Extinction

[beryl]

Gene:
. I agree with you wholeheartedly. I was disappointed to see people carry my work to silly extremes. One wanted to develop one of my charts for a specific RI (he couldn't solve my formulas because they are implicit - I did the whole thing with a slide rule!). For colored stones it is less important than for diamonds because light return is not the most important thing. Here is the preface I wrote for reprints I made via Xerox in 1987:

. "There has been much correspondence from many countries, all favorable; however there has been a tendency to over-rate the value of this work. Before gem-cutting, my hobby was mathematics and I worked in true research; it was automatic that I would approach faceting in this academic way*.
. I am satisfied that I proved WHY the well-known 'best' slopes are best and that there are others which have probably been 'discovered' often. We contributed nothing new ... just showed WHY!
. We now cut stones only for profit and focus on those factors which most affect salability ...
... GOOD NATERIAL - color & clarity count most
... GOOD POLISH ---- important in brilliance
... FLAT FACETS ----- important to scintillation
We use mostly traditional cuts and facet slopes."
* A recent hobby interest in computers produced many interesting results. When shown to a British expert he said, in essence, "So what?"

. Al Fargo, the faceting man at Raytech, visited a club in Texas who were examining facets by magnification and missed that that they were not flat! They were using 'Ultralaps' = thin mylar discs which bulged-up slightly at the leading edge.
. Glenn Vargas pointed-out that Brazilian stones then had all facets rounded on the same edge because they flop them down onto the lap (hand-held jamb-peg method then).
. On a TV marketing show you will see the round-off edges of gems as they revolve in the light.
. A few years ago I visited a faceter's club. One admired local 'expert' showed a faceter head he made displaying the tilt angle to 3 decimal places; it varied as you walked near it! Nothing on a faceting machine can reproduce that accurately! Anton Vasiliev, who cuts commercially, agrees with me that anything beyond one decimal place is absurd.
. My computer interest produced a ray tracing program (in BASIC!) in 1986, which I am told was the first. Garry played with it at my home in 2000. I gave copies to the Russians and later asked them to include my multiple ray function in theirs (DiamCalc), which you will see in some of my illustrations here - a fun tool used by few.

. You are right about the crystal orientation and direction of best color - one of my favorite questions. Beryl & tourmaline generally have best color crosswise to the length of their prismatic crystals - hence the long step-cuts. Corundum generally has best color looking down the c-axis, so they are generally but round; however, synthetic boules (in the 70's) were seeded on a bias and split lengthwise so a rectangular-cut ruby or sapphire was generally synthetic then.
. Garry: this is one of the significant differences from diamond-cutting.

 

[beryl]

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

Texas: That is silly; you buy a stone because you like it. Our opinions have nothing to do with it except to say watch out for soft stones (opal, etc) and how they are mounted (no projecting points to catch on clothing).

 

[beryl]

[quote="Harriet|1294467565|2817373. 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.[/quote]

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? [/quote]
Harriet:
. Thanks for the input.
. I told the lady that emeralds don't come in the color she wanted. She left. I could have said nothing and sold her the tsavorite, but I don't do that; a lie can come back to bite you.
. I am curious about the stones in your logo. The colors look unnatural; are they imitations or a color reproduction problem?

 

[Michael_E]

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.

At one time this was true, but it has already changed to the point where there is really very little guesswork involved in know what a particular design in a particular material will look like in a specific lighting environment. This is because the way light works CAN be reduced to a set of equations. Complicated equations, certainly, but with the the speed and accuracy of computers and advanced software you can tell very precisely what will happen with any stone in any environment.

The big problem comes in the time required to do this and how many different environments you are going to have to model to assure yourself that the stone you have will work well in all of them, (many, of course, only work well in a few environments). That's where experience comes in handy, (as a time saver only), since you've already "modeled" many different stones in many different lighting environments by looking at them in a critical way and "know" what will happen when cutting a rough stone. Sometimes though, our memories are faulty or we want to believe in something which isn't true, (such as there being some sharp pinnacle of cutting quality called, "Ideal"), and so come up with all sorts of stuff to support our wish to believe what we want to. We can't do that any more since we can get very accurate answers to questions regarding how light will be changed as it moves through a gem and can predict exactly what will happen with almost any cut, any stone and in any environment.

 

[beryl]

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.

Gene: I agree with you, and so does Gabi Tolkowsky, a famous diamond cutter. You would be delighted by his presentation of the sounds made by a diamond as it is rotated in a light and the reflections are converted to musical tones.
. Attached is pic of Gabi & me at Moscow Conference 2004.

