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 Post subject: A crowning achievement, a 1.5 carat round indicolite.
PostPosted: Wed Apr 26, 2017 8:33 pm 
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Joined: Thu Sep 25, 2008 10:33 am
Posts: 840
Location: Mars PA
I did what I would not have done in my youth, but then wisdom is suppose to come with age. Should cutting the cleanest stone I can, while making a reasonable effort to retain weigh, be the goal of my cutting effort? (I am not worried about the dichroic nature of tourmaline in this effort.) The results of my latest effort cutting a blue tourmaline from Nigeria that is NOT noticeably dichroic, would call into question the dropping of the angles of the crown's mains, to eliminate shallow flaws that do not endanger the girdle of extend under the table. The flaws I am talking about break the surface of the gemstone.

Now let me step back and talk about the specific piece of rough that sits, cut, in a nice plastic box. It was a very transparent crystal section that was broken at both ends. the breaks bulged as if the section wanted to be a nodule and were not perpendicular to each other. Only one was perpendicular to the sides of the crystal. It was awkward to cut because it really was too thick to efficiently make a round with its table perpedicular to the c axis and not deep enough to make anything with its table parallel to the c axis. This is a very common shape of tourmaline rough today. There is no reasonable way to get a high yield from cutting such a piece of rough, But it has a medium light tone value without any darkness that can be associated with indicolite and I am glad to get it.

After grinding a rough round by hand and grinding a flat on one of the ends of the crystal I dop the stone, Then I use the machine to establish the girdle and rough out the pavilion. Now I have to make a decision on whether I will leave in a small flaw that is about 1/3 the distance down the cylindrical preform or grind it out. Grinding it out would certainly be a bigger cost in weight loss so it is rejected. Now the effort comes down to keeping the small flaw from being in the girdle while making a deep stone. I always use 41 degrees, unless it is a darker tone value, so I cut in my mains and see that I have the depth to horizontally split the mains with a 51 degree round of facets. It will be close, but if I didn't try to cut it close, I would have lost a lot of beautiful tourmaline over the years.

So I complete the cut and polish the pavilion. The tourmaline polishes beautifully and it really firers up under my incandescent work light with a color shift to a cyan color similar to paraiba. I will test it for copper with my spectrometer, but I doubt that it has that chromophore. Now I transferred the stone and quickly answered the question about judging my ability to eliminate the flaw. I started to produce a crown that was high because I had lots of depth. Then I went to my standard 40 degree mains which eliminated almost all of the flaw from under the main, but it still marked the break facet. I have failed.

Now in the past I would have dropped the mains angle to no less than 25 degrees. But here is where I parted ways with the past. Even though it marks the break facet, I finished the round as cut. I changed my ways for a variety or reasons. I was not sure that I could make a flawless stone even if I dropped the main's angles. I have grown tired of gemstones with thin crowns and don't make any rounds with mains less than 35 degrees. And finally I did not think that the shallow flaw in the break facet would really impair the beauty of the gemstone. (If it had been centered under a main, where a prong would cover it, I would have certainly left it without debate.)

I don't know how it balances out between a more beautiful gemstone overall and being able to declare the stone flawless in the marketplace, but to me the gemstone is much nicer by retaining the flaw. By eye and at reasonable angles of observation, I don't even notice the flaw, but the vivid blue flasher with a nice crown is a real treasure. So raise a cheer for the beauty of tourmaline and keep working to make the most beautiful gems you can with the flawed rough, we all have to deal with.

Bruce


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