**Calling Zeolite opposed bar cut questions

[mercoledi]

Hiya! I saw in an earlier locked thread that this is your favorite cut. I've only seen a few in person but I love orderly, geometric looks. How does one judge an opposed bar cut? Have you bought any online? Do you have any favorite vendors or favorite base materials for this cut?

Thanks!

 

[Lady_Disdain]

I am not Zeolite, but I hope you don''t me piping up!

I think Jeff Graham is the master of the opposed bar cut - his Smith Bar design was developed when he was asked to cut a stone for the Smithsonian collection. I know that Gene has cut some nice opposed bars stones as well. There was one available on his site a while ago.

You should look for very precise cutting: very good meet points and the facets should line up very exactly. I can''t say mich about angles and suchlike, but I would recommend that you look for the good precision cutters.

Tourmaline is one of the materials most often cut in the opposed bar design (the crystal lends itself to long cuts, such as emerald cuts or the OB). Have you seen a bicoloured tourmaline in a opposed bar cut? It is one of my favourites, as the little squares make the colour separation very defined.

Best of luck!

 

[mercoledi]

Hi Lady Disdain,
Nope, I don''t mind at all, I''ll take information anywhere I can get it! I''m considering a tourmaline from Gene at Precision Gems to get my feet wet. Is that the Gene you were referring to? He has a few at different price points and it isn''t immediately clear to me what the differences are besides carat weight and color of course. Jeff Graham has a tourmaline currently available, but it''s $5.5K which is outside of my budget (by an order of magnitude

).

Thanks for the info! I''ll keep my eyes open for a bi-color too.

 

[Lady_Disdain]

I''ve had that problem with Jeff as well. I really want one of his stones, but he is just out of my budget. Gorgeous cuts and excellent rough, though.

Yes, I meant Gene at Precision Gem! He is an excellent craftsman.

 

[zeolite]

Date: 2/23/2009 3:30:18 PM
Author:mercoledi
Hiya! I saw in an earlier locked thread that this is your favorite cut. I''ve only seen a few in person but I love orderly, geometric looks. How does one judge an opposed bar cut? Have you bought any online? Do you have any favorite vendors or favorite base materials for this cut?

Thanks!

Hi mercoledi,

An opposed bar cut is more difficult than an emerald cut, but is still quite easy to do. It is very forgiving about height. The design is easy to adjust for flat thin long crystals, and also for tall long crystals. It is best suited for tourmalines, since they tend to come in long crystals, but I also like it for garnet. I think it is well suited for any gem material.

I''m not sure how many I have, perhaps 35 gems in this cut. Besides the cutters popular on this site, I have one by Phillip Youngman, and one by Steven Avery, both superb, award-winning cutters. No, I''ve never bought any cut gems online.

Facet meets aren''t a problem for this cut, because there aren''t any. There are usually 6 bottom bars (3 on each side of the long keel) and usually 7 opposed bars on top (always an odd number, to have a top small table center facet). Some very long gems, like my color change diaspore, shown here, has 9 top bars.

General issues to look for is no window (bottom facets not cut too shallow), no chips on the long bottom keel, and good polish. But most cutters do this design with a taller dome on top. This gives a brighter center reflection, but it is dark in a broad area on either side of the bright center. I prefer a flatter top dome, which gives a much broader center reflection. But almost no one cuts it this way. Also, the steep side facets (on the top) provide a sharp edge, which can chip easily. I prefer a thin line of transition facets all along the top, to avoid this possible chipping.

Here are two of my recent threads, showing the opposed bar cut:

https://www.pricescope.com/community/threads/blue-tourmaline-blue-gems-tone.106915/
https://www.pricescope.com/community/threads/guess-those-gems.107329/

 

[mercoledi]

Thanks Zeolite!

I''m looking at two tourmalines, both blue-green, both from Afghanistan. One is slightly bi-color. How important is depth in this cut? I looked at Jeff Graham''s plots for the Smith bar but couldn''t work out the overall depth. So if I''m worried about darkness and windowing in particular, are there specific questions I should ask?

Thanks so much for all of the information.


Lady- your new spinel is an amazing color!

 

[zeolite]

I''m looking at two tourmalines, both blue-green, both from Afghanistan. One is slightly bi-color. How important is depth in this cut? I looked at Jeff Graham''s plots for the Smith bar but couldn''t work out the overall depth. So if I''m worried about darkness and windowing in particular, are there specific questions I should ask?

I''m not sure what you''re asking when you mention depth. Are you concerned about mounting or about windowing and darkness? I very much dislike trying to buy gems at a distance. I like to examine it in person, on an overcast day (with light coming from all directions) or on the north side shadow of a building on a sunny day. That way I can judge the cut, depth of color and windowing.

The opposed bar gem can be very shallow and still not window if the crown is very low (shallow). All this means is that the pavilion angles are correct. An opposed bar cut can be very deep and still be excellent optically, if the pavilion angles are correct, but it has a tall crown.

In the left (green) diaspore picture above, there is a vertical dark band on the right side. That is not a window, that is extinction. The pavilion is cut quite correctly, but the extinction is due to light not coming in at wide enough angles. On an overcast day, that bar wouldn''t be there. It is a paradox that well cut gems show extinction more than poorly cut ones, assuming that both gems come from the depth of tone material. And extinction get worse in dark toned materials, no matter how well or poorly it is cut.

