Coming Soon!!! Gem Identification Instrument Give-Away. Donated by W.Wm.Hanneman
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Fri Mar 22, 2019 11:07 am

All times are UTC - 4 hours




Post new topic Reply to topic  [ 20 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: emerald
PostPosted: Wed May 09, 2007 4:41 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
So I was going through the beryls, doing 30 second scans to see if any interesting absorption features popped out.

I started with ... goshenite, boring, would have gotten more absorption from a glass of water. ... morganite, boring ... aquamarine, boring

... and then I came to emerald :D

mineral box 827, stone 1 (of 2)
beryl var. emerald
location: Santa Terrinha, Estado Goias, Brazil
mass: 0.20 g
(click to see larger image)
Image

mineral box 827, stone 2 (of 2)
beryl var. emerald
location: Santa Terrinha, Estado Goias, Brazil
mass: 0.45 g
Image


A close-up of the red region of stone 1:
Image

A close-up of the red region of stone 2:
Image


So I'm diligently reading the gemology project wiki about emeralds, and I'm like ... "hmmm ... my emeralds are from Brazil, so maybe they are vanadium beryls instead of chromium emeralds."

Comparing with the wiki spectral data for the chromium emerald, I see in stone 1 an absorption doublet in the red. It is shifted 10 nm down from where the fluorescent doublet in ruby appears, but that could be due to different surrounding atoms in the crystal. Also in stone 1, I see an absorption in the orangey-red, but the wiki article says there should be two absorptions in that region. Finally in stone 1, I see the characteristic broad absorption through the orange-yellow.

The statistics in the spectrum for stone 2 are not very good... I threw out all the violet region data because it was so wildly up and down. This was a very difficult spectrum to collect, running three days non-stop. Just goes to show, whereas a gem collector may choose the larger of two comparable stones, it is a lot easier to collect spectra on smaller stones.

Anyways, the major difference between the two stones is that the absorption line in the orangey-red of stone 1 apparently shifted position in stone 2... maybe one stone is missing one of the orangey-red absorptions and the other stone is missing the other absorption. For stone 2, one can argue that this isn't much of a dip, but I can see this absorption line clearly in my spectroscope.

Also noteworthy for stone 2 is the much higher transmission in the IR region beyond 700 nm. What's that all about?

So which are these emeralds, chromium or vanadium? I'm going to consider the doublet in the deep red diagnostic and declare them chromium emeralds. At least until I see a typical vanadium emerald spectrum.

The wiki article also mentions an absorption at 477 nm for synthetic or high-quality natural emerald. I don't see anything in the blue, so apparently I have cheap natural emeralds.


Last edited by Brian on Mon May 14, 2007 3:15 pm, edited 2 times in total.

Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed May 09, 2007 5:45 pm 
Offline
Platinum Member

Joined: Sun Oct 16, 2005 12:42 pm
Posts: 2591
Hi Brian,

You can indeed expect the doublet in the red and two other lines in the orange, with a general absorption of the yellow. That is an ideal situation of course and that might vary from stone to stone. I'm sure your stone is a nice natural chromium emerald.

The project is by far not done yet, so don't refer to it as the ultimate resource (yet :) ). There is still so much to be done but someone is working on the spectra. Luckely not me.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed May 09, 2007 6:25 pm 
Offline
Gemology Online Royal Princess

Joined: Mon Oct 17, 2005 10:56 am
Posts: 6462
Location: The frozen north prairie :-/
Hey Brian, do you have a Chelsea filter you can check the emerald with? Chromium will show a red reaction under the Chelsea, whereas vanadium won't. :)

_________________
IIJA Registered Gemologist
GIA Graduate Gemologist


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed May 09, 2007 10:29 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
Chelsea filter? What do I look like, a gemologist? :roll:

And thanks Doos! Between this forum, the wiki, and gemca's site, I'm learning a lot about gem spectroscopy.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sat May 12, 2007 1:28 am 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
You know, mucking around late at night, sometimes you find interesting things. Like how you should be able to recognize chromium-rich emerald without accounting for every line, or how emerald shows different lines in different directions:
http://yey.be/Winter.cgi

Anyways, I have yet to post the second emerald spectrum because my light went kaput overnight during a measurement. No worries, I fixed it, and it is running right now.

