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Identification and characteristics of PHT (‘HPHT’) - treated sapphires - An update of the GRS research progress

IntroductionA series of peculiar events that commenced in 2015 brought a degree of uncertainty to the Sri Lankan gemstone industry. Sri Lanka is known as the world’s foremost producer of ‘heated’ corundum. So far only corundum of higher clarity has been used to produce clean sapphires. It was noticed that a few buyers began purchasing previously ‘untreat-able’ rough at uncommonly high prices. A rumor spread that a new treatment was developed and fears emerged that a new type of diffu-sion treatment could be responsible. Shortly afterwards some sapphire vendors were selling ‘heated’ sapphires below the already decreasing market prices of heated sapphires, and rumors were confirmed that a new type of sapphire treatment had been discovered. While specula-tions that HPHT-treated sapphires being responsible for the price de-crease of heated sapphires are unsubstantiated, it can not be ignored (see Fig. 29.) due to the coincidental occurrence of events. The ap-pearance of this new treatment came at a critical time of political and economical changes in Asia. If it did not negatively affect the market, it surely did not help.

In 2009, a Korean company called HB Laboratory Co. Ltd first suc-cessfully treated corundum with a new HPHT technique. The new heat treatment executed at elevated pressures is similar to how diamonds are being HPHT-treated, although the applied pressure is significantly lower. The applied pressure during the HPHT-treatment for diamonds is larger than 50 Kbar (Dobrinets, Vins and Zaitsev., 2013) whereas the applied pressure during the HPHT sapphire treatment is approximately 1 Kbar (Ref. 1, 2). At present GRS categorizes the new material with the treatment code ‘PHT’ (pressure, high temperature), however for the purpose of this publication and aligned with previous publications on the subject we continue to refer to this treatment as HPHT.

The Korean HB Laboratory Co. Ltd. modified the traditional HPHT diamond apparatus into a mold press machine suitable for lower tem-perature and pressures. They currently treat corundum with atmospher-ic pressures just below 1 Kbar and temperatures between 1500-1800°C for less than 30 minute soak times. The experimental work from Korea shows (Choi et al., 2018) that the treatment is applied mainly to two

Adolf Peretti 1,2,3,4 *, Maya Musa 1, Willy Bieri 1, Edward Cleveland 2, Ishtiyaaq Ahamed 3, Matthias Alessandri 4, Lawrence Hahn 4

1 GRS Gemresearch Swisslab AG, Baumschulweg 13, 6045 Meggen, Switzerland / * corresponding author, e-mail: adolf@peretti.ch2 GRS (Thailand) Co., Ltd., Unit 501-506, Silom 19 Building, Soi 19, Silom, Bangrak, Bangkok 10500, Thailand3 GRS Lanka (PVT) Ltd, 471 3/1 Galle Road, Colombo 3, Sri Lanka4 GRS Lab (Hong Kong) Limited, 19/F, Man Hing Commercial Building, 79-83 Queen’s Road Central, Hong Kong, China

categories of sapphires; unheated rough and already conventionally heat treated sapphires (multi-step treatment). The treated product has been commercially available since 2013. An influx at GRS lab was seen in 2015.

The advantages of this new technique are significant when compared to traditional heat-treating methods. The traditional heat-treatment meth-ods require high temperature soaking times between 1500-1800°C for many hours or even several days. The new elevated pressure method reduces the soaking time to less than 30 minutes similar temperatures. Another key competitive advantage is the healing effect on fractures and surface reaching feathers. The process of crack-healing and im-provement of clarity has also been reported by Choi et al., 2018.The Korean Hanmi Gemological Institute Laboratory (GIG) first re-ported in November 2011 on blue sapphires having unusual infrared spectroscopic characteristics. The HPHT-processed gemstones show a strong absorption band centered around the 3047cm-1 in the infrared

Fig. 1. A series of faceted HPHT-treated sapphires in a color range from pastel, vivid (royal blue) to deep blue in different shape and sizes (4-14ct).

Fig. 2. View into the New Diamond Technology production factory in St. Petersburg (Russia). The facility comprises of over 30 cubic presses (photo courtesy of NDT). Full article see Ref. 6. This image is for informative purposes only to show the large scale application of new HPHT technology. It is not implied that this particular machines are used for the treatment described in this article.

