Welcome to AGTA GTC's Laboratory Update for November 1, 2005

In this message

  1. Introducing Our Instruments: The Raman Spectrometer
  2. LMHC Update
  3. AGTA GTC on the Web

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Introducing our Instruments: The Raman spectrometer

When the first Raman Imaging Spectrometer (Figure 1) arrived in a gemological laboratory in 1994, the only available publication with Raman reference spectra was a catalogue by the authors Pinet, Smith and Lasnier, which appeared in 1992. Today manufacturers, laboratories and individuals have compiled vast databases of Raman spectra for gemological use.

Raman Spectrometer

Figure 1. Renishaw Raman spectrometer
The Raman spectrometer is extremely useful in the non-destructive identification of inclusions in gems, as well as fingerprinting rare gem materials. It also can be used to identify certain types of fillers used in treated emeralds, along with HPHT treated diamonds. This is just one of the advanced gem-testing instruments in residence at the AGTA GTC.

The Raman Effect
In 1927, Sir C. V. Raman discovered an extremely faint type of electromagnetic scattered radiation. This was an amazing feat due to the small amount of scattered radiation present (about one hundred millionth part of the incident light). He would later receive a Nobel Prize for discovering this effect, which today is known as the “Raman Effect.”
     The Raman Effect occurs when light interacts with electrons. Light photons striking electrons may be reflected without any loss of energy, something termed “Raleigh scattering.” The photon can also have part of its energy absorbed by the electron, an effect known as “Raman scattering.”

Mineral Identification
Raman scattering also occurs on the molecular level. As an intense light source (such as a laser) strikes a molecule, electrons in the molecule begin to vibrate. The pattern of these vibrations is determined by the types of atoms present and their arrangement. Since each mineral has a unique mixture of composition and/or structure, the resulting Raman scattering will be specific to each mineral (Figures 2 and 3).

burma sapphire raman spectrum

Figure 2. The Raman spectrum for sapphire.

raman spectrum benitoite

Figure 3. The Raman spectrum for benitoite, which resembles sapphire. Differences in composition and structure produced a unique Raman “fingerprint” for each mineral.

     The basic application of Raman spectroscopy in gemology is the fast, non-destructive identification of unknown materials. While there are less expensive ways to identify unknown mineral samples, occasionally a situation arises when only Raman spectroscopy will help. Take for example the case of separating diamond and synthetic cubic zirconia. While this is normally an easy separation, imagine if the gems are mounted on the face of a wristwatch where it’s impossible to get direct access to the stones? A quick and easy determination could be made by examining the Raman spectra of the materials in question (Figures 4 and 5).

diamond raman spectrum

Figure 4. The Raman spectrum for diamond.

cubic zirconia raman spectrum

 

Figure 5. The Raman spectrum for synthetic cubic zirconia.

     A situation could arise where a large parcel of red rough needed to be separated before being sent out to be cut. Using the Raman spectrum as a means of identification, this could be done easily. For people studying gem materials such as maw-sit-sit, which is a rock composed of ureyite, jadeite, albite, chromite and other minerals, the separation and identification of the individual components is facilitated using Raman spectroscopy.

Inclusion Identification
In the past, inclusions had to be exposed to the surface in order to be identified. This was often impossible, so the inclusion remained unidentified. Raman spectroscopy has the capability to identify solid, liquid and gas phases even when they lie within a gem. Thus it has proven to be an indispensable tool for the non-destructive identification of inclusions (Figure 6 and 7).

zircon inclusion in jadeite

Figure 6. A white crystal inclusion seen in a jadeite cabochon.

zircon raman spectrum

Figure 7. The Raman spectrum for zircon.

Treatment Identification
Treating gemstones to enhance their appearance is an ancient practice. The value of gemstones can sometimes vary greatly between treated and untreated gemstones. For this reason, modern laboratories are always asked to determine whether a gemstone has been altered to improve its appearance. Raman spectroscopy has been proven to be of great value in the identification of treatments involving oils, resins or plastics, (Figures 8 and 9).

opticon raman spectrum

Figure 8. The Raman spectrum of Opticon filling a cavity in emerald.

cedarwood oil raman spectrum

Figure 9. The Raman spectrum for cedarwood oil, a common oil used to enhance the appearance of emeralds.

     As the number of reference spectra increases, the use of Raman spectroscopy to identify unknown materials will grow until it enjoys the widespread use of other techniques, such as the well known x-ray powder diffraction method of identifying minerals. The non-destructive nature of Raman spectroscopy makes it especially important to gemology because specimens are often so valuable.

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LMHC Update
On October 20–22, 2005, AGTA GTC Lab Director Lore Kiefert attended a meeting of the Laboratory Manual Harmonization Committee (LMHC) in Lucerne, Switzerland. The LMHC consists of seven of the world's major laboratories. They include the AGTA Gemological Testing Center (USA), CISGEM (Milan), GAAJ (Japan), GIA (USA), GIT (Thailand), Gübelin Gem Lab (Switzerland) and SSEF (Switzerland). For several years now, representatives of these laboratories have met to discuss the harmonization of identification procedures and wording on gemstone reports.
     Issues discussed at the meeting included the definitions of padparadscha sapphire and Paraíba tourmaline, lab nomenclature for emerald and ruby. As the LMHC reaches agreements on these issues, documents will be available from our website at this link:

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AGTA GTC on the Web
A number of clients have asked us to consider making our gemological bulletins available to a wider audience. Towards that aim, over the past few months we have built a website specifically for the AGTA Gemological Testing Center. It is now live and offers a complete archive of our e-mail bulletins, along with a full description of the lab and its services.
     See it at http://www.agta-gtc.org or link from AGTA’s regular site, www.agta.org.

The new AGTA-GTC website offers the most up-to-the-minute gemological news, along with a full description of the lab and its services.

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The AGTA Gemological Testing Center provides the industry and the public with a complete range of lab services, including gemstone identification, origin determination and pearl identification. Located in New York City, the laboratory is equipped with the latest, technologically advanced, investigative equipment.The AGTA GTC is committed to providing excellent service, superior value and outstanding quality. A complete list of services and detailed pricing information is available on our website, www.agta-gtc.org. Please contact us with any questions.


American Gem Trade Assocation Gemological Testing Center
18 East 48th St., Suite 502
New York, NY 10017, USA
Tel: 212-752-1717; Fax: 212-750-0930
E-Mail: info@agta-gtc.org; Web: www.agta-gtc.org
© 1999–2005 American Gem Trade Assocation Gemological Testing Center. All rights reserved. Users may download this information for their own private, non-commercial use. Any other reproduction of this document (text or graphics) without the express written consent of the AGTA GTC is strictly prohibited.
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