Tourmaline and black outs...

[Indylady]

Do tourmalines black out easily? I found a very pretty green tourmaline that I like, but some of the pictures make it seem dark. I''ve noticed this with a few other green tourmalines that I''ve seen. Is that something that is common with green tourmaline, or is it just because the ones I''m seeing aren''t good quality?

 

[Barrett]

are you speaking of extinction?

 

[jstarfireb]

Date: 5/24/2010 9:55:12 PM
Author: amethystguy
are you speaking of extinction?

...which would be a result of tourmaline''s relatively low RI, right?

 

[lelser]

Many green tourmalines are black along the C axis. If you think of the table as the A-axis, the long side as the B and the line from end to end as the C, the C will often not let any light pass through.

There''s a number of ways to cut tourmaline that minimize the effect of that black C, but if the stone isn''t cut exactly right you''ll get darkness reflecting up onto the stone. It''s a form of extinction, a bit different.

The RI isn''t a problem - after all, lots of stones have a lower RI but cut terrific bright stones.

I''ll almost never buy dark C tourmaline anymore since it limits my options for cutting. That said, I''ve got a doozy on the machine now, which IS being cut correctly

Sometimes they are just too damn pretty to walk away.

Cheers,

Lisa

 

[VapidLapid]

I have a green tourmaline that blacks out. It has a wide open c axis. It was precision cut. Sometimes it is one of the loveliest stones I have ever had and other times it is a black hole. It is at its worst when it is sitting in the box on the cushion. I expect that once it is set it will behave better. I have another green one, also precision cut, with a closed c axis that doesnt black out.

 

[Indylady]

Thank you Amguy, Jstar, Lisa and VL!!

Lisa, your explanation has helped a lot. I thought that it might have something to do with the C-axis, but didn''t remember enough to piece together the puzzle.

VL- That''s exactly how this tourmaline is! Its the perfect medium green in most shots...and then the a deep, deep blackish forest color in the 3/4 shot. This tourmaline was also precision cut.

 

[VapidLapid]

like this?

 

[VapidLapid]

and then this

 

[Indylady]

Exactly! The difference is wild...like a color shifter, only a "shade shifter".

 

[Barrett]

Here..nice little quote
"Observation has shown that extinction is present to a certain degree in all faceted gemstones, including diamond, no matter how well they are cut. Individual stones will posses varying degrees of extinction, as well. " and "As the stone is manipulated when face-up, the amount of extinction will remain relatively the same. The effect is also observed in some emeralds, often in tourmalines, many of the garnets, including tsavorite, and some spinels"
Now I recommend all PS'ers read R Hughes page here..it's really a good read and helps understand things..I fall back on it all the time to reaquaint myself with it
http://www.ruby-sapphire.com/brilliance_windows_extinction.htm

 

[chictomato]

Hi Indylady, my Chrome tourmaline does not black out in different light condition, it could be the concave faceting though.

 

[chrono]

I find that most green tourmalines tend to black out under certain lighting; part of the reason is the material itself (closed C axis) and the cutting (even if precision cut depending on how the cutter cuts it). Chrome tourmalines do not do this as much or if at all, I’ve noticed. Perhaps it is due to the chromium but I do not know for sure.

 

[iLander]

Date: 5/24/2010 10:28:19 PM
Author: lelser

Lisa

Just curious; why don''t you have the prices and stone sizes on your website? Just wondering why people do that . . .