Even so, with the very test spectrum of the second stone, I could identify the chromium from the significant doublet in the red. However, the spectrometer's measurement of the line (or maybe a couple lines) in the orangey-red is very weak in this particular stone. It is interesting though, in using a SPECTROSCOPE, I have a much easier time seeing the orangey-red line than I do seeing the doublet in the deep red. In conclusion, even though the orangey-red line is much weaker in the spectrometer, it is much easier to see in the prism spectroscope.

Since I don't have an OPL, I was wondering how these two lines compare in that instrument.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sat May 12, 2007 7:55 am 
Offline
Platinum Member

Joined: Sun Oct 16, 2005 12:42 pm
Posts: 2591
Hi Brian,

Yes emerald is known for having different spectra in the ordinary and extraordinary vibrational directions (some other gems show that aswell). I updated the wiki a few days ago to state that. Will have to wait for the images though.
The image on the wiki is for the ordinary ray if I'm correct.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon May 14, 2007 3:19 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
Well, added the second emerald's spectrum, such as it is. Each new spectrum unleashes more questions. Each stone of the same type is slightly different than the one before.

Anyways, these spectra are collected using unpolarized light. And using an arbitrarily chosen path through the stone.



Next up, zircon. Looking through my prism spectroscope, I was surpised to see major differences between the natural zircons and the one I tried to heat.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Fri Jun 01, 2007 12:09 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
A repeat of emerald's spectrum, using stone 2 from mineral box 827 (0.45 g mass). The new spectrometer was used to collect the spectrum... which means I don't have to chop off the blue part this time.

Here's the complete VIS-IR spectrum (click for larger picture):
Image

In the IR, we see a big, broad absorption dip across the 750 - 900 nm range.

And here's the visible spectrum only:
Image

The chromium doublet around 684 nm is merged into one dip. There are a couple shallow absorptions at 661 nm and 637 nm. So apparently this emerald does show a couple bands in the red along with the chromium doublet. Of course there is the widespread absorption everywhere but the green and the edge of the red.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jun 06, 2007 8:34 am 
Offline
Valued Contributor
User avatar

Joined: Mon Oct 17, 2005 2:52 pm
Posts: 241
Location: France | French Riviera
Funny stuff regardinf emerald spectrum.
Generaly beryl can show an absorption in the NIR region (I dont remember exactly :roll: may be near 967 nm).
I presume that thi feature is due to water in the beryl lattice channels.
Thus if you see this feature you are sure your emerald is not a flux synthetic stone :)
Brian, can you do the spetrum until 1000nm in the NIR?
It will be interesting to verify the presence of this peack..

The peack near 475 nm can be very discrete in a spectrum from a spectrometer like our.
It is more easy ( to my sense) to see this peack in a prism desck-spectroscope.
Nevertheless, we can see this discrete feature in your spectrum near 475 as a shoulder 8)

_________________
Regards

Jean-Marie Arlabosse
GemInterest.com


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jun 06, 2007 11:19 am 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
Your wish is my command. :wink:

Of course, I have already recorded the spectrum beyond 900 nm. But the light intensity is so low and decreasing above 900 nm that it can lead to an "apparent" steady increase in %T that I don't necessarily trust. So I usually don't present that range. However, the spectrum is clean enough to observe any significant absorption that is present:

emerald spectrum from 400 - 1000 nm (click for larger image):
Image

And bang, an absorption at 957 nm, close to your prediction! Good job. :D

As for the shoulder at 475 nm, it is mighty difficult to see. I agree completely that some lines are easier to see through the spectroscope, compared with the digital spectrum... if they are very, very narrow they will not resolve in the digital spectrum.

But ok then, some slight absorption at 475 nm, suggesting either a somewhat fine natural emerald or synthetic, and the absorption at 957 rules out the synthetic. Cool 8)

This emerald may be my favorite. It's not be very big, but it has a nice color when shining the fiber optic light through it. There is some kind of visible-to-eye black inclusion through its middle, almost cutting it in half.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jun 06, 2007 4:10 pm 
Offline
Valued Contributor
User avatar

Joined: Mon Oct 17, 2005 2:52 pm
Posts: 241
Location: France | French Riviera
Brian wrote:
..As for the shoulder at 475 nm, it is mighty difficult to see. I agree completely that some lines are easier to see through the spectroscope, compared with the digital spectrum... if they are very, very narrow they will not resolve in the digital spectrum..

