Learning from a strongly dichroic, vivid blue round, tourmaline.

One thing you learn early in the tourmaline game is that you can not cut a practical gemstone without including at least some principle axis (c axis) color.  An emerald cut with steep ends, on the order of 70 per cent can somewhat reduce the impact of an optically closed c axis (closed ends), but if the concept is pushed to 90 degrees and the complete illumination of the c axis, face up, the gemstones are not practical.  Now the commonly suggested procedure for semi closed ended tourmaline has been to treat the material as if it closed ends.

Now to the present and the recent completion of a gemstone that questions the validity of needing to minimize semi-closed c axis tourmaline threw the use of steep ended emerald cuts.  The piece of rough that started this adventure was quite included, strongly dichroic and had lost all of its crystal shape in the depths of some water source.  The pebble was large enough (about 22 carats) to present problems seeing down the dense, blue c axis.   90 degrees to the c axis (a.b axis) was also blue, but with only a moderate tone level.

With a highly to moderately included piece of rough, my first priority is to remove the worthless unacceptable flaws.  I don’t necessarily eliminate them in the beginning, but I at least marginalize them.   Then I grind the nascent table and begin to rough in the girdle.  In the case, with this pebble, I even used my faceting diamond saw to try and save one end of the pebble for a small round.  But most of the pebble turned into mud.  Finely I had two things, a good view of the wonderfully vibrant blue c axis color and a piece of rough that would be very difficult and wasteful to turn into an emerald cut.  And not only that, the rough was still quite included with minor wispy feathers.  Emerald cuts generally do not work well with included material, especially where it is under the table.  There just is not enough scintillating flash to distract the eye from those flaws that can not removed and still get a decent/any stone.

So now that the rough demanded a round with its table parallel to the c axis, I removed excess material from the piece, to enable me to accurately center and dop the preform.  I have cut many tourmaline rounds that show both dichroic colors face up in the gemstone.  Depending on their relative tonal values, you can get alternating pie shaped wedges of color or what appears to be an eye.  This blue piece of rough was dichroic that I was sure that I was going to get a blue eye with the a/b axis color, but how useful the c axis color would be in making the round beautiful was in question.  It could be too dark to be displayed under any acceptable light source.

The results of the labor with this pebble, that was really very close to cabochon material, was a round with the best Indicolite, blue tourmaline, color I have ever seen.  To check out my memory on great blue color in tourmaline, I went threw the collection and found an exception long ratio emerald cut that displays the pure c axis color of the round in very rich uniform manner.

Putting my analytical eye to work, I compared the beauty of both gems and they are both beautiful.  The emerald cut needs decent to above average light to really flash, which is the principle way that it show its color.  It a/b color is a greened blue that can only be seen when you turn the stone sideways.  It is both eye clean and with fine crystal.  The round, as I have already described its color distribution, varies under different lights.   It is still significantly included with one coarse inclusion still remains close to the girdle.  But all these remaining problems with the round evaporate when it responds in its great blue fashion under all reasonable levels of light.  The eye may come and go, but the round is responsive and vivid even from a distance.  (The remaining wispy feathers have next to no impact on the beauty of the round.)

Summing up the point I was try to make about cutting strongly dichroic tourmaline.  Rounds can be a great cut if the material’s c axis has a good color that is compatible with the a/b axis color.  And that the semi open ends of the rough are not too dark.  The added benefits of a round being better at hiding flaws and being thinner are nice, if needed.

Bruce

 

 

About Bruce Fry

I was born in Summit, NJ in 1947 and graduated from Summit High School in 1966. I graduated from the Colorado School of Mines in 1970 and after spending another year in graduate school, I left to see the world of Brazil. After spending some more time discovering myself, I ended up working for the Commonwealth of Pennsylvania for 32 years as an Air Quality Engineer in the Department of Environmental Protection. I retired in 2007 and took up faceting gemstones again after a long hiatus that reached back to my twenties. I had started cutting cabochons when I was 13 and bought my first faceting machine when I was 15, but ran out of money and time until I retired.
My great love in gemology is tourmaline and the collection presented here represents my effort to get as much beauty and variety in the colors of tourmaline as I can. I was particularly lucky in being able to get unheated cuprian tourmaline before copper was discovered in gem grade tourmaline from Mozambique.

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