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region (see Fig. 20e) and literature (Ref. 1, 2). It was widely believed to be the key identification signature of this new treatment.

Since March 2015, GRS has issued over a hundred gemstone reports for HPHT-treated sapphires from almost as many different clients. This shows that the material has penetrated the market. We recently updated our nomenclature on the new process and would like to docu-ment some of the important features and observations recently made.

Materials & MethodsA total of 159 HPHT-treated gemstones were submitted to GRS from 74 different clients between 2015-2018. In addition, 128 faceted and pre-shaped samples from different stages during the treatment process were available from the manufacturers. The faceted samples include six large faceted gemstones of vivid blue color between 5-12cts. These six gemstones have GRS gemstone reports from June 2015 and were resubmitted and retested in October 2018.

Following the GRS internal standard procedures, all the gemstones are analyzed by advanced techniques, including FTIR, UV-VIS-NIR, LIBS, Raman spectroscopy and XRay fluorescence spectrometry (ED-XRF).

To improve the scientific data regarding the new treatment, an innova-tive method consisting of a micro-Raman mapping has been applied on a study-case sample. The instrument used is the Renishaw InVia con-focal micro-spectrometer, equipped with a Leica microscope (5x, 20x, 50x and 100x objects), a solid crystal 100mW laser source working at 514.5 nm and a charge-coupled device air-cooled detector (CCD). The instrument calibration was carried out before each analysis section by checking the position and intensity Si band at 520.6 ± 0.1 cm-1 of the standard internal silicium sample.

The mapping process was carried out on a HPHT-treated Sri Lankan sapphire cabochon, not re-polished after treatment but broken in half. This stone is considered the case-study sample.

The map grid area was 30x30 µm for a total of 142 spot Raman analy-ses in static motor mode. In order to balance the signal against noise, at least 5 cycles of 50 s were performed for each analysis point, focusing on the source using a 100x magnification object. After the map result, longer micro-Raman spot analyses (50 cycles of 10 s) were performed to improve the sample characterization of several inclusions.

Additionally micro-FTIR analyses were carried out on the same case-study sample. The instrument used is the Spotlight 200i micro-ATR coupled with Frontier FTIR spectrometer by Perkin Elmer. The analy-ses were performed in transmission mode, in order to trace an absorp-tion profile of the sample in the OH spectroscopy range. To balance the signal against noise, 256 cycles were performed for each analysis point at 4cm-1 resolution.

The UV-VIS-NIR spectra were collected using a quadruple-channel Czerny-Turner spectrometer and two broadband light sources, record-ing the spectra in the wavelength range from 240-1100 nm.

Finally, fluorescence images were acquired on the study-case sample using the DiamondView IIDGR a DeBeers Group Company instru-ment, equipped with a 225 nm light source.

The instruments used for this study are installed at the GRS headquar-ter in Meggen (Switzerland), Bangkok (Thailand) and Hong Kong (China).

Comparison of Results after HPHTHPHT-treated stones show obvious deposition on the surface and a type of synthetic overgrowth. Some facet edges are brittle and cavities and cracks are graphitized.

Cracks or fissures are partially healed with pinpoint feathers. Some

shiny internal feathers are bending in 3 dimensions. A brown ‘burned’ type coloration appears in the cracks arising from fluid feathers (see Fig. 6, 7, 11, 12, 13).

The data from samples submitted in 2015 was compared to the re-col-lected data in 2018 of the same samples. It appears that the color is stable, no color fading and no reduction of absorption bands were ob-served.

LIBSTwo samples that were cut in half and LIBS measurements were tak-en. No traces of Li or Be were found and there were no indications of Ti-concentration enrichments towards the edges of the stones. How-ever, a high concentration of Li was found on the surface of the sap-phire. This opens the question of low concentration Li diffusion into the sapphires which we are currently investigating using LA-ICP-MS on a larger sample base.

