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 Post subject: red transmission in natural emerald
PostPosted: Thu Oct 25, 2012 3:21 pm 
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so - red transmission in synthetic emeralds is from the chromium - and i've seen a lot of that.
however, i've never seen a natural emerald display red transmission.
has anyone ever seen this?

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 Post subject: Re: red transmission in natural emerald
PostPosted: Thu Oct 25, 2012 6:13 pm 
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Please tell us precisely what you are referring to with "red transmission".
Thanks.


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 Post subject: Re: red transmission in natural emerald
PostPosted: Thu Oct 25, 2012 7:43 pm 
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As Mrs. B. said: what the heck you are talkin´about precisely?

A filter reaction? HarHar

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 Post subject: Re: red transmission in natural emerald
PostPosted: Thu Oct 25, 2012 8:08 pm 
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when looking at the stone through the scope, darkfield, transmitted light produces a reddish color.

this post http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=187 has a nice photo of this phenomenon (just not through the scope):
the best example (through the scope) that i have on-hand is the in Photoatlas of Incl. Vol.3, p458, upper left.

in my expericence, it's very stong in flux grown emeralds through the scope. but i've never seen it exhibit strongly in natural emeralds

here are some pics i took with a pinpoint fiber optic source.
you can see the concentration of reddish color right where the fiber optic light is brightest.
Image

Image

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 Post subject: Re: red transmission in natural emerald
PostPosted: Thu Oct 25, 2012 8:42 pm 
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sorry. that first one was a poorly phrased, inadequate post.

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 Post subject: Re: red transmission in natural emerald
PostPosted: Thu Oct 25, 2012 9:14 pm 
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I'm still unclear. You are seeing red when you shine transmitted light in a dark field environment. What is the source of that transmitted light? The emerald you've pictured appears very pale. Have you checked it with a spectroscope for Cr absorption? Have you checked it with UV light to see if it is fluorescent at all?

Both natural and synthetic emeralds can fluoresce red when viewed with LWUV.
This has to do with chromium as the primary chromophore and a lack of iron. It is far more common to see red fluorescence in synthetic emeralds, such as Gilson

Ux4 said he noted a red glow when he inspected some synthetic emeralds with a penlight. He was looking for confirmation.
Others, including Gemça (a very respected mineralogist and gemologist) , could not confirm these findings.

That being said, you are reporting you have an emerald with an unusual anomaly. :?:


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 Post subject: Re: red transmission in natural emerald
PostPosted: Fri Oct 26, 2012 5:56 pm 
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(sorry for the delay, i'm now back in the store with the stone)
it is very weak to LW and my SW is broken.
in the spectrum i managed to squeeze out, the Cr 680 is there.
Image

once i get my photo setup completed (viewtopic.php?f=57&t=16749), i really want to shoot the visual red transmission that i'm talking about. it's a very distinct color difference to the normal, black extinction when you're looking at a stone in the scope. the best photographed example is in the photoatlas vol.3 p458, upper left photo.

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 Post subject: Re: red transmission in natural emerald
PostPosted: Fri Oct 26, 2012 6:04 pm 
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Thanks for your well documented follow through.
Perhaps Brian can weigh in on what's happening here, and Nikolas can revisit it.


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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 1:23 pm 
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In the spectrum, note how more red light is being transmitted, compared to green light. Also note in the pictures how the red color appears to come from the deepest part of (longest light path through) the stone. This suggests the observation arises from to our old friend, the "usambara" effect. Chrome tourmaline quite often exhibits this effect. Here are some spectra and pictures of usambara tourmaline. And here is an article by Elise Skalwold about the effect.

In general, the effect occurs in gems (or dyes or leaves) where both red and green light are transmitted through the material, with more red light being transmitted. If the material thickness is small, even though red light is being transmitted, the material appears green because our eyes are so much more sensitive to green light. But as the thickness of the material increases, eventually the amount of transmitted green light falls below some threshold for vision, and we see the red light transmitted through.

Thus, I imagine a full boule of this synthetic emerald would appear red.


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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 2:01 pm 
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The emerald pictured above looks natural to me.


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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 2:13 pm 
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Sorry, I wasn't trying to identify whether it was natural or synthetic. Just that a thicker slab of the material would appear red.


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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 2:25 pm 
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I don't mean to repeat myself (I've brought this up before), but here's a good example of that phenomenon, as seen in pumpkinseed oil. Green when thin, red when thick:
Image

That said, I don't think I've ever seen an emerald that appeared red in any thickness (though I'll admit I haven't been up close and personal with too many really large and really transparent emeralds).

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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 2:30 pm 
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You can bring up this picture again and again Scarodactyl, it's awesome!


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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 3:10 pm 
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yipes- that pumpkin oil photo is amazing! (and you should trot it out even more in late october)

under about 80x, i manged to resolve some 3 phase inclusions with very angular (calcite) guests.
tricky because of the mounting. you can see in the photos, the plane of any informative inclusions run vertically, near the edge the stone. needless to say, we'd rather not unset and rest the stone.

for the same reason, i can't really get a better spectrum.

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 Post subject: Re: red transmission in natural emerald
PostPosted: Mon Oct 29, 2012 3:19 pm 
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There should be a bottle of pumpkin seed oil in every kitchen - extremely healthy and tastes wonderful (Can send you recipes per PN if wanted) :-)

Back to the effect described above. I still have no clue and never observed it personally.

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