Bicolor gems almost always have many tiny visible inclusions, and even if it is flawless, the tone is usually so light that it has weak color. A well cut, flawless, intense color bi-color gem is exceedingly rare! Whatever the price for such a gem is asked, it is really too low. It is only that low because of relative low demand. People just don''t realize how rare that is, and not enough people bid up the price to where it should be. It is like top red spinels vs ruby. Ruby gets the high price but spinels of that quality are much more rare.

I would ask him to examine the gem in widely diffused light and ask if it windows. Most good cutters won''t allow that to happen.

 

[mercoledi]

Interesting. I wasn''t sure if there were overall depth parameters/prefered proportions like there are for RB diamonds. I know that colored stones are cut to maximize the color, but some poorly cut ones are really really shallow and window something awful. I think I''ve decided to get the one that''s slightly bi-color (I don''t think it''s a strong transition, but it sounds interesting), from a well respected cutter and is rated as having great color. I can return it if I really don''t like it. I was asking specifically about extinction because I have a few tourmalines and some of them are darker than I''d like in certain conditions. I suppose that always happens, and extinction probably contributes to contrast. I also prefer darker colors so it sounds like that contributes to it as well.

I''ve heard that it''s better to have tourmalines with open C axes, how important is this in an opposed bar?

Thanks for all of your help, I''ve really learned a lot!

mercoledi

 

[zeolite]

I've heard that it's better to have tourmalines with open C axes, how important is this in an opposed bar?

Tourmaline crystals can be a short, stubby shape or a rounded nodule, but by far the most common shape is a long, pencil shaped crystal. In all colors, the long C axis (parallel to the length of the crystal) is the darker color. The color across the short dimension is lighter in tone, and often a different color. This is called dichroism (meaning 2 colors).

About 95+% of blue tourmalines are too dark, and the c axis blocks all light (it is "closed"). In iron green tourmalines, perhaps 70% block all light. The other 30% usually have a yellow green or olive color which is not desireable. Chrome tourmalines (which are actually colored mostly by vanadium, and sometimes chromium), usually have an open C axis.

So in blue and most green toumalines, all of the reflected light comes from the sides. That is, the table is always cut parallel to the long C axis. The ends, instead of be cut at a reflective 45 degrees, are cut at a steep 70 degrees so that they don't reflect the undesireable olive color.

Pink, red and the rare orange tourmalines, on the other hand, almost always have the table cut at right angles to the c axis. That is, all pink and red tourmalines are cut to show the darker color. Sometimes a cutter will try to get more yield (a larger gem) by cutting the table of a pink crystal parallel to the c axis. In this case, he gets a pastel gem with lots of brown in the pink (a very undesirable color in my opinion).

Getting to your question, an emerald cut on a tourmaline with an open C axis will be brighter than with a closed C axis, but only if the cutter chooses to cut a reflective (45 degree) end facet. Why would the cutter not cut a reflective 45 degree end facet? When he does not want to reflect an undesireable color such as olive green, yellow green, or brownish pink color.

Also the opposed bar cut shows mainly square facets, and a reflecting 45 degree cut on the ends, introduces a v shaped facet at each end. So an open C axis is great for emerald cuts with a good dichroic color, but is of little value for an opposed bar cut that wants to show square facets. I hope I didn't lose anyone with this long explanation.

 

[mercoledi]

Wow, that was extremely informative. Thanks so much for taking the time to write it all out, I really appreciate it!

 

[mercoledi]

One last question- why are rough axes a b and c when all other axes are x y and z?

 

[zeolite]

I don''t know the answer to that.

 

[mercoledi]

One last bar cut question, are these images fairly representative of how this things behave in real life? The only ones I've seen were both massive and mounted. I like how this stone looks in most of these images. This is not the exact stone I'm looking at, but I want to make sure that my expectations are in line with reality. All the vendor images look like magic neon static, and I doubt that the stones have that appearance all of the time. This is what I mean by neon static. This is one of Gene's tourmalines for sale.

 

[zeolite]

One last bar cut question, are these images fairly representative of how this things behave in real life?

Opposed bar cut gems that you are viewing in person, should look considerable better than these two examples that you gave here. In the first picture, Auctiva, the gem is poorly focused and poorly lit. In the second picture, the precisiongem link, the lighting is excessively contrasty. The bright areas are all washed out in color (not what your eye would see). That might be how the gem looks in direct sunlight, but direct sunlight is about the worst lighting you can have for enjoying gems.

If you click on the two links I gave (about the 5th message in this thread, above) or look at the double diaspore picture, that is how an opposed bar gem should look in diffused (overcast day) lighting.

Gem photography is quite difficult, because the contrast range is so high, from bright facets to black extinction. Your eye can see an incredibly wide range of different brightnesses, but no present camera can record that wide contrast range. I don't mean to put criticism on the vendors of the two links above. And opposed bar cuts are unusually difficult to light evenly, as Robert Weldon (former GIA gem photographer) told me when he was trying to photography my color change matched tourmaline pair that was published in a Gems and Gemology article.