Welcome to AGTA GTC's Laboratory Update for May 17, 2005

In this message

  1. Inside Out: The Symmetry of Crystals and Inclusions
  2. Meet our Staff: Garry Du Toit
  3. The AGTA GTC in Las Vegas

Inside out:
The symmetry of crystals and inclusions

One of the joys of gemology is the examination of fingerprint inclusions. Particularly when viewed in reflected light, they can shine the light fantastic.
     Just how did these fascinating inclusions form? At any point after a crystal grows, it may fracture. Given the proper conditions, that fracture may later heal closed, leaving a scar-like inclusion typically known as a “fingerprint.” Rubies and sapphires often contain gorgeous examples.
     The healing process involves exposure to a combination of heat and solvents. In the ground, elevated temperatures and solvents produce healing of fractures via corundum-containing solutions. Dissolved nutrients (solute) may come from solvents dissolving surrounding crystals, the exterior of the crystal itself, or the interior walls of the fracture. This dissolved nutrient material then regrows on the walls of the crack, “healing” it closed. But an internal scar remains, something we term a “fingerprint” inclusion (see Figure 1).

Figure 1.  Anatomy of a healed fracture
A well-healed fracture in a sapphire lying roughly parallel to the basal plane. The healed areas appear dark, while the undigested fluids are highly reflective. Note that the pattern of healing relates to the underlying crystal structure, with angles of healed areas following the underlying crystallographic structure (in this case, at 60/120°). Photo © Richard W. Hughes/RWH Publishing & Books

    The brightly-colored areas are actually trapped fluids. The cavities they reside in are tiny negative crystals, with miniature crystal faces that face inward, rather than out. Since they are crystal faces, they will have a symmetrical relationship to one another.

Figure 2.  Formation of a fingerprint
The healing of a crack in a crystal, resulting in secondary cavities (‘fingerprint’).
A. A fracture develops during or after the crystal’s growth.
B. Healing begins. Growth solutions flow into the fracture and/or the inner walls of the crack are partially dissolved, beginning the healing process.
C. Healing continues. Dissolved nutrients are re-deposited on the inner walls of the crack as the healing proceeds.
D. Eventually the fluid-filled cavities become more angular in shape, turning into fluid-filled negative crystals arranged in a fingerprint pattern. The fluid that remains behind has been leeched of its nutrients. These pockets containing exhausted growth solutions are smaller along the inner edges and bigger near the outer edges of the original crack. (After Roedder, 1962) Illustration © Richard W. Hughes/RWH Publishing & Books

Figure 3.  External symmetry…
In a perfectly formed corundum crystal, such as those shown above, one can clearly see the symmetry of prism faces is two-fold, while that of the basal pinacoid faces is three or six-fold. This will be reflected in the appearance of certain inclusions within the gemstone, such as fingerprints. Illustration © Richard W. Hughes/RWH Publishing & Books

Figure 4.  …equals internal symmetry
Secondary fluid inclusions (healed fractures, or ‘fingerprints’) often display the symmetry of the underlying crystal structure in the healed areas. Above is shown a healed fracture in a Thai ruby which formed parallel to the basal pinacoid. As the c axis (3-fold symmetry) runs perpendicular to this face, the healed (dark) areas display distorted hexagonal or triangular (60/120°) outlines. Vertical lines cutting through the fingerprint are repeated twinning striations. Photo © Richard W. Hughes/RWH Publishing & Books

Figure 5.  Rectangular symmetry
A fingerprint in a Sri Lankan sapphire. Here the fingerprint has formed along a prism face (parallel to the c axis), and so the healed (dark) areas show rectangular (90°) outlines, indicating the two-fold symmetry at right angles to the c axis.
Photo © Richard W. Hughes/RWH Publishing & Books

Figure 6.  Outer triangulation…
A thin polished slice of an Australian sapphire, looking parallel to the c-axis (parallel to the prism faces. The three-fold symmetry is clearly visible. Photo © Richard W. Hughes/RWH Publishing & Books

Figure 7. …equals inner triangulation
Decrepitation halos surround minute primary negative crystals in the pinacoidal plane of a basaltic Thai/Cambodian ruby, creating a sea of highly characteristic thin-film fluid inclusions. One can clearly see the three-fold symmetry of these distinctive features. Photo © Richard W. Hughes/RWH Publishing & Books

    From the above, one can clearly see that fingerprints are far from random, but have patterns that relate intimately to the underlying atomic structure. Far from being flaws, they are just what the word suggests – fingerprints – features that are unique to that gemstone alone. No two are ever alike.

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Meet our Staff:
Garry Du Toit – Laboratory Manager

Garry Du Toit came to gemology in the 1980s, following extensive travels around the world (including an epic trek to the famous Kashmir sapphire mines). He began his studies at Bangkok’s Asian Institute of Gemological Sciences and upon graduation began working at AIGS to develop a line of gem testing instruments. He later went on to work as Research Gemologist at the AIGS lab, a position he held from 1991–1998.
     Garry joined the AGTA GTC upon its inception in May 1998. His responsibilities include:

  • Identification of all gem materials submitted for examination and the production of reports that describe the findings of the investigation.
  • Advanced gem-testing techniques, such as energy dispersive X-ray fluorescence, x-radiography, Fourier-transform infrared spectroscopy (FTIR), UV-visible-near infrared spectroscopy and Raman spectroscopy.

     His specialty is Raman spectroscopy, for which he is considered one of the world's experts in its application to gemology. Garry has also co-authored a number of papers in publications around the world.

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The AGTA GTC in Las Vegas
Once again, the AGTA GTC will be participating in The JCK Show - Las Vegas 2005, offering a range of gemological services, such as:

  • Identification reports for all kinds of gems, including the identification of clarity enhancement fillers
  • Country-of-Origin reports for ruby, sapphire and emerald.

    The AGTA Pavilion has special dates and times! The AGTA Pavilion opens and closes one day before the main JCK show. The AGTA GTC Mobile Laboratory, located in the AGTA GemFair Cultured Pearl & Jewelry Pavilion, is open Thursday, June 2nd-Monday, June 6th. Hours are 10:00am to 6:00pm on June 2nd and 9:00am to 6:00pm from June 3rd-6th. The AGTA GTC Mobile Laboratory is planning to deliver the reports to clients within 1-2 days and the services are going to be available during the show time.
     But even better than submitting gemstones during the show, is to submit them to the AGTA GTC prior to the show. By doing so, you'll have the gemstones in your showcase, ready for sale with reports. Gemstones submitted by Wednesday, May 18th, will be returned by May 27th just before the show. Hurry up and have your gemstones tested today!

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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. The laboratory, which is located in New York City, 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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