Open C-axis?

[Arkteia]

I read about open C-axis in tourmalines but did not realize the effect it had till I saw two tourmalines side-by-side, one with open C-axis and one with closed one. My question is, if one of my stones has beautiful, deep, saturated color but definitely has closed C-axis. I was trying to read about it on another forum, and there was such a divergence of opinions about closed and open C-axis in darker tourmalines... Can something be done to remove the effect? A recut? I have very few tourmalines, but many of you like these stones, and have many, so I hope someone here can give me an advise...

 

[VapidLapid]

Light cannot propagate down a closed c-axis. Nothing can open a closed C, it is the result of the atomic conformation with the impurities or variable elements in that stone. As they can be wonderful in color, the thing is to find cuts that play up the a and b axes. Bar cuts are especially good for closed stones.

 

[chrono]

Sorry, as far as I know, no amount of cutting can do away with a closed C axis. The lapidary can, however, cut in such a way as to minimize its effect by choosing the right design.

 

[minousbijoux]

Yup, as the others have said. When the cutters buy the rough, they know whether its open or closed. If they cut a closed C, then it means it will be dark with less light refraction. I wonder if you could have a closed C in lighter stones? I've never seen it. But hopefully, one of the cutters can answer that question...

 

[bobsiv]

This is a little more complicated than just open or closed; open means that light can travel through the c-axis and closed means that it won't. But there are all degrees of transparency, so a stone can have a 'dark', or 'slightly open' c-axis. The lighter toned stones with a dark c-axis are the tricky ones. You can find them in lighter blues pretty often, and the low price and great color in the a-b axes make them hard to resist. But they're often pretty disappointing once they're cut. As for a stone with a totally closed c-axis, it will always look dead no matter how it's cut (in my opinion).

 

[Jksonner1]

Open or closed?How does one know?

 

[wordie89]

Is there a tutorial on the axes? Very interesting topic.

 

[minousbijoux]

You can see whether its open or closed by holding it up to a light source. The rough and/or stones I've seen with closed C axis are typically the tubular shaped rough (I don't know the proper name for this) like tourmaline crystals. If you hold a piece of rough and look at a light source through it, you should be able to see through it across it or horizontally and if you turn it, then be able to see light through it up and down or vertically. But, if you turn it and try to see light through the length, from end to end and you can't, then its a Closed C axis.

 

[innerkitten]

Good question.

 

[minousbijoux]

IK, you don't think my explanation was perfectly clear?

 

[innerkitten]

Actually it was great. But I hadn't reached your post. I wish I had something to try it on.

 

[Roger Dery]

Hi all,
Hope to shed some light on how Tourmaline's are viewed by gem cutters. This is a recently imported 10gram crystal of Namibian Tourmaline. We are viewing it through the 'A' axis. The crystal is backlit by two 13-Watt OttLites.


Here we are viewing the same crystal through the 'B' axis. Usually, there is little if any color difference between A and B.


Here we are viewing the same crystal through the 'C' axis. Often, the C axis has at least some color differential between the A/B sides.


To my eyes, the A/B axes are showing more blue. While the C is less blue and a hint of yellow-green. If the table is placed on the C axis, and the stone is cut to proper proportions, it is likely to finish out richer in color than the photo of the rough shows. Though, this is NOT always true - even if I might like it to be!

Hope you find this helpful.

 

[chrono]

Roger,
If the table is the C axis, wouldn't the ct weight loss be greater? But yes, colouration seems richer and more even, even though it is yellowish green and you'll lose the blue seen on the AB side. Can the graduation of colour on the AB side be considered a bi-colour?

 

[Jksonner1]

I just got a handful of large tourmaline and will take pictures on the different axis and post. Some definitely are bule green looking through one way and very green another. This is interesting

 

[Roger Dery]

Roger,
If the table is the C axis, wouldn't the ct weight loss be greater? But yes, colouration seems richer and more even, even though it is yellowish green and you'll lose the blue seen on the AB side. Can the graduation of colour on the AB side be considered a bi-colour?

Hi Chrono,
If the table is placed on the 'end' of the crystal, in other words, perpendicular to the 'C' axis, it is very likely there will be greater weight loss. Of course, this always depends on the original shape, whether it is degraded in form, and if inclusions or hue affect the decision as to where to place the table.

There is much to consider, and it may depend on the needs present. Such as, am I faceting the piece for overall beauty and size? Or, am I faceting to fit a particular need?

Example:
1- a very clean, well-shaped, 3-gram piece of [open C-axis] Tourmaline may produce a 4-1/2 to 5-1/2 carat finished stone - when cut using the A/B axis as the table. Generally speaking, these tend to be elongated in nature - and would often be seen in emerald cuts, baguette style, pear shapes, rectangular cushions and the like.

2- if the same piece was faceted to accommodate the C axis as the table, the return may only be in 2-3/4ct to 3-1/2ct range. This arrangement is more likely to produce rounds, trillions, square cushions and other highly symmetrical designs.

The above example is notable in that roughly only 15% to 20% of facet-able Tourmaline crystals have an Open C Axis. And, as most custom faceters know, there is normally a premium paid on open C's.

As to your question regarding "Can the graduation of colour on the AB side be considered a bi-colour?" - technically yes, but in practice it would not have been obvious enough to try and capitalize on it. For the piece I photographed above, it is likely I will cut it in half and facet two stones, knowing they will be different colours.

I hope you someone finds this useful

 

[chrono]

Thanks for the reply, Roger. I never considered splitting that piece of tourmaline rough into two piecees.

 

[Roger Dery]

Thanks for the reply, Roger. I never considered splitting that piece of tourmaline rough into two pieces.

....as I chuckle to myself, I too, would rather not slice up the rough.

But because the colour differential is not significant enough to warrant faceting as one piece, the next best option is to create two stones of slightly differing hues. And, sometimes we are buying under less than ideal situations such as the Neu Schwaben site shown below:

The photos are from last Wednesday, and the temps were over 35*C with lots of sun. So they obliged us to 'go below' and cool off, as seen here:

As cutters we like to think we always go with the best piece possible, but even this is not always do-able. The 11-gram rough shown in the example up-thread is not the norm as to what is found here. So even though the hue is not consistent throughout the stone, I feel very fortunate to have arrived un-announced and still come away with some significant finds.

Of course, leaving the country and clearing customs legally was another hair-raising feat.

 

[minousbijoux]

Roger welcome back - can't wait to hear what you found and how Namibia turned out for you since its not on your regular route.

Now I have a question for you after Chrono's question. Why would you ever want to use the C axis as the table - that is, have the c axis be perpendicular to the table? Wouldn't it make no sense in a closed C stone because you would not be able to see down in, or in the case of the stone you have, you would be looking down into the yellowish green portion instead of the prettier bluish green? Am I missing something in the description? I would think you'd always want to orient the c axis to a position where it would not be as visible, since clarity and color are key. What am I not understanding?

Thanks.

Also, when you're fully recovered, will you come back with some great pics of your trip - the little mini-travelogues are always my favorite threads

 

[Roger Dery]

Roger welcome back - can't wait to hear what you found and how Namibia turned out for you since its not on your regular route.

Now I have a question for you after Chrono's question. Why would you ever want to use the C axis as the table - that is, have the c axis be perpendicular to the table? Wouldn't it make no sense in a closed C stone because you would not be able to see down in, or in the case of the stone you have, you would be looking down into the yellowish green portion instead of the prettier bluish green? Am I missing something in the description? I would think you'd always want to orient the c axis to a position where it would not be as visible, since clarity and color are key. What am I not understanding?

Thanks.

Also, when you're fully recovered, will you come back with some great pics of your trip - the little mini-travelogues are always my favorite threads

Hi Minou,
Thanks for asking. I arrived back Saturday evening, having left Namibia on a flight at noon Friday. The flight Friday night from Johannesburg to Atlanta is roughly 15 hours. So, I'm here, but not at my best. Still have a stack of mail, packages and emails to work through, so later this week I should be able to share some. What I can say.... it was hot and dry, though no bugs!