. I cut for pleasure, which is also in the eye of the beholder. One of my favorites is andalusite with its trichroism and inclusions. To me, inclusions do not detract, they add interest. I make up whatever shape the crystal (and its color) tells me, with no intent to sell.

 

[PrecisionGem]

Michael_E, can your software model in fine needles in the stone? Can you model in the purity of the crystal? These types of things factor into the stone. The software programs I have seen that try to show how a finished stone will look are helpful not always completely realistic.

Some of the best cutters I know, don't use any rendering software.

 

[beryl]

Just found this pic I made in 2004 and probably posted on 'Diamond Talk' at that time. This is my 'thousand words'.

 

[Michael_E]

Michael_E, can your software model in fine needles in the stone? Can you model in the purity of the crystal? These types of things factor into the stone. The software programs I have seen that try to show how a finished stone will look are helpful not always completely realistic.

Some of the best cutters I know, don't use any rendering software.

Yes, it can model everything that you'd want to add to it. BUT, and that's a BIG BUT, it is often not time effective to mess with it because the computer in my mind has already seen the effects from the more difficult to model features of a stone and so it it not worth the time to do the modeling for those features. What it is worth using for is to maximize the quality of design in any particular cut stone. Let's see, here's a quickie that I did for a fellow on GO. He had constraints in the depth of an aqua and so I changed the design from a barion brilliant cut to something with a small set of keels with barion like break facets. The attached picture shows the barion on the left and the modified stone on the right. This saved a couple of mm in depth while maintaining a similar face up profile. The picture shows these in the same lighting conditions and allows a comparison which is quite realistic, assuming similar rough. I'm not saying that this is perfect, but it sure beat trying to go by some rules of thumb for angles.

 

[Michael_E]

Just found this pic I made in 2004 and probably posted on 'Diamond Talk' at that time. This is my 'thousand words'.

Bruce, your writings on line are what had initially interested in finding out more about what is going on. I appreciate your thoughts on this and your prior writings as well as anything else that you comment on in the future. My views on you diagram are that the shadow portion of this drawing means nothing when it comes to how a stone looks to that viewer. What you have labeled obstruction is only really an obstruction if there is a single light source behind the viewers head. In a real world scenario there is a whole hemisphere in front of the stone which can be reflected back to the viewers eyes. The viewers head is only part of that lighting environment and so the "obstruction should be labelled, "head reflection" since what the viewer sees is a dark reflection of their own head. Other areas outside of that cone from the stone to the head, can provide light to be reflected to the eyes of the viewer and where that light comes from and it's quality is even more important than head reflection in a well cut gem.

The objective of cutting a nice looking stone is the balance the contrast of the viewers head reflection, (dark), with the areas outside of that cone, (light). With that in mind it becomes kind of limiting to say that for any stone there is some pinnacle of perfect cutting which one can call "Ideal" and I have actually cut stones which were purposely so far off of an "Ideal" mind set that one would think that they were bound to be dogs. In fact they had turned out to be very nice looking stones and upon examination it becomes clear that there is much wider range of possibilities for stone cutting, which can produce a nice looking stone, than had previously been thought possible. As an example here's a tanzanite which has a Critical Angle of 36.28° whose pavilion was cut at 31°. It was a nice looking, bright, stone that looked much better than it's weird cutting angles would indicate and showed no more extinction that a "standard" cut tanzanite.

With the broadening of what a cut gem means, (concave cutting, bubbles, "V''s", curved girdles, carvings and all sorts of combinations), it is becoming more and more critical that a cutter utilize all of the means available to foresee what the end product of their labor will become... trial and error or rough guidelines just don't cut it, (nice pun, huh?). Your thoughts?

 

[PrecisionGem]

Michael, do you have a photo of the cut stone to compare with your rendering? I typically find that the actual stones look nothing like the renderings from programs such as BOG.

 

[Michael_E]

Michael, do you have a photo of the cut stone to compare with your rendering? I typically find that the actual stones look nothing like the renderings from programs such as BOG.

Not on these stones. I only use BOG in a coarse way to get close and then do the renders using Maxwell Render. I initially was using this to show people what there pieces would look like assembled, but it works so well with gems that I've been using it to help with stone design. It's pretty funny that you'd want a picture, since I've seen very few pictures that really look like the stone they're trying to capture. Not surprising since the look of the stone is just as dependent on the environment it's in as the qualities of the stone. I do agree with you and Bruce about inclusions, they can really make a stone much better than without. I'll try to show some renders compared to cut stones next week when I have more time.

 

[VapidLapid]

This stone is very extinct.

