AGTA GTC's Laboratory Update for May 17, 2005
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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 |
Figure 2. Formation of a fingerprint The healing of a crack in a crystal, resulting in secondary cavities (‘fingerprint’). |
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. |
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 |
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!