 

[LD]

Roger

Thank you for your fascinating pictures/posts above. Like Minous I'm wondering why you would want the table on the C axis which, although I understand would give a better saturated stone, the colour (to my eyes) is less desirable than the A/B axis which has more blue. Would market preferences not drive to a certain extent which way you would cut the stone? For example, would there be a price differential (at the point of sale after cutting) for a 5ct blue/green against a 3ct yellowy green?

 

[chrono]

A third who is pondering the same question: 5 carat lighter saturated but pleasant blue green vs 3 ct better saturated yellow green.

 

[Jksonner1]

Still perfecting my wonkie picture taking system sorry...Closed C Axis I think, no? Not much of any bluish comes through either although it is slightly bluish in the right light.

 

[Jksonner1]

Please disregard my dirty fingers haha...playing in the dirt.

 

[Jksonner1]

Roger, I am very interested in what you do and how it is that takes you to environments as pictured. I would love to talk to you further privately. I am currently in Afghanistan with a primary job, however gems and artifacts have become an obsession of mine..some very interesting stuff.

 

[Roger Dery]

Q = Roger welcome back - can't wait to hear what you found and how Namibia turned out for you since its not on your regular route.

A = Actually, since some time has passed and I focused on other areas of Africa, I worked in Namibia off-and-on from 1999 to early 2001 developing 2 gem cutting centers. One in the northwest in a town called Karibib, and the other in an even smaller town way in the south called Keetsmanshoop. I’ve known for some time that fine roughs were being mined in Namibia, but since trade reports claimed there was little production, it was not an easy task to line up a good travel partner/buyer to go with me. (As some of you already know of me, I am no longer interested in taking such risks as traveling to these kinds of places – alone.)

So yes, east Africa is the easy track due to favorable conditions [at present], but I am fairly certain this will unlikely be the case in 3 to 5 years. Perhaps I can share more on this as regulations are adjusted in the future, as they will have a cause and effect to us custom cutters.

Now I have a question for you after Chrono's question.
Q = Why would you ever want to use the C axis as the table - that is, have the c axis be perpendicular to the table?

A1 = the C axis on Tourmaline is also its Optic axis. Since Tourmaline is doubly refractive, and has fairly strong Dichroism, it creates a scenario whereby looking directly into the Optic axis creates a brighter stone. When using the A/B axis, the strong doubling of the pavilion facets, (especially the culet meets) can reduce the brightness or liveliness of the gem.

Q = wouldn’t it make no sense in a closed C stone because you would not be able to see down in?

A2 = True, it would not be sensible to attempt this in a Closed C Axis Tourmaline. Someone new to faceting may attempt this, but is unlikely an experienced cutter would do so.

Q = Or, in the case of the stone you have, you would be looking down into the yellowish green portion instead of the prettier bluish green?

A3 = generally speaking, yes under this scenario, you would be arranging the stone so that the yellowish green would have dominance. However, this is most accurate at the girdle line. Once the facets are tapered in some facet arrangement around the crystal structure, it will capture at least some of the blue-ish green as well. It will still make for a nice looking gem. Many buyers here on PS have purchased Tourmalines that were faceted with the C axis perpendicular to where the table was placed – and not be aware of it. This, would be the knowledge and skill of the cutter coming into play.

Q = Am I missing something in the description? I would think you'd always want to orient the C axis to a position where it would not be as visible, since clarity and color are key. What am I not understanding?

A4 = you would always want to orient “with” the C axis when the C axis is [dark or closed]. In other words, you would be looking INTO the A/B axis as is typical when viewing a normal emerald cut. Open C axis Tourmalines are acquired as a premium. Often, when buying from experienced rough suppliers, they will either charge a serious premium for open C, or force a cutter to buy a parcel of mixed goods just to get the open C’s.

