Welcome to AGTA GTC's Laboratory Update for May 2, 2005

In This Issue

  1. Idol's Eyes: Demantoid Cat's Eyes
  2. Inclusion Corner
  3. New Laboratory Fee Structure
  4. Let's Make a Deal
  5. Record Day at the AGTA GTC

Idol's Eyes: Chatoyant Demantoid

Demantoid, a variety of the garnet species andradite, is one of the rarest members of the gem garnet family, and in fine quality may command high prices. The most appreciated and well-known demantoid garnets traditionally come from Russia, but some demantoid is also found in Italy, Namibia, and Iran. Colored by a trace amount of the element chromium, it is one of three green gem garnets, the other two being tsavorite, grossular, and the species uvarovite.
    In March 2005, the AGTA Gemological Testing Center received two Iranian demantoid cat's eyes for analysis (figure 1) from Mr. Makhmout Douman of Arzawa Mineralogical Inc. These stones have briefly been described in the Gem News International section of Gems & Gemology (Spring 2004), but without supplying any gemological data on this material. We therefore took the opportunity to do a more thorough study on these two unusual gems.

Gemological Data
The two demantoid cat's eye cabochons weighed 7.03 ct. and 4.98 ct. respectively. The specific gravity was determined hydrostatically and was found to be 3.84 for both stones. Refractive index was over 1.81, the stones were inert to long- and shortwave UV radiation. With the hand spectroscope, the stones showed a strong absorption in blue and yellow, plus chromium absorption in the red. This data is in the known range for andradite.

Figure 1. The pair of unusual demantoid cat's eyes examined for this report. Photo by Min Htut, AGTA GTC

Microscopic Properties
The cat's eye effect was caused by long parallel irregular cracks or tubes as well as ultrafine, parallel to sub-parallel fibrous inclusions that appeared to be some form of mineral (possibly chrysotile). En-masse, these chatoyancy-causing features resulted in the stones appearing partly whitish to yellowish and translucent (Figure 2) in oblique surface-reflected fiber optic illumination or sunlight. Distinct transparent zones contained fewer fractures and inclusions and sometimes appeared as phantom growth layers or concentric structures (Figures 2 & 3). One macroscopic phantom zone was observed as a dark ring on the flat side of the larger cabochon (Figure 2), which was a more transparent area that followed the cuboctahedral shape of the original garnet crystal. Surrounding this dark ring on both sides, a higher occurrence of inclusions was clearly visible, so that these zones looked more white to yellowish and reflective.
     Some minute white particles parallel to growth planes were also observed. When the 7.03 ct. demantoid cabochon was examined in diffused transmitted fiber optic light the phantom zones that appeared dark and more transparent now displayed a more intense concentration of green color (Figure 3) than did the surrounding zones containing higher concentrations of inclusions. This distinct color zoning shows variability in the chemistry and a cyclic availability of the coloring element(s) chromophore(s) during the host garnet's growth. Another feature observed under these lighting conditions was that the more brightly colored areas contained small intersecting cracks with a random arrangement (Figure 4).

Figure 2. Macroscopic phantom zones in the 7.03 ct. demantoid cat's eye consist of relatively inclusion free dark areas bordered by inclusion-rich chatoyancy-causing reflective zones that follow a cuboctahedral shape of the original garnet. Photomicrograph by John I. Koivula, AGTA GTC/microWorld of Gems

Figure 3. In diffused transmitted fiber optic light the darker phantom zones in the 7.03 ct. demantoid displayed a more intense concentration of green color, showing a cyclic availability of chromophore during the host garnet's growth. Photomicrograph by John I. Koivula, AGTA GTC/microWorld of Gems

Figure 4. The more brightly colored areas in the demantoid cat's eyes contained random patterns of small intersecting cracks that were most easily seen using diffused transmitted fiber optic illumination. Magnified 50x. Photomicrograph by John I. Koivula, AGTA GTC/microWorld of Gems

Chatoyancy, Demantoids and Rarity
While recent new discoveries of demantoid in Iran and Namibia and rediscoveries in Russia have made these beautiful gems more widely available in recent years, these are the first two demantoid cat's eyes we have had the opportunity to examine. The future availability of chatoyant demantoid in not known. Considering the popularity of phenomenal gems as a general group there will most certainly be a demand for fine demantoid cat's eyes should suitable rough be found and fashioned. 

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Inclusion Corner

One of the distinct pleasures for any laboratory gemologist is the observation of a beautiful inclusion pattern, particularly one that has never been encountered before. The photomicrograph above clearly shows the trigonal (three-fold) symmetry of this particular sapphire. The blue zones are essentially free of any visible exsolution rutile, while the lighter colored brownish areas contain wedge-shaped zones of minute light scattering mineral dust, presumably exsolution rutile. Such a pattern and distribution of "rutile" aids the gemologist in determining the authenticity of the sapphire and determining that it has not been heat treated. The small primary three-phase carbon dioxide fluid inclusion just above the shortest of the three blue lobes provides welcome additional proof of natural, untreated origin. Photomicrograph by John I. Koivula, AGTA GTC/microWorld of Gems

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New Laboratory Fee Structure
AGTA Gemological Testing Center has considerably expanded its highly qualified team of gemologists and support staff. Three internationally renowned gemologists, Dr. Lore Kiefert, Richard W. Hughes and John I. Koivula joined the AGTA GTC earlier this year. The recent expansion has greatly improved turnaround times. Customers should expect results within 5-7 business days.
    Concurrent with our commitment to quality and service comes the need to continually monitor our pricing structure. As we do annually, we have completed a review of the method in which we charge for our services. We must remain committed to the quality and integrity of both the reports and the process through which we produce them. Accordingly, we have realigned our prices for services and we have established a new fee structure for the 2005-2006 fiscal year. New fee schedules will go into effect on Monday, May 2, 2005 and will be posted on our website at www.agta.org. If you would like a copy of the new fee schedule sent to you please contact us at info@agta-gtc.org, or call 212-752-1717.

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Let's Make a Deal
From Monday, May 2nd through Friday, May 13th, customers submitting six or more stones at the same time will receive the Identification Report on the least-expensive gemstone free of charge.

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Record Day at the AGTA GTC
The AGTA Gemological Testing Center celebrated a record day on April 21, 2005 where we received the largest number of gemstone submissions in our six-year history. Indeed the quantity of stones taken in exceeded the previous record by a whopping 22 percent. We would like to thank all of our customers for helping to make the AGTA GTC the finest colored gemstone lab in North America.

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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
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