AGTA GTC Laboratory Update
Further Characterization of Sapphires Recently Treated in Bangkok
April 19, 2002 – Following our initial examination and reports on certain sapphires, that have been treated using a new technique in Thailand, we continue to examine these stones. Our focus recently has been, (1) to gain a better understanding of the causes of their color, and (2) to investigate any other changes that may have taken place to the structure of the stones.
During this period we have been working closely with GIA Gem Trade Laboratory and Dr. John Emmett (Science adviser to the AGTA-GTC Board of Governors), as well as many other individuals and Laboratories around the world. These include SSEF (Swiss Foundation for Research in Gemmology), Gübelin GemLab, GIT (Gem and Jewellery Institute of Thailand), and GAAJ (Gemmological Association of All Japan).
Visits to Bangkok during February and April have resulted in more information and a host of samples on which to carry out our investigations. Indeed these investigations now encompass most corundum varieties, including the Mong Hsu ruby, blue, green, yellow, orange and orange/red treated sapphires, as well as the original pink/orange treated stones.
GIA continue to carry out series upon series of chemical analyses using the latest technologically advanced equipment to establish which elements might be contributing towards the induced coloration. A short report on the status of the chemical analyses is located here.
AGTA GTC have been examining each gemstone for any structural changes that may have taken place at the elevated temperatures used in Thailand to create this new product. In addition, AGTA-GTC has also examined the chemistry of numerous treated stones using standard gemological laboratory equipment – Energy Dispersive X-Ray Fluorescence (EDXRF).
The following is a brief outline of some of the information gained since our last report.
It should be noted that research and consultation is continuing and anyone with further observations should make contact with AGTA-GTC at info@agta-gtc.org.
Chemistry
In addition to the usual trace elements detected by EDXRF in faceted corundum, the chemistry of the treated pink/orange stones thus far examined, consistently reveal the presence of both Ca and Zr. While the Zr may be related to surface breaking inclusions, it is interesting to note that we identified a tiny grain of synthetic cubic zirconia on the surface of one gemstone. The Ca, on the other hand, has no clear explanation other than it being a possible additive to a flux used during the treatment process.
Structure
Examining the surfaces of several pink/orange stones prior to being repolished but after treatment, an unusual surface structure is observed. In parts, the surface appears as if it has dissolved (see e.g. figures 2, 3, and 4), as evidenced by the smoothly undulating surface. While in other parts partial crystal growth seems to have occurred (see e.g., figures 1 and 2). These observations are consistent with the sapphire being partially dissolved by the flux during the heating process and then re-deposited from the cooling flux (re-crystallized or synthesized) on the surface of these treated sapphires.
Examinations carried out on other colored sapphires said to be treated in the same manner as the orange/pink stones (but this time fully repolished and ready for the market) revealed similar features.
The surfaces of three treated yellow stones were found to be covered in what might simply be described as a "spider web" pattern of randomly oriented and irregularly shaped ?blocks?. If the surface is examined in reflected light alone a slight ghosting of these blocks is seen behind the surface polishing lines (see e.g., figures 5 and 7). With the gemstone held in exactly the same position and point of focus, if transmitted or dark-field illumination is used the individual blocks at and below the surface become obvious (see e.g., figures 6 and 8).
In these treated yellow sapphires, the corundum appears to have been partially dissolved by the flux during the heating process. Then, it has been re-deposited from the cooling flux on the surface in such a quantity that only repolishing to a considerable depth would remove this synthetic corundum layer.[1]
Similar re-deposition structures have been observed in the surface to near surface of treated orange/red sapphires and ruby from Songea. These include stones obtained for research purposes and those in the market.
Numerous briolettes of blue, orange/pink, and yellow treated sapphires have also been examined for indications of dissolution and re-deposition of corundum. All three colors readily revealed these characteristics. Figures 9, 10 and 11, detail some of the observations.
During the examination of a treated orange/red Songea rough sapphire, we noted a yellow material over part of the surface. An examination of the yellow area revealed a host of tabular corundum crystals that were consistent with the appearance of synthetic corundum, see figures 13 and 14. Therefore, within our sampling of this product we have observed what appear to be two clear stages in corundum surface re-deposition (from irregular blocks to distinct crystals) as well as internal (parting plane) ?healing?.
Summary
In addition to the issues surrounding the cause or causes of color (e.g., bulk or surface diffusion of beryllium leading to trapped hole centers) in these new treated sapphires. There is also the equally important issue that surrounds the re-deposition of the corundum that is dissolved by the flux during the new process carried out in Thailand. We are presently working to characterize this situation more fully. These observations are also causing us to re-examine the results of previous heat treating practices.
Click on pictures below to enlarge.
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| Figure 1: Re-deposited corundum on the surface of a treated orange/pink sapphire. | Figure 2: Re-deposited corundum on the surface of a treated orange/pink sapphire |
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| Figure 3: The dissolved surface of a treated orange/pink sapphire. | Figure 4: The dissolved surface of a treated orange/pink sapphire |
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| Figure 5: Faceted treated yellow sapphire showing indications of re-deposition, recrystallization, or synthesis of corundum (same position as figure 6 but in reflected light). | Figure 6: Faceted treated yellow sapphire showing indications of re-deposition, recrystallization, or synthesis or corundum (same position as figure 5 but in transmitted light). |
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| Figure 7: Faceted treated yellow sapphire showing indications of re-deposition, recrystallization, or synthesis or corundum (same position as Figure 8 but in reflected light). | Figure 8: Faceted treated yellow sapphire showing indications of re-deposition, recrystallization, or synthesis or corundum (same position as Figure 7 but in transmitted light) |
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| Figure 9: Parting planes showing indications of dissolving and re-deposition of corundum. Seen here on the surface of a treated orange/pink briolette cut sapphire. | Figure 10: Large globule of redeposited corundum showing evidence of being in close contact with other stones during the heating process. Seen here on the surface of a treated orange/pink briolette cut sapphire. |
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| Figure 11: Within a parting plane in a treated orange/pink briolette, in the area that probably once housed boehmite, are found these dog bone-like voids or possibly glass, encased in recrystallized or “healed” corundum. | Figure 12: Within a parting plane in a treated orange/pink faceted gemstone, in the area that probably once housed boehmite, are found these dog bone-like trapped voids or possibly glass, encased in recrystallized or “healed” corundum. |
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| Figure 13: Yellow synthetic corundum crystals on the surface of a treated orange/red sapphire from Songea. Scale is in microns. | Figure 14: Yellow synthetic corundum crystals on the surface of a treated orange/red sapphire from Songea. |
[1] Any material that has been dissolved and then reformed in the manner described here, in accordance with internationally accepted nomenclature standards is a "synthetic material." See CIBJO rules. "Synthetic stones are artificial products having essentially the same chemical composition, physical properties and structure as that of their naturally occurring counterparts."