Infrared SpectroscopyThe stones tested show various different types of FTIR absorption, the classical strong absorption band centered around 3047cm-1 (Fig. 20e) as well as FTIR spectra more reminiscent to those of conventionally heat-treated sapphires (see Fig. 20a-d). Similar spectra have been published previously (Ref. 1, 2) but not those shown in Fig. 20a-c. When we measured the FTIR spectra at different positions of the sapphire (Fig. 20d), we discovered significantly different FTIR spectra from the core to the rim. This finding suggest that some features are only formed in near-surface regions and only a broad-band was found in a specific sector of the HPHT-treated sapphires. This observation is yet to be statistically verified with more samples but certainly indicates challenges for the identification of the new treatment using FTIR spectroscopy exclusively.

Raman SpectroscopyA variability of different minerals as well as a amorphous non Ra-man-active coating could be identified on the surface of the HPHT-treat-ed samples (unpolished). See Fig. 23, 25, 26 with Raman results.

UV-VIS-NIR SpectroscopyIt has already been reported (Choi et al., 2018) that the UV-VIS-NIR absorption characteristics of the HPHT-treated sapphires are compa-rable to those of conventionally heat-treated sapphires (Fig. 21a, 21c, 22b). However, we also found different types of absorption spectra with broad absorption bands around 400nm that are not explained by Fe3+ (around 388nm and 450nm) absorption bands (see Fig. 21b and 21d). Variability of the absorption spectra covers the whole range of conventionally heated origins with different chemical compositions of carbonatic and pegmatitic geological environments (non-basal-tic). UV-VIS-NIR spectra are not indicative for the identification of this particular treatment but rather for the detection of conventional heat-treatment only (with some exceptions, see Fig. 21a-d, Fig. 22a-c.)

ConclusionHPHT-treated sapphires contain a unique set of inclusion features that help identify them against other types of conventional sapphire heat-treatments and their unheated counterparts from different sources (countries of origin). Pinpoint feathers indicate healing and staining of internal cracks and synthetic inclusions (graphitized material) are formed (Fig. 6 and 7). Brownish cracks are formed around pre-existing fluid inclusion feathers (Fig. 12 and 13). FTIR spectroscopy supports the identification by means of microscopic studies but its application is problematic due to variation within the sapphire (rim to core). A ‘black box’ identification method using solely FTIR spectroscopy is not recommended (see different types of FTIR spectra Fig.20d).

The treatment produces a particular brittleness that is surprising when compared to the normally very durable corundum (see Fig. 10). The pinpoint feathers act as a kind of tarnish of otherwise highly reflect-

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Fig. 3. These faceted sapphires were subjected to HPHT-treatment and displayed dull and non-reflective surfaces (‘tarnish’) after treatment. For microscopic details of the surfaces see Fig 3.

Fig. 4. Surface of HPHT-treated sapphire as seen in the microscope. Some geometric recrystallization or synthetic overgrowth was seen.

Fig. 6. Tarnish of a crack with pinpoints. This fissure may have been reflective prior to the treatment.

Fig. 5. A facet of the sapphire exposed to HPHT-treatment developed a carpet of pinpoint crystallites (identified by Raman spectroscopy as corundum, graphite, carbonate and rutile). The rutile crystallites were geometrically oriented on the surface of the stone (see picture).

Fig. 7. Cavities and cracks are filled with synthetic graphitized material or residual contamination from the HPHT-treatment.

Fig. 8. Example of a HPHT-treated sapphire (sample from Nov 2016) with vivid blue (royal blue) color saturation (a) face-up and (b) immersed in water (see figure on the right).

Fig. 9. HPHT-treated sapphire showing internal blue color zoning without color concentrations along the edges and the girdle as it would be expected from conventionally Ti-diffusion treated sapphires.

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Fig. 11. A specific type of badly healed fissure within a HPHT-treated sapphire as seen under the microscope. Notice the formation of pinpoints and partial healing that helps tarnish the reflectivity of an otherwise highly reflective crack (GRS designation ‘tarnished crack’).

Fig. 12. Fluid inclusion feathers with brownish appearing cracks. Fig. 13. Fluid inclusion tubes in magnification and formation of brownish cracks.

Microscopic observations

ing cracks. This type of crack-healing is a special aspect of this pro-cess and is generