Gem Cutters... explain the intricacies of gem cutting..

[katbadness]

Inquiring mind wants to know more...

On this thread on the Rocky Talk forum, Rich Sherwood said:

"Light return performance is largely determined by the relationship of the crown angle to the pavilion angle."

Now, I'm assuming that the same principle applies for colored gems, as well. However, I'm also assuming there are other things that's involved in determining how a colored gem rough is cut.

I posted my 2 cents of what I understand about colored gem cutting on the tanzanite thread.

But now I'm *really* curious...


What are the other factors involved (other than light/dark of body color of a rough)? How does a cutter determine how to best use the rough to obtain best looking gem?

One thing of a particular interest for me, is there such a thing as an interplay between the refractive index and the angle by which the cut is angled to obtain the most optimum cut for the material? Or am I not understanding what I was explained correctly?

(Hmmm... this probably warrants me looking into learning how to cut gems!!

)

 

[strmrdr]

The color and shape of the rough determines the cut more than anything.

Light rough cut it deep to bring out the color.
Dark rough cut it shallow as to not darken it too much.


select the shape that maintains the most weight from the rough while achieving the above and put some facets on it and sell it.

 

[valeria101]

Did you go through the formal description of visual perception that went into the make of light return analysis fro those H&A diamonds? (meaning the stylized facts standing for "brilliance", "contrast", "scintillation" and "fire" ?) I do not recall seeing such a lineup for colored stones anywhere. I know OctoNus also makes a piece of software for colored gem cutting... but the abridged version of their research is nowhere to be found (aside the user's manual, I suppose).

Not that such concepts are not appropriate, but there seems to be a different set than for diamonds.

You are right to mention the RI and the angles - the refractive index determines them. However, I think that the ideal proportions for diamonds went a step further to pinpoint what sets angles deliver the most light. More often, one would mention what should be avoided so as not to produce a window, with little concern about what (stricter) proportions would make the stone most brilliant. Whether this is good or bad... no idea. Would you be looking for a "sparkly" sapphire? Or a "fiery" one?

The more common language would describe "no darkness" and "strong color in low light" and "flashes of color" as visual qualities of a gem. At least this is what I most often encounter. These attributes are more related to the properties of the rough than the cutting. Of course proportions play a part - but wider ranges deliver the desirable look. Besides, there is much variation in the facet pattern of cut gems with no one so overwhelmingly dominant as the RBC (even those ovals come with as many versions of facet patterns).

There are lots of references for the basics of cutting (and a long way from paper to the hands on approach, if I am not wrong).

This thread could become an interesting PS "asset" if anyone can (and would) give a low-down, but since I am not professional cutter, I will not go that far

 

[mogok]

Hello Kat,

I'm not a cutter even if I've spend 4 months learning about that in Burma when I was also studying gemology.
For colored stone there is an important relation between the RI and the angles to be cut: If you cut a colored stone as a diamond: You will have a low brilliancy. The reason is in 2 words: Critical angles.
When the light arrives from one material to an other one, there are 2 solutions:
-1) the angle between the light and the normal to the separation plane is smaller than the critical angle: In this case the light will penetrate the material.
-2) the angle is larger than the critical angle: So the light will not penetrate the new material and will be reflected.

As the critical angle goes smaller for a stone in the air in the air when the RI goes higher. The light will have more chances to be reflected when arriving on the pavillion of a gemstone if the stone is a diamond: RI: 2,417 than on the pavillion of a quartz: RI: 1,54.
Colored stones are so much more subjected to the "window" and "Extinction" phenomena, especially if cut by a diamond cutter than does not know the crown and pavillon angles to optimise the brilliancy of a spinel or a ruby...

Of course optic rules are not all. A colored stone cutter will have to deal with the orientation of the table in order to find the best color in a dichroic stone: For a sapphire the stone final color can be greenish blue, blue or violetish blue depending of the orientation of the table.

Then there is the repartition of the color within the stone: It is very rare for example that a sapphire get an even color. So this is an other factor of importance: To place as much color as possible near the culet area in order for the stone to show a final color as even as possible. A blue sapphire completly colorless with just a blue dot near the culet will appear completly even blue... Such stones are called "ottu" in Srilanka.
Color zoning can also be a problem if you dont want to have a "zebra" like sapphire.

