Welcome to AGTA GTC's Laboratory Update for October 12, 2005

In this message

  1. Seeing in the Dark 3: The Green Filter
  2. Introducing our Instruments: The UV-VIS Spectrometer

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Seeing in the Dark 3: The Green Filter

Careful readers will remember our two-part series, "Seeing in the Dark," on the use of UV fluorescence for the detection of treated gems.
     To refresh your memories, many heat-treated rubies and sapphires will display chalky short-wave (SW) fluorescence. This reaction is practically never found in untreated corundums. It is actually the colorless portions of the stone that fluoresce (a reaction similar to Verneuil synthetic sapphires). Since colorless areas follow the original crystal's growth structure, the fluorescence will follow the same pattern as the gem's color zoning. Figure 1 illustrates this reaction in ruby.

mong hsu ruby

Figure 1. Let it glow
In this heat-treated and flux-healed ruby from Möng Hsu, Burma, thin, zoned patches of chalky blue fluorescence float on top of the chromium-based red body fluorescence. These chalky blue zones are a strong indication of high-temperature heat treatment. Photo: Richard W. Hughes/AGTA GTC

While the reaction is quite obvious in stones such as blue sapphire, which usually have no other fluorescence, the situation is complicated in ruby, where the strong red body fluorescence can mask the chalky reaction, making it difficult to see.
     During the writing of those articles, the AGTA GTC's technical advisor, Dr. John Emmett, suggested the use of a Schott BG-12 green filter to isolate the chalky blue fluorescence that is so important in the identification of heat treatment. Thus we ordered one up and, lo and behold, it works! Placing the Schott BG-12 green filter between the eye and gem cuts out the red glow, transmitting just the chalky fluorescence that is diagnostic in detecting heat treatment (see Figures 2 and 3).

no filter ruby

Figure 2. Without the green filter
In this heat-treated and flux-healed ruby from Möng Hsu, Burma, thin, zoned patches of chalky blue fluorescence float on top of the chromium-based red body fluorescence. These chalky blue zones are a strong indication of high-temperature heat treatment, but are largely masked by the red body fluorescence. Photo: Richard W. Hughes/AGTA GTC

green filter ruby

Figure 3. Enter the green filter
In the same stone from Figure 2, a green Schott BG-12 filter is placed over the gem. It removes the red fluorescence, thus making the chalky blue areas far easier to see. Photo: Richard W. Hughes/AGTA GTC

In the coming weeks, we will be stitching together all of our previous writings on the use of UV fluorescence as a gemological tool and placing them into a single article that will appear on the AGTA GTC website.

dingbat
Introducing our Instruments: The UV-VIS Spectrometer

The visible portion of the electromagnetic spectrum has long played an important part in gemology. Traditionally gemologists have used hand held spectroscopes to observe the visible spectrum. Today the hand held spectroscope is still used but in modern gemological laboratories computer driven spectrometers which operate from the ultra violet through the visible range to the infra red region are the norm. AGTA GTC has two such instruments. One is a larger model, which is used in the laboratory on an everyday basis and the second is a smaller portable model, which is used in the laboratory and for onsite testing (Figures 1 and 2).

UV Vis

Figure 1. The Unicam UV 500 is the spectroscope that AGTA GTC uses for gathering spectra for the determination of origin of gemstones.
Photos: Riccardo Befi © AGTA GTC

UV-Vis Spectrometer

Figure 2. The Zeiss MCS 500 from the AGTA GTC portable laboratory is helpful in collecting spectra for pearls and diamonds as well as spectra for determination of origin.

Origin determination
Increasingly, country of origin determination has become an important service provided by gemological laboratories. The UV-VIS spectrum plays an important part, along with trace element chemistry, infra red spectroscopy and microscopic observations, in establishing the country of origin of blue sapphires. Because the UV-VIS spectra for blue sapphire from different sources have subtle differences, they can provide valuable clues to the gemologist (Figure 3 and 4).

sapphire spectra

Figure 3. A typical UV-VIS spectrum of a sapphire from Kashmir region of India Figure 4. A typical spectrum of a sapphire from Myanmar (Burma).

Treatments and techniques
The UV-VIS spectrum is very helpful in detecting certain methods of treatment even when there is no microscopic evidence to indicate treatment. One treatment is the heating of cuprian blue tourmaline such as the material which is found in Paraiba, Brazil. When this material is heated, chemical changes take place that result in a change of the visible spectrum (Figures 5 and 6).

paraiba unheated

paraiba heated
Figure 5. The UV-VIS spectrum of an unheated cuprian blue tourmaline shows a broad band centered at approximately 515nm. Figure 6. The UV-VIS spectrum of a heated cuprian blue tourmaline clearly shows a small peak at 415nm which has developed due to chemical changes caused by the heating process while the broad band at 515nm has disappeared.

It has long been known that the UV-VIS spectrum contains a large amount of information that is of help in determining whether or not a diamond has been treated. Simple room temperature spectra reveal some of this information but by cooling the diamond using liquid nitrogen much more information becomes evident (Figures 7 and 8).

diamond room temperature

diamond cooled
Figure 7. The UV-VIS spectrum of a diamond taken at room temperature. Figure 8. The UV-VIS spectrum of a diamond taken at low temperature showing more and more well defined peaks.

Sometimes gathering low temperature spectra can be accomplished by using very unsophisticated techniques. Using the Zeiss UV-VIS spectrometer, with the diamond placed in a Styrofoam cup with liquid nitrogen, satisfactory spectra are recorded (Figure 9).

Figure 9. A simple technique using a Styrofoam cup filled with liquid nitrogen makes the collection of UV-VIS spectra of diamonds at low temperature possible.
Photo: Riccardo Befi © AGTA GTC

dingbat
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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