*thank you for asking excellent questions.
**I will be working on photos in the coming weeks, and hopefully pull together a travelogue.
***and yes, I will respond to the follow up questions, but must work on photography the balance of the day.

I hope the above info is helpful in how you view what custom gem cutters must work through to produce fine looking gemstones.

 

[chrono]

Roger,
What do you foresee happening in 3 to 5 years' time that will make East Africa more of a challenge? And a big THANK YOU for answering our questions. It helps us understand the dilemma and decisions lapidaries are required to make in order to get the best out what he or she has to work with.

 

[Roger Dery]

Roger, I am very interested in what you do and how it is that takes you to environments as pictured. I would love to talk to you further privately. I am currently in Afghanistan with a primary job, however gems and artifacts have become an obsession of mine..some very interesting stuff.

Jksonner1,
I am just a simple gem cutter who prefers to locate his own roughs through lower level options in source countries. And, hopefully, along the way I am able to lend some assistance to those less fortunate. You are welcome to google my name and I'm sure you'll learn more than you ever wanted to know about me and my history and exploits.

 

[PrecisionGem]

Just to add to Roger's excellent answers, cutting a stone is always an act of balancing compromises. This is where the skill comes in, making these decisions. In the rough stone pictured, if the stone were to be cut along the A/B axis, the stronger green with the slight yellow would still be introduced into the stone. This can be somewhat eliminated by cutting the stone with steep angles on the C axis. However, generally this creates a shape harder to for a jeweler to work with and can limit the options for the finished piece. Likewise, cutting the stone along the C axis, the more blue as seen from the A/B will be introduced into the finished gem.

Stones that show 2 or more different color on the various axis can be tricky to work with, but can also produce very beautiful and interesting finished stones.

 

[minousbijoux]

Roger: Thank you so much for taking the time to answer our questions. I really do understand the nuances much more clearly as a result of your response. Wow, with something like tourmaline, it must be a huge challenge trying to figure out what orientation in cutting will lead to the prettiest stone.

JKSonner: if I were you, I would try and reach out to Jim Rentfrow, another lapidary. He has been working on and off in Afghanistan over the last few years. He is back now (I'm pretty sure) and would be an excellent resource for you if you haven't already been in touch with him.

 

[Roger Dery]

Roger,
What do you foresee happening in 3 to 5 years' time that will make East Africa more of a challenge? And a big THANK YOU for answering our questions. It helps us understand the dilemma and decisions lapidaries are required to make in order to get the best out what he or she has to work with.

Hi Chrono,
There is much that could be discussed, but given the nature of public forums and my need to protect some folks on the ground in developing countries - I'll just mention a few things that are safe for me to share.

Ruby and Gemfields, Inc.
The firm Gemfields out of London, is moving into a controlled sales scenario with Ruby rough. It may appear DeBeers-like in that they will control the auctions of the rough. If there is little rough for buyers like me now, it will only move in the wrong direction.

Beneficiation
I normally only see this term used in conjunction with African initiatives. At its most basic motives, it is to increase employment (and employ-ability) to native Africans. For the gem trade as an example, in Tanzania there have been just over 600 locals trained in faceting. At present, according to gov't estimates shared during last April's show and conference in Arusha, there are only 400 still working in the field.

Yet, Tanzania is a significant producer/exporter of uncut gem materials. The majority of roughs are allowed to be exported to two major cutting centre's, Jaipur and mainland China. The locals of course, would like a piece of this action and rightly so. Because regulations tend to take a wide swath in their effect, smaller operators like me will be swept aside and discounted. In other words, when regulations are put in place as to who is allowed to export, I'm sure it will be the big players who will have the advantage.

For me, I am trying to be pro-active and searching out new sources, and renewing previous options. This is why we engineered a trip into Namibia last week. And from this trip, a few magical things happened that even took ME by surprise. I unfortunately cannot share the details here, but I will at some point in time when it is appropriate and all the pieces of the puzzle come together.

I hope you find this useful.