 

[Harriet]

Harriet:
. Thanks for the input.
. I told the lady that emeralds don't come in the color she wanted. She left. I could have said nothing and sold her the tsavorite, but I don't do that; a lie can come back to bite you.
. I am curious about the stones in your logo. The colors look unnatural; are they imitations or a color reproduction problem?

You're welcome!
What colour did she want?
They're all natural. The Rubellite and the Mahenge spinels are redder in person, but I have no idea how to use Photoshop.

 

[beryl]

. I think the point of “Faceting Limits” is being missed by some; it would perhaps become clear if they read it.
. It is not just proximity of the viewer’s head, or pavilion and bezel slopes, but the combination of all 3 which cause the viewer to see himself.
. Attached here is an FL diagram for CZ. The shaded areas are combinations of pavilion and bezel slopes which produce a divergence of less than 10° between the incident and returned rays, whereby the viewer might typically see himself.*
The vertical area at the right is for table-to-table rays. The upper curved diagonal is for table-to-bezel rays (and vice-versa). The lower curved area is for bezel-to-bezel rays, but these rarely occur in a typical faceted gem.
. For any combination of pavilion and bezel slopes within these areas the viewer will probably see himself. The edges correspond to 10° divergence, points along the centerline of the area correspond to 0° divergence, and other points are somewhere in-between.
. To check my method, I compared the historically-evolved ‘best’ combinations against these charts. They all fell outside of these shaded areas. This confirmed my method.
. In all cases they fell very near the edges of the upper curved zone - all near the upper edge (Zone A), except stones RI 1.6 to 1.7 (topaz, tourmaline, peridot, spodumene, etc.), which fell near the lower edge (Zone B). This is discussed in the Antwerp paper, soon to be presented here (I hope).
. Although stones can be cut in any of the bright areas, history chose these edges. Why?
Garry Holloway suggested the answer 25 years later – because it results in contrast: between mains and breaks, which are only about 2° apart (on a round brilliant) and also changes from light to dark as the stone, viewer, or light source is moved. This makes a stone more attractive to a viewer. Long discussions of this in ‘Diamond Talk’ forum (before PriceScope) led to the general recognition that ‘brightness’ and ‘brilliance’ are two different things.

* This angle varies with the size of the viewer’s head (or coif or hat) and distance from the gem, but I found that with my head about a foot away, 10° was a good approximation.

 

[beryl]

Michael E.
. Your point is well-taken that the viewer's head obscures only light from behind his head. However, if those are the rays that the cut returns to his eye, then he will only see in himself in the areas served by those facets.
. "Facet Designer", software by Anton Vasiliev, shows that each light you see comes from a different source (and where those sources are). Only some of them will be obscured by the viewer. The attached illustration, from a related Russian article by Anton, shows white dots on the sphere which are the directions of light sources seen in each tiny reflection; notice how close some of these are to the viewer's head. I have blocked all spots, except one-at-a-time, to find where certain reflections come from - very interesting.

 

[beryl]

. As a point of possible interest, here is an illustration from my ‘GemRay’ program, 1986, written in T-Disc BASIC on my Toshiba T-100 computer. There were no common languages then (not even a mouse); this was written for Toshiba by a little company in California called Microsoft. I used it for 12 years and missed the era of MS-DOS. When it died and nobody knew how to fix it*, I bought a ‘modern’ computer, with mouse and ‘Windows’. I rewrote this program in Q-BASIC on the IBM-type computer and added a few features – available to anyone.
. Garry Holloway visited me in Dec 2000 and had fun playing with it and I sent copies to the Russians on ‘Diamond Talk’. I asked DiamCalc to add this multi-ray and double-view feature to their software; I can still do some things with this that I can’t in DC.
. Note the rays which appear to hit the viewer’s eye. Actually, all of the vertical lines are from the viewer’s eye and therefore represent rays going to the viewer’s eye. Note that some of them apparently come from his head and therefore appear black = what you folks sometimes call ‘extinction’.
* Local Toshiba ‘experts’ said there was no such thing; my son-in-law later fixed it for less than a dollar. Sadly I threw it, disc drive, and printer away and cannot retrieve things now.

 

[beryl]

. In the above illustration, notice (on the left) that some of the returned rays cross their sources, although they appear to be divergent in the big picture (at the right); these rays correspond to pavilion/bezel slope combinations below the upper shaded area (Zone B) in the 'Faceting Limits' diagram. The truly divergent ones are in Zone A. This is discussed in the Antwerp paper.
. Note also that the pavilion and bezel each appear to have two slopes; this is because the plane was chosen through the breaks and stars.
. Would you believe --- 24 years ago! Peter Yantzer of AGS calls me 'the Martian' because I arrived too soon.

 

[beryl]

The objective of cutting a nice looking stone is the balance the contrast of the viewers head reflection, (dark), with the areas outside of that cone, (light). With that in mind it becomes kind of limiting to say that for any stone there is some pinnacle of perfect cutting which one can call "Ideal" ...

. Michael: You have said it well.
. Pre-2000 Garry Holloway was saying that there was a continuum for cutting good diamonds with the bezel slope decreasing by 4-,5-,or 6-1 for each increase in pavilion slope. The 'experts' laughed at him but Garry is a master of observation and he was 'proved' correct when he saw that slope in my "Faceting Limits" diagram and the charts by Moscow State Univ. You can see a similar slant In the CZ-diagram I attached here. The top of the upper dark zone has a slant of -6:1 - from 41° to 29° bezel slope (-12°) for 40° to 42° pavilion slope (+2°). Any cut along this line should have good contrast. Considering the clarity of CZ, I would suggest cutting right along this line. For more saturated color-stones I would cut above this line. For fancy stones forget the whole thing except critical angle, and you have shown that can also be violated with your tanzanite (same post). BTW what is tanzanite, generically? I forget: it was coming in as I was going out.

 

[Harriet]

. I am curious about the stones in your logo. The colors look unnatural; are they imitations or a color reproduction problem?

Beryl, is this a question or an insinuation?

 

[Garry H (Cut Nut)]

. I am curious about the stones in your logo. The colors look unnatural; are they imitations or a color reproduction problem?

Beryl, is this a question or an insinuation?

Or beryl, perhaps your screen needs recallibration? I run my lap top through a desk top keyboard and big screen - and the big screen has them jumping out of their skins ultra fluoro. The pinks are like my Ideal-scope avatar.

 

[Harriet]

Garry,
Thanks mate.

Here's a LWUV photo of one of my pieces:

 

[Garry H (Cut Nut)]

The objective of cutting a nice looking stone is the balance the contrast of the viewers head reflection, (dark), with the areas outside of that cone, (light). With that in mind it becomes kind of limiting to say that for any stone there is some pinnacle of perfect cutting which one can call "Ideal" ...

. Michael: You have said it well.
. Pre-2000 Garry Holloway was saying that there was a continuum for cutting good diamonds with the bezel slope decreasing by 4-,5-,or 6-1 for each increase in pavilion slope. The 'experts' laughed at him but Garry is a master of observation and he was 'proved' correct when he saw that slope in my "Faceting Limits" diagram and the charts by Moscow State Univ. You can see a similar slant In the CZ-diagram I attached here. The top of the upper dark zone has a slant of -6:1 - from 41° to 29° bezel slope (-12°) for 40° to 42° pavilion slope (+2°). Any cut along this line should have good contrast. Considering the clarity of CZ, I would suggest cutting right along this line. For more saturated color-stones I would cut above this line.

For fancy stones forget the whole thing except critical angle, You mean pale fancy colored diamonds Bruce?
and you have shown that can also be violated with your tanzanite (same post). BTW what is tanzanite, generically? I forget: it was coming in as I was going out. Zoisite
[b]http://en.wikipedia.org/wiki/Tanzanite[/b]

 

[beryl]

Harriet:
. It was a question and you answered it. I don't insinuate.

Garry:
. I was talking about fancy-shaped colored-stones (not diamonds).

 

[yssie]

The objective of cutting a nice looking stone is the balance the contrast of the viewers head reflection, (dark), with the areas outside of that cone, (light). With that in mind it becomes kind of limiting to say that for any stone there is some pinnacle of perfect cutting which one can call "Ideal" ...

. Michael: You have said it well.
. Pre-2000 Garry Holloway was saying that there was a continuum for cutting good diamonds with the bezel slope decreasing by 4-,5-,or 6-1 for each increase in pavilion slope. The 'experts' laughed at him but Garry is a master of observation and he was 'proved' correct when he saw that slope in my "Faceting Limits" diagram and the charts by Moscow State Univ. You can see a similar slant In the CZ-diagram I attached here. The top of the upper dark zone has a slant of -6:1 - from 41° to 29° bezel slope (-12°) for 40° to 42° pavilion slope (+2°). Any cut along this line should have good contrast. Considering the clarity of CZ, I would suggest cutting right along this line. For more saturated color-stones I would cut above this line. For fancy stones forget the whole thing except critical angle, and you have shown that can also be violated with your tanzanite (same post). BTW what is tanzanite, generically? I forget: it was coming in as I was going out.