But in some case the spectrometer can be more efficiant than our eyes.
Take a look at your post on zircon in this research rubric ;-)

Brian wrote:
..But ok then, some slight absorption at 475 nm, suggesting either a somewhat fine natural emerald or synthetic, and the absorption at 957 rules out the synthetic. Cool 8)


Yep.. the 957 nm feature rule out ...just flux (anhydrous) synthetic emerald (Chatham, Gilson, etc.) .. not hydothermal synthetic emeral :?
As for natural, synthetic hydrotermal emerald show the NIR absorption feature related to water.
Image
In this case we have to study very closer the inclusionw.
To be diagnostic we can do FTIR spectrum. . etc.
;-)

_________________
Regards

Jean-Marie Arlabosse
GemInterest.com


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jun 06, 2007 4:48 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
Gemça wrote:
But in some case the spectrometer can be more efficiant than our eyes.
Take a look at your post on zircon in this research rubric ;-)


And of course, our eyes would never see the 957 nm absorption line. :lol: But in the visible, as you show in the zircon thread, if the dip is very shallow then the digital spectrometer can pick up what the eye can't see. And if the dip is very narrow, the eye can pick up what the spectrometer doesn't see well.

But it is interesting to see that for the synthetic emerald, the 475 nm feature is also a difficult-to-discern shoulder as well. So I should have been looking for a shoulder in the first place, rather than a dip.

Here is, I think another interesting point about this material. I think one could use the visible spectrum to basically grade the color of an emerald. The green "peak" looks like a simple Gaussian "bell" curve that could be characterized by its peak position (average value of the bell curve) and full-width at half-maximum (standard deviation of the curve).

One could measure spectra of stones that people consider to be ideal in color and then come up with an averaged ideal peak position and FWHM. Then other stones could be characterized or ranked by how much their peak positions and FWHM differ from those properties of the ideal colored stone.

For example, my emerald's green peak is at about 520 nm, while the peak in your synthetic emerald looks to be around 505-510 nm. So which is closer to the ideal color? Is mine a shade more yellow than ideal, or is yours a shade more blue than ideal (betting mine is shade too yellow)? Also, the FWHM of my emerald's green peak is larger than yours, so my emerald color wouldn't be as "spectrally pure green".

Also, do you think the large absorption in the natural emerald across the 750 - 900 nm range, compared to the nearly flat spectrum of the synthetic emerald over that range, has any significance?


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Thu Jun 07, 2007 3:20 am 
Offline
Valued Contributor
User avatar

Joined: Mon Oct 17, 2005 2:52 pm
Posts: 241
Location: France | French Riviera
Brian wrote:
..
Also, do you think the large absorption in the natural emerald across the 750 - 900 nm range, compared to the nearly flat spectrum of the synthetic emerald over that range, has any significance?


I don't know :oops: ,
I have to do more emerald and synthetic emerald spectrums and gain in knowledge to point out the potentiel intrest of the behaviour of this zone :P .

I agree with you concerning the resolution/sensibility of the spetrometer vs eye in a spectroscope !
To my sense it is very important to never forgive basic instruments... :wink:

_________________
Regards

Jean-Marie Arlabosse
GemInterest.com


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Thu Jun 07, 2007 6:40 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1451
Gemça wrote:
I don't know :oops: ,
I have to do more emerald and synthetic emerald spectrums and gain in knowledge to point out the potentiel intrest of the behaviour of this zone :P .


Sounds like a good excuse to re-measure my other emerald.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Fri Jun 08, 2007 1:48 am 
Offline
Valued Contributor
User avatar

Joined: Mon Oct 17, 2005 2:52 pm
Posts: 241
Location: France | French Riviera
LOL :)

Yep.. more we have spectrum with the same behaviour..more we can do a generality..until the day of the big suprise arise!
Like for this hydrothermal synthetic emerald (russian) under the orange line
Image

As you can see the colombian emerald have similarity with hydrothermal synthetic emerald.
I think the 700 - 1000 nm could not help for discrimination.

I have printed the orange line to show that, some time, we can have spectrum very different of we can expect.
This stone is an hydrothermal synthetic emerald => confirmed by gemmological data and FTIR study.
I really want to know what is the chromophoric element:
May be a mix of Cr3+ and/ore V3+ and/or Cu2+ and/or other ??
If you know something on this kind of hydro. synth. emerald..please let me know [-o<

_________________
Regards

Jean-Marie Arlabosse
GemInterest.com


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 20 posts ]  Go to page 1, 2  Next

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock