It is with great sadness I'd like to announce Dr. Hanneman passed away on December 12, 2020. His legacy will live on forever!
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 Post subject: Raman/PL results: Characterizing emerald types
PostPosted: Sat Jan 19, 2013 5:10 pm 
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Careful observation of exact chromium photoluminescence peak locations allows to make distinction or produces strong evidence in order to discriminate between natural and most synthetic emeralds. Additionally, natural emeralds can be characterized as geologically belonging to schist- or non-schist type.


The strong intensity of Raman spectrometer's 532 nm laser beam is capable of causing noticeable photoluminescence reactions in gems which are traditionally considered as non- or weak luminescent materials. Among these PL reactions we can have the diagnostic luminescence pattern of chromium ion in beryl; Emerald or Cr- bearing aquamarine are readily identified by luminescence reaction of chromium ion occupying aluminum site in beryl structure. Some emeralds having low chromium concentration are mainly colored by vanadium, but chromium content in these stones is still high enough to cause the luminescence reaction.

Image

Other impurities like magnesium, titanium and zinc occupying the same octahedral position in the beryl structure influence the position of detected chromium PL lines. A careful measurement of exact position of the major chromium PL lines allows to characterize the sample as belonging to one of the three major groups: synthetic emerald, natural schist type emerald and natural non-schist type emerald.

Synthetic emeralds

Many synthetic emeralds contain relative high Cr concentration compared to natural stones. The first clue about synthetic origin is the PL reaction's high intensity; many synthetic emeralds saturate the spectrometer already with the minimum exposure time of GemmoRaman-532 (1 millisecond). The sample must be oriented slightly off from the laser aperture in order to get readable results. This is not a diagnostic behavior but gives a very strong indication about the synthetic origin. PL peaks for both flux and hydrothermal synthetic emeralds occur at relatively low wavelengths compared to natural stones.

Natural emeralds

Natural emeralds can be characterized as belonging to two main types of geological occurrences; schist- type and non-schist type. This method combined with inclusion studies and absorption spectroscopy is very useful in order to determine the geographical origin of emeralds.

Picture1: Photoluminescence peaks of chromium in beryl structure. All spectra have been normalized, trimmed to 4050-4300 cm-1 and background corrected. Most of the spectra are vertically shifted in order to visualize the difference between major groups. Colored bars indicate the peak location variation in each major group.

Image

Examples of Type I (non-schist) emeralds: China (Malipo), Colombia (Chivor, La Mina Glorieta, Las Cruces, El Diamante, El Toro, La Vega de San Juan, Coscuez and Muzo), Nigeria (Jos, Gwantu), Tanzania (Sumbawanga)

Examples of Type II (schist) emeralds: Afghanistan (Panjshir Valley), Austria (Habachtal), Brazil (Carnaiba/Sotoco, Capoeirana, Itabira, Santa Teresinha), Russia (Malishevo, Perwomaisky, Mariinsky, Aulsky, Krupsky, Chitny, Tsheremshansky), Zambia (Kafubu area), Madagaskar (Mananjary, Ankadilalana), Zimbabwe (Sandawana), South Africa (Transvaal), Spain (Franqueira), Pakistan (Mingora Mines), Mozambique (Morrua district)

References:

Inessa Moroz, Michael Roth, Micheline Boudeulle and Gerard Panczer: Raman microspectroscopy and fluorescence of emeralds from various deposits. Journal of Raman Spectroscopy 31, 485–490 (2000)

Further reading:

Microscopic, chemical and spectroscopic investigations on emeralds of various origins Le Thi Thu Huong

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 Post subject: Re: Raman/pl Results: Characterizing emerald types
PostPosted: Sat Jan 19, 2013 9:46 pm 
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Thank you Alberto,

The depth of knowledge here is amazing and
I appreciate every opportunity.
I do not understand as much of this as I intend to
but I am getting there.

The link is also appreciated really nice photos of
inclusions also.

Harold


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 Post subject: Re: Raman/pl Results: Characterizing emerald types
PostPosted: Sun Jan 20, 2013 1:50 am 
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Alberto wrote:
Careful observation of exact chromium photoluminescence peak locations allows to make distinction or produces strong evidence in order to discriminate between natural and most synthetic emeralds. Additionally, natural emeralds can be characterized as geologically belonging to schist- or non-schist type.
g

Now, can you tell us how this correlates with the "more simple" Hanneman-Hodgkinson Synthetic Emerald and support filters?


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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Mon Jan 21, 2013 5:56 am 
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Very interesting, thanks for sharing!

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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Mon Jan 21, 2013 11:34 am 
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Question:
How many samples have been tested?


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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Mon Jan 21, 2013 12:45 pm 
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We tested 20 samples – the spectra you can see in picture 1 – likely we will add more stones once we will have time (which is actually the only thing we desperately lack…. :( )

ciao
albé

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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Tue Jan 22, 2013 12:49 pm 
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hi Alberto

Very interesting observations. What are actual wavelength values (nm) for peak pairs in each of the three cases?

Thanks, Brian


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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Tue Jan 22, 2013 1:34 pm 
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Hey Brian,

These are the wavelength for main peaks of the actual scans

Synthetic: 683.67 - 683.81 nm
Non-schist: 683.95 - 684.11 nm
Schist: 684.43 - 684.72 nm

Note: it would be good to look for visual comparison and not to the numbers only. Since this measurement is very sensitive to calibration of the spectrometer, each user should create his own personal set of reference spectra.

ciao
albé

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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Tue Jan 22, 2013 3:11 pm 
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Alberto, thanks for the numbers!

Alberto wrote:
it would be good to look for visual comparison and not to the numbers only. Since this measurement is very sensitive to calibration of the spectrometer, each user should create his own personal set of reference spectra.


Indeed, that is the plan. This would be a very nice research project for a student, as I have all the parts (laser, spectrometer, and low-pass filter) lying around. If only I can beg, borrow, steal enough different types of emeralds to build up the database.

And also, perhaps the student could take on Bill's challenge. Bill, where to purchase the filter set you speak of?


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 Post subject: Re: Raman/PL results: Characterizing emerald types
PostPosted: Wed Jan 23, 2013 1:33 am 
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Bill, where to purchase the filter set you speak of?

Send me your address.


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