Would this not depend on what colour? A yellowy/greeny material that is absorbing blue/purple will return avg refraction wavelengths nearer the normal - to force rays returning toward the viewer into a sufficient divergent angle (to avoid colliding w/ your head obstruction) for different absorbances a steeper bezel for a given pav seems more/less productive, looking at your gemray model.?

 

[beryl]

"Would this not depend on what colour? A yellowy/greeny material that is absorbing blue/purple will return avg refraction wavelengths nearer the normal - to force rays returning toward the viewer into a sufficient divergent angle (to avoid colliding w/ your head obstruction) for different absorbances a steeper bezel for a given pav seems more/less productive, looking at your gemray model.?

. I do not understand your question and, if I did, would not be able to answer it

 

[yssie]

Q: more saturation would require cutting above that line regardless of stone colour?
A: yeah, well, apparently already got that

 

[Treenbean]

I find this all very interesting. Some of it is hard to follow at times. If this is true, that extinction is commonly a result of the viewers head causing a "disturbance in the force". How do you tell truly what stone is extinct? And does it matter, since we all pretty much look at gems the same way ie: with a loupe, resting on our hand etc. Is this a suggestion that there will one day be a change in how colored stones are evaluated? I hadn't heard about ASET prior to 8 years ago (but then and again I wasn't gem crazy then).

 

[beryl]

. I am not familiar with the term 'extinction' as it has been used here in threads pertaining to colored-stones. I assume that it refers to areas which do not return light. One reason is viewer 'obstruction' (AGS says 'obscuration' is correct), where rays returned to the viewer's eye come from his head or body - that is what I discuss. Of course, there is the case of strong absorption by a stone with too-deep color and perhaps there are some other causes.
. I do not believe in looking at gems with instruments, but I am not a jeweler. Gems should be pretty to the naked eye. I understand that instruments are necessary for measuring features to compare to standards for appraisal purposes and diamond cutters heed these criteria for maximum earning. My son (a jeweler) says he can now order a diamond and know what he's going to get, so the 'improved' standards have been beneficial in that way.
. Faberge did not consider cost - he just made things that were pretty (because money was no object in tsarist Russsia). At a gem show a head I carved in tigereye (a few cents material cost) got more praise then a cameo I made with opal (50 dollars material cost); the most praise was for a carving of "End of the Trail" (indian on horse) from obsidian (volcanic glass worth only pennies).

 

[gsellis]

Related to bow-ties and extinction, Bob Keller has a good article on it on his site in the faceting section from a cutting viewpoint http://www.rockhounds.com/rockshop/gem_designs/bow_tie_blues/. I have seen obstruction producing a similar effect, and at times when observed, can be identified by the additional marker of color (flesh tones are an example - or a camera ). I even had a nice blue flash in a stone, errr... no wait, that is my shirt.

And to Michael's description, I quickly made an assher go extinct and captured the test trace through the stone. Right out the side.

But remember, getting light into the stone, and how it comes out cannot be all assigned to having it come straight back to you. Other functions are to lighten or darken the color, let structures in the stone handle the return, or let the design present an image. So perfect return is not always the goal.

 

[LD]

. I am not familiar with the term 'extinction' as it has been used here in threads pertaining to colored-stones. I assume that it refers to areas which do not return light. One reason is viewer 'obstruction' (AGS says 'obscuration' is correct), where rays returned to the viewer's eye come from his head or body - that is what I discuss. Of course, there is the case of strong absorption by a stone with too-deep color and perhaps there are some other causes.
. I do not believe in looking at gems with instruments, but I am not a jeweler. Gems should be pretty to the naked eye. I understand that instruments are necessary for measuring features to compare to standards for appraisal purposes and diamond cutters heed these criteria for maximum earning. My son (a jeweler) says he can now order a diamond and know what he's going to get, so the 'improved' standards have been beneficial in that way.
. Faberge did not consider cost - he just made things that were pretty (because money was no object in tsarist Russsia). At a gem show a head I carved in tigereye (a few cents material cost) got more praise then a cameo I made with opal (50 dollars material cost); the most praise was for a carving of "End of the Trail" (indian on horse) from obsidian (volcanic glass worth only pennies).

Absolutely 100% correct and this is why this thread is so confusing. The terminology is used differently in diamond world and coloured gemstone world!

Extinction = a gem cut with areas that do not return light - no matter what you do or how you look at it or where the light is/isn't.
Head obstruction = as it says on the tin! Move the head, the obstruction's gone.