Color zoning and orientation of the table were my 2 main headhakes while trying to maxinise the beauty of a sapphire! And then you have to make the anlge as good as you can... May be you begin to understand now why I have some respect for some "potato" cut native cut sapphire that are to me piece of art when I see the result!
Then you have to deal with the presence of inclusions and of course the shape of the rough you have in order to get a stone with the best clarity and the more weight as possible without to loose too much symetry.

That's the reasons why when I see a fine color sapphire with a slightly irregular shape and a natural near the girdle: I usually dont say: What is that potato junk? But I buy it... and I dont recut it!
Well dont get me wrong: I dont mean here that all native cut are pieces of art... But some are very optimised that they should get more respect from cuuters orientated markets.

Hoping to have helped,
All the best!

 

[WinkHPD]

Another fine post by Mogok, whose writing is itself a piece of art in so many ways.

Some of those potatoes are not so well cut as you stated, and it is these that I eagerly bought when I was at the tables in Chantaburi (I am sure I am spelling this incorrectly). I deliberately bought poorly cut items to make into better ones.

I will not begin to claim to understand the cutter's art,only to claim that I am fortunate to work with Richard Homer who used to actually teach the GIA cutting class, after writing the curiculum.

Here is an example of what can happen when you correct the relationship between the RI of the stone and the critical angles that Mogok spoke of.

Wink

P.S. This one was actually pretty good compared to some of the others, but I do not have their pictures here at home. This will do for tonight, time for bed...

 

[WinkHPD]

P.P.S. The deepening of the color and saturation is a common happening when Richard cuts a stone. Keeping the color in play definitely lets it absorb more color before exiting the stone.

 

[strmrdr]

Revisiting RI and critical angles.

It takes a fairly wide swing in RI to change the angles much.
For example zircon RI:1.81-2.024 and sapphire - RI:1.76-1.77 usualy work very well with cuts designed for diamonds at RI:2.417.
This is one reason that sapphire makes great side stones because it can be cut to match the shape and type of cut of the mainstone and still perform well.
Most cutting diamgrams will make the split around an RI of 1.65 with different angles and cuts for those above and those below.

But there is more to it than just RI and critical angles dispersion comes into play also as does cutting axis.

Some types will have increased dispersion or stronger double refration depending on what axis they are cut on.
This is due to the shape of the cystals inside the stone and how they tend to align themselves.
Zircon is one example.

It would take 3 or 4 books to even more than scratch the surface of this subject.

Until one of the real experts confirms the above dont take it as 100% fact Im just learning myself and could be off on a few minor points.

 

[mogok]

----------------
It takes a fairly wide swing in RI to change the angles much.
For example zircon RI:1.81-2.024 and sapphire - RI:1.76-1.77 usualy work very well with cuts designed for diamonds at RI:2.417.----------------

Hum...
I will come back to some definition work:

If I refer to the definition of the critical angle from my old optic course in university, I get:

CA= sin-1 (nr/ni)

With CA= Critical angle,
nr: Refractive index from refractive medium
ni: Refractive index from incident medium

For diamond to air we will have:

CA= sin-1 (1.000/2.417) = 24.4 degres

For glass (lets say RI=1.520) to air we will have :

CA= sin-1 (1.000/1.520) = 41.1 degres

For sapphire (RI=1.77) we will have:

CA= sin-1 (1.000/1.77) = 34.4 degres

Ruby and sapphire critical angle is 50% more important than diamond. Its not that close!
If you cut a sapphire as a diamond that get 80% brilliancy and hope to get the same result i think that you will have some surprises: This important critical angle in sapphire explain why many stone have both window and extinction... Extinction is typical in many sapphire cut too deep as the light after being reflected by the pavillion for the first time arrive on the second time inside the critical angle and then pass through the stone instead to be reflected to the crown. Some cutters in Burma call that "shaddow", some people who understand less cutting mistake that for "tone" especially in spinel for which the critical angle is 35.55 with RI= 1.72.

"Dispersion" does not change with the axis: Dispersion is a constant to my knowledge within a gemstone: Dispersion is the difference in the refractive index of a material at the B and G Fraunhofer wavelengths of 686.7 nm and 430.8 nm and is meant to express the degree to which a prism cut from the gemstone shows fire or color. Dispersion is a material property.

"Fire" on the other hand depends on the dispersion, the cut angles, the lighting environment, the refractive index, and the viewer...

I think strmdr that you mean "fire" when you say that "Some types will have increased dispersion or stronger double refration depending on what axis they are cut on"

"Dichroism" which is I think what you mean when you say "stronger double refraction" for sure shows variations depending of the way you will orientate the table of the stone compared to the "C" axis and also depending of the length of the travel of the light in different direction inside the stone. There is I think a good explaination of that for ruby and sapphire on "ruby and sapphire" from Richard Hugues page 83.

But you are right this is not simple, and this is the reason why many good diamond cutters are very bad sapphire cutters.
I'm sorry so say that but if you cut sapphires as you cut diamonds, I will never give you one of my stones to recut...

I just dont like these sapphire with "mirror like pavillions".
Sapphire is a stone that get very nice brilliancy and life with pavillions cut "breast like" and not "pyramid like"...
I dont know... I just like my stones to be a little bit bulgy, like a nice women breast!
Why to recut that?

Ok: To make money sometimes. That's what a gem dealer is: A guy that try to optimise the money he can make with a stone.
A very good gem dealer of my friends use to tell me: Vince, you will never be a good dealer as you love your stones too much... Some days I feel that he is right.

Whatever!

All the best, and hoping to have helped a little bit again...

 

[strmrdr]

mogok,
Im not a cutter but looked into it some and have studied it a little.

first the easy one:
Some sites on the web list dispersion == ability to cause "fire"
Further reasearch shows that it isnt that simple and your currect as dispersion
is only one aspect of fire and is a material property.
The other aspects of fire are what can be affected by what axis its cut on.

""Dichroism" which is I think what you mean when you say "stronger double refraction"" currect.

Now the harder one:
From my reading and from my own computer simulations.
The angles that are the best with diamonds also are near the top with sapphire mainly iv studied the RB shape in the simulations.
Most of the cutting diagrams also bear this out with a wide range of RI that will work well with the given set of angles/shape.
I dont know on this one.
I think a cutter will have to settle that one.

Thanks for the lesson mogok I really enjoy your posts and learn from them.

 

[katbadness]

This has been a very enjoyable and *very* enlightening discussion for me.
Thank you Mogok, for explaining some of the intricacies of gem cutting.
Wow!!

I'm still hoping somebody like Michael E would find his way to this thread and give his 2 cents.

 

[strmrdr]

----------------
On 5/25/2004 5:40:24 PM katbadness wrote:

This has been a very enjoyable and *very* enlightening discussion for me.

Thank you Mogok, for explaining some of the intricacies of gem cutting.

Wow!!


I'm still hoping somebody like Michael E would find his way to this thread and give his 2 cents.

----------------

I dropped him an email hopefully he has time to stop by.
He has been very busy lately.
I agree this thread had been fun :}

 

[Michael_E]

katbadness,
You've picked a very intricate subject. There are a whole number of variables that are taken into account when you're looking at cutting a specific piece of colored gem rough. These variables change greatly depending on whether you're cutting a high value material like sapphire or emerald, or a much lower value material like amethyst or aquamarine. If you're cutting the high value materials, then your primary concerns are weight retention and beauty. Weight retention determines much of the overall geometry, such as shape, crown height, pavilion depth and the type of faceting performed on both the crown and pavilion. Weight retention and beauty can be opposing parameters depending on the initial shape of the rough and the desired finished shape. I won't get into the weight retention/ beauty thing since I tend to err on the side of beauty.
In cutting a colored stone for beauty, light return is only part of the equation. The reason for this is that the light around a stone, (this includes diamonds too), and the stone itself are not static, they are both constantly moving. If any stone is cut to get "maximum light return", then you also need to tell me where the light is coming from in relation to the stone for that commment to have any meaning. Usually it means if the light is diffuse and entering the stone from many angles AND you're looking at the stone directly through the table facet. Since this is the way that most round gems are designed to be looked at and measured then it really doesn't matter if it's a diamond or an amethyst, they will both reflect similarly. The big difference comes when you tilt the stone or change the angle that the light enters the stone. When tilted the diamond will continue to reflect light to your eye, even up to an angle of tilt approaching 60 degrees. If you tilt an amethyste to anything over 5 degrees or so you'll end up with a purple window through the side opposite you. By changing the crown and pavilion angles and the shape of a colored stone you can tilt it a little more before it windows.
One of the beauties of the cuts that Wink has shown by Richard Homer is that the curvature of the facets gives you a constantly changing angle relative to the light entering the stone which makes the windowing less apparent and makes the stone look brighter all of the time. At the same time it changes the way that the stone flashes, making the flashes smaller and smoother since the light rolls across each facet instead of being turned on and off when the stone is tilted.
This last paragraph is probably confusing so I'll just say that I do cut a round stone to the same angles as a diamond, UNLESS it's very light or very dark or very expensive. It just depends on each stone. If you've got any questions about a particular stone or cut, that would be much easier to answer in a short paragraph.

 

[mogok]

Well strmdr, I would like to see your computer simulations... and some gemstones samples following the cut CAD designed. I'm very interested by that because (may be it will make you laugh: Cutting is my weak point...)
Micheal is completly right when he explain the fact that a stone has to be seen moving. But when we grade a stone in a laboratory or in a gemstone color grading course, this is not really the same.

Color: is grade looking perpandilular to the table.
Brilliancy: is grade giving the higher result tilting the stone with a maximum of 10 degres compared to the vertical to the table.

probably one of the most important thing is to give the overall grade:

"life" which is a term that covers the quality of fire (if present), scintillation (but its very subjective as some people like the stone to have a lot with many small facet on the pavillon and other people like me are not real fans of that), color as the presence of a shaddow (from tone or from cut) will "kill" the stone, brilliancy inluding especially the ability for this brilliancy to remain even if you tilt the stone.

As Michael said very correctly stones have to be seen in movement and life reflect the color and cut quality of this stone when you move it. A stone "with a lot of life", some people say "Open" will usually have beauty and then value...
A stone with a window can have a lot of "life"...I've several very living "potatoes", but of course poor symetry will usually "hurt" the stone if not kill it... But "Perfect mechanical and very technically cut" can also be as bad as "Native potato cut"

About cutting style, the best rubies, sapphires and spinels I've seen were not brilliant cut...(and the worst also: lol) I dont speak here about cutting quality but about life result.

There was a cutter in Mogok that has sadly died few days ago and that was probably one of the best cutter to my knowledge in the world. His nickname in Burma was "guillotine". He was send in his young in Belgium to study cutting after being choosen by general Ne Win (former dictator of Burma and ruby expert and lover) because of the fine cut he did for a sapphire. He was the fist Burmese cutter to go to study in Europe and came back with both a double cutting cultural knowledge. This guy has spend his life cutting stones there for beauty... No weight retention, he has make cry several dealers and gem owners that were expecting larger stones so his name. His pavillions were art pieces, I nerver saw one of his stone cut as a diamond (I dont have seen many of his stone also, but the few I've seen were very very fine with beautiful pavillions). Several of the best rubies and sapphire cut during the last 30 years passed in his hands, and I know several swiss gem dealers that send him to recut some stones cut in Idar Oberstein...
May be there is some legend also in that: Possibly as Burma is a place were gems and gem people are very specials...

Whatever i'm not really a cutting expert, this is even I have to say one of my weak point compared to other people which are going regularly to Burma and were professional cutters before to become buyers.
I try to learn from them, and I try to teach what I learn then in the gemological school I work in...
Cutting is aart and I agree with strmrdr and Michael that this subject is difficult to cover in one paragraph.
All the best,

All the best, hoping again to have helped with my "AIGS colored stone grading" teaching aspects.

 

[Richard M.]

Regarding the angles and dispersion discussion:

I facet colored stones nearly every day. I can pass on some of the views of prominent American faceters on some of the topics you’ve raised.

Most diamonds are cut with pavilion (culet) angles at 41 degrees and crown angles at 34 to 35 degrees. Corundum is generally cut with pavilion mains at 42 and crown mains at 36 to 39 degrees depending on the cutter’s preferences. Some feel crown angles nearer 36 degrees increases what we call “tilt-brilliance,” meaning that the stone remains brilliant with no windowing when tipped as much as 30 degrees from level.

Caveat: American faceters are a contentious lot and there are often rowdy arguments about which angles are “best.” They don’t even agree whether the name describing themselves should be spelled “faceter” or “facetor.”

Diamond cutters don’t have to worry about double-refraction (dichroism); corundum cutters do. Diamond cutters have to worry about dispersion (the term we frequently use in the U.S. for “fire”); corundum cutters don’t. Careless orientation for dichroism and low crown angles are probably why diamond cutters don’t do too well on corundum by your estimate.

Dispersion is of course a constant for specific gem materials, 0.044 for diamond, and only 0.018 for corundum (and most visible dispersion is masked by the color of the corundum). The further a light ray travels through a medium and the more it is bent, the more it will be separated into its spectral components. Traveling through a solid substance like diamond, each wavelength of white light has its own individual index of refraction, separating red from violet rays by a set amount. A diamond’s crown angles must be calculated to preserve this wavelength separation to the maximum when light emerges to the eye.

Rays leaving a gem at normal will show the least dispersion. The closer emerging rays approach the critical angle, the greater the dispersion. Rays entering a gem far away from normal are dispersed on entering and further dispersed when they emerge from the crown. Diamond crown angles are calculated to maximize both brilliance and fire.

Most gems cut in the colored stone trade aren’t very fiery. Some of the lighter colored garnets like Mali display nice fire and andradite/demantoid has dispersion higher than diamond, 0.057. The lighter saturated green demantoids and yellowish andradites cut real rainbows as do benitoite, zircon, sphalerite, sphene and a few others.

Katbadness and others who are thinking they might take up faceting should carefully study the information presented at

If specific questions remain I’m sure the cutters here will try to answer them. Many excellent books on faceting are in print. An oldie but goodie is “Faceting for Amaterus” by Glenn and Martha Vargas.

HTH,

Richard M.

 

[Richard M.]

For some reason the Preview function wasn't working when I posted the previous so I didn't know the link was missing. Rather than trying to post a live link, copy and paste the following:

http://theimage.com/faceting/index.htm

Richard M.

 

[valeria101]

What software do you use Strmdr? I am somewhat not fond of ray trace, siding with Mogok's saying that the typical lighting is diffuse, not directional. Do you know of any "ready made" alternatives?

As for a (approx.) quick read on the subject, I definitely like the manual of GemCad (LINK).

Since this is the "Colored Stones" forum, it begs a Q (I think)

What makes a cut diagram more suitable than others to enhance the color (= hue & tone & saturation) of gem material? Te orientation of the rough likely says all there is to say about apparent hue position...but how about the other two factors?


#1 In theory, the longer the light path inside a gem, the stronger the apparent saturation. This would explain your preference, Mogok, for rounded pavilions and the higher cut crowns of colored stones, I would think.

#2 And there is one more side of this discussion which seems to link the apparent tone and cut proportions, which I understand (and hear about) less. For example, better "light return" would introduce lighter colored flashes within an otherwise too dark stone... and avoiding them would make lighter colored material appear darker in tone. Does this work, or if just another theoretical "myth" ?

Of course, all my consideration for whoever makes faceting speak with little symmetry and all those intricate constraints of the material - I can easily model a classic cut using grammar school math, but one of those "native cut potatoes" would take much more than that even on paper

 

[valeria101]

----------------
On 5/25/2004 11:43:06 PM Richard M. wrote:



Some feel crown angles nearer 36 degrees increases what we call “tilt-brilliance,” meaning that the stone remains brilliant with no windowing when tipped as much as 30 degrees from level.

----------------

Gee, I was definitely awaiting this on the H&A check list

Am I wrong saying that those ideal diamond proportions do not take this factor into account ?

 

[mogok]

Thanks a lot Richard for this very good link and your very informative post, as I say I'm not a cutter even if I've spend some time syudying that in the past, but your informations about the different angles suitables for diamonds and sapphire follow quite well what I was thinking without to be able to express it that clearly and nicely... (I've to say that I'm also not a native english speaker... sorry) I will take note and compare a little bit...
One point you have express whatever hit on my "gemology teacher logic"

----------------

Dispersion is of course a constant for specific gem materials, 0.044 for diamond, and only 0.018 for corundum (and most visible dispersion is masked by the color of the corundum). The further a light ray travels through a medium and the more it is bent, the more it will be separated into its spectral components.
----------------

Hum, spinel is RI: 1,72 with a dispersion around 0.026 but sapphire is RI: 1.76-77 with a dispersion only 0.018. So more the light is bend does not mean that the dispersion will follow the same way... Ok: i'm picky! But this thing distrubs be for long time even if I've never taken the time to search for an explaination about it.
In spinel dispersion is clearly visible even if the stone is very colorful. It is masked but visible. You are perfectly right when you speak about the cutting related problem for dispersion as spinel sometimes looks like dichoic beacause of the dispersion and the way the stone is cut...especially in pink spinels that can have violet and orange looking facets due to that.

One more question Richard, you say that you cut colored stones everyday: Do you cut spinels? Do you have examples?

I'm many times worry about understanding the reason why I find several time that some native cut stone I found in Burma looks better than many "fine cut pieces" I see in the west... I guess that there is the quality of the material, but I'm all the time wandering about other factors.

All the best,

 

[mogok]

----------------
but how about the other two factors?

#1 In theory, the longer the light path inside a gem, the stronger the apparent saturation. This would explain your preference, Mogok, for rounded pavilions and the higher cut crowns of colored stones, I would think.

#2 And there is one more side of this discussion which seems to link the apparent tone and cut proportions, which I understand (and hear about) less. For example, better 'light return' would introduce lighter colored flashes within an otherwise too dark stone... and avoiding them would make lighter colored material appear darker in tone. Does this work, or if just another theoretical 'myth' ?

Of course, all my consideration for whoever makes faceting speak with little symmetry and all those intricate constraints of the material - I can easily model a classic cut using grammar school math, but one of those 'native cut potatoes' would take much more than that even on paper

----------------

Well Ana,
I think that Richard have given you more clearly that I could some answer for the first point:
"Most diamonds are cut with pavilion (culet) angles at 41 degrees and crown angles at 34 to 35 degrees. Corundum is generally cut with pavilion mains at 42 and crown mains at 36 to 39 degrees depending on the cutter’s preferences. Some feel crown angles nearer 36 degrees increases what we call “tilt-brilliance,” meaning that the stone remains brilliant with no windowing when tipped as much as 30 degrees from level."

The angles given here make the stone "more generous" in shape than an "anorexic diamond"... You are also probably right when you say that to maximise the lengh of the travel of the light inside the stone will make the color looks more saturated. But the "Ottu sapphire" case comes to disturb this point a little bit... or add the point that the best color have to be placed at strategic places in order to get the finest final color that will maximise the value of the stone!
Because whatever this is the cutter work to maximise the value of the stone with an optimisation of the "symetry- brilliance- clarity- color- weight" equation for a given stone...

Ana on the second point you raise I have one objection:

Tone is a component of the color of a stone: It cannot be removed by cutting. A stone with high tone will keep its hight tone whatever the cut. Spinels dealers in Burma have a special technique to evaluate the presence of the absence of tone in a given stone: They bring the stone to the eye, just few millimeters from it and they looks through in the light: If the red stone shows a ligh dark area in the center then this rough will have "high tone" even if the rough stone looks fine color. If the stone looks even color without dark core then the stone will be bright without this "shaddow"... For fun I use to compare color in spinel with the "force" (star wars style) Some spinel has fallen on the dark side of the force meaning that they have too high tone. Some other one can get true beauty and brigh color: "Jedi Spinels". Only "Jedi spinels" can get real top gem quality!
Try it!
This is one small spinel trader secret i've learned in Burma...

Now when you cut a spinel or an other colored stone with the wrong angles, some light will leave the stone by the side and this will make the stone looks darker... The "apparent tone" will be too high.
There are many bad cut stones in Burma or in the west, that's the reason a good cutter can make some money in gem business by recutting the "so so stones" in the market and optimise their beauty and value.

I'm a gemologist, I'm more focus on the quality of the material (color, inclusions,...) than on cutting conceptions... Cutting or recuuting is a very hard job as it needs much more experience than I have.
After the 4 months I've spend cutting stones in Burma, I knew how I could spoil a gem. I've decided to stop going in this direction... But I still like to understand whatever is behind.I like "native potatoes" because of the quality of their material. Its incredibly difficult to find top quality material nowadays especially in Thai markets...
When you find some, you just dont care how it is cut: You are already so happy to have got the stone with the color and the clarity you were dreaming about! This rarity of the material makes you think twice before to give your pretty little things to somebody for recut.
Currently I dont know well enough any cutter to give him my stones. I love them already despite their cutting imperfections... So why to recut them?

Anyway I collect stone for my pleasure, I dont put them in jewelry, I dont show them... Or very very rarely to very selected persons. Burmese style in fact...

That's the point.
Crazy? Probably...

 

[strmrdr]

Ana,
Gemcad, Raywin, Gemray, Gemflick
http://www.gemcutter.com/pioneer.htm

Of those I play with raywin the most.

Datavue2 is a database of cutting diagrams and available from the same place.

 

[Richard M.]

Mogok (re: dispersion)

Mogok,

Your story about “the guillotine” made me laugh out loud although I’m sorry to hear of anyone’s death. The war between the gem dealers and the cutters continues! Believe me, it hurts very much when I see how much precious rough is lost in cutting.

Also you write very well in English. I only wish my French compared. What language do you use teaching in Bangkok?

I can understand why you sometimes prefer “potatoes” to stones cut strictly for brilliance. I do too now and then, depending on several factors. I think one reason American cutters go for brilliance is that they and their customers have been conditioned to cutting standards for diamonds instead of color. It’s only in very recent years that many American jewelers have grudgingly begun emphasizing colored stones at all. Even now they often pitch colored stones as being “diamond cut,” meaning round brilliant with proper angles, as a sales incentive, instead of emphasizing the unique qualities of colored stones.

If you’re in the business of selling, you sell what your customers want or someone else does. In general, American jewelry customers are unsophisticated about color. They are not willing to forgive and even treasure the “birthmarks” you and many others of us in the business appreciate. They usually demand diamond-like clarity and brilliance in colored stones even though color is an entirely different ballgame.

You’re right about dispersion and R.I. If what I wrote was strictly true there should be a one-to-one correlation between R.I. and dispersion values. There isn’t, although all high-dispersion minerals also seem to be high R.I. with a few glaring exceptions like obsidian (0.41), gypsum (0.33) and dioptase (0.36), using Schumann’s tables.

My tables show a much different value for spinel than yours, 0.20 instead of 0.26 as you state. There’s a much smaller difference between 0.20 and 0.18 although the R.I. difference between spinel and corundum is substantial.

This is a very interesting question, and one that’s also occurred to me. Something else must be going on here but the information I posted is based on what I’ve been taught. I’ve scoured my personal library for more information but dispersion seems to be a topic most texts only skim over. My old GIA lesson says: “In general, dispersion in gems increases with increasing refractive index. There are a few exceptions to this as can be seen by comparing the refractive index and dispersion tables…” There’s no explanation for why this occurs. You’re the expert here…maybe there’s a professional research paper possibility.

I facet spinel but am restricted to rough that reaches the U.S., generally the least popular colors or “inky” stones. All the choice rough is cut at the source, either in Burma-Thailand or by on-site buyers of African rough. High-end rough usually costs as much or more than finished stones. Now and then I can buy a ‘lumpy’ finished red or 'flame' stone to recut but your Asian faceters are very crafty. When I remove objectionable flaws and establish good symmetry the stone loses as much of 50% or more of weight. I cut only for my own needs.

If I can find a piece of obsidian with decent transparency and light enough color I think I’ll try to facet a few stones as a dispersion experiment. That 0.41 dispersion number astonishes me! It's almost as high as diamond's.

HTH

Richard M.

P.S. Is anyone else having problems with Preview or am I doing something wrong?

 

[mogok]

Aaarg!
Richard you are right! My favorite reference, "Color Encyclopedia of Gemstones" by Arem say dispersion 0.020!
I've no idea why my memory gave me this 0.026! May be I get old... Thanks for the correction! I appreciate as I could have repeat the same stupid mistake elsewhere...
I was sure that the dispersion was higher than corundum as it is quite easy to see the dispersion in spinel and in corundum its a challenge.

Anyway you are right about the dispersion, it is an interesting point: When I was studying in Burma, the gemology course were FGA style: It means that we had to know everything by heart: RI, SG,... for all the stones (Studying in GIA was hollidays compared to that for the theory part). In order to help us for the SG and RI I was tought a rule proposed by "Gladstone and Dale" at the beginning of XX century that link for liquids the RI and SG. If there is a direct and perfect relation in liquids the rule does not hold on solid state and mainly... with the stones that show a high dispersion: Diamond, andradite, zircon, benitoite (and also topaz).
For the rest this rule is close to: RI=SG/5+1
It was very useful anyway as the exceptions are not that much in order not to make big mistakes with these datas and to spot stupid results...

Dispersion, you are right, has raised several time questions to me but I've never really taken the time to seach for answers.

Anyway, in AIGS (Asian Institute of Gemological Sciences) in Bangkok, I teach in english of course as there we have some Thai language classes and English speaking classes. The English classes are very funny as we have people from all around the world: young and old, with or without experience that come to study with us.
Its a very friendly environment... Are you interested to spend some time here to study our ruby and sapphire collection?

Anyway thanks for your comments on my english, but I'm sure that I'm better in frenchglish than in english...

All the best,