the vein is only 4 feet wide, 8 feet deep, and 20 feet long, in a soft brown schist on a low hillside).

 

[beryl]

. Ta-daaaa! I found Anton's article "Emeralds of the Main World Origins) in Russian Gemological Bulletin No.11, 2004 (in collaboration with E.P.Melnikov and G.N.Pilipenko) - and that I had nearly completed phase #1 of the translation - enough to know what he's saying. I must yet look up some unfamiliar words - mostly foreign locations. I will not bother to rephrase it in English form at this time.

 

[diagem]

DiaGem:
. Are these 'Zambian' emeralds the same as the ones I knew of in the 70's as 'Sandawana'.

I am not sure as I am distantly familiar with this fact within the last decade...

 

[T L]

DiaGem:
. Are these 'Zambian' emeralds the same as the ones I knew of in the 70's as 'Sandawana'.

I am not sure as I am distantly familiar with this fact within the last decade...

No, those are two separate locations for emeralds in Africa. Sandawana emeralds are typically very small. Zambians are larger.

 

[beryl]

Tourmaline Lover: Thank you. In skimming Russian study I see only Zimbabwe mentioned in Africa.
. I recall India emeralds as being very dull. I recall Russian article as saying that Ural emeralds are yellower (iron if I recall). It has been several years since I read it.
. Funny story. One day in our store a woman wanted a lion's-head ring with an emerald in the mouth. Each one I showed her was not green enough, so I put in a tsavorite; she said that was just the right color but wouldn't buy it because it wasn't an emerald.

 

[PrecisionGem]

Gary, Bruce and DiaGem, thanks for the interesting posts. I see Bruce you are a mechanical engineer too. To often we can over analyze and over calculate things. I see this in cutting colored stones. Many cutters are very deep into the machines, and debates for ever on polishes, some try to calculate for ever the optimal angles. My findings, in reality boil down to the material. I'll get a piece of rough, that I know will cut a spectacular stone, and as long as the cutting angles are within certain ranges will produce a fine stone. What's more important is the orientation and the stone it's self. A dark stone with plenty of extinction, no matter how you cut it will like like crap. To me the whole head shadow thing is salesman talk to try to talk your way out of a stone that doesn't perform. As a few of the non trade people pointed out above, they want to look down at the stone on their had with out lights perfectly positioned or a white piece of paper taped to their head with a hole in it.

Sure some cuts will work better with dark stones, some with lighter stones, and some with various materials, and this is where the experience of the cutter comes in. I don't think it can be resolved to a set of equations. I think cutting is more of an art than that, very few artist will reduce an oil painting to equations.

As far as answering the question about why you don't see certain stones often in rounds etc, it has a lot more to do with the shape of the rough than anything else. A lot of tourmalines grow as long crystals, and often have a dark axis, these stones lend them selves to opposed bar cuts, certainly not rounds, but that doesn't mean that other tourmalines can't produce fantastic round stones.

 

[texaskj]

Y'all are making hy head hurt. I'm just going to ask for opinions here before I buy another colored stone.

 

[Harriet]

Tourmaline Lover: Thank you. In skimming Russian study I see only Zimbabwe mentioned in Africa.
. I recall India emeralds as being very dull. I recall Russian article as saying that Ural emeralds are yellower (iron if I recall). It has been several years since I read it.
. Funny story. One day in our store a woman wanted a lion's-head ring with an emerald in the mouth. Each one I showed her was not green enough, so I put in a tsavorite; she said that was just the right color but wouldn't buy it because it wasn't an emerald.

I have been told that the Sandawana emeralds are yellowish compared to the bluish ones from Zambia. I had the pleasure of handling 2 of the former and they were yellowish.

Out of curiosity, what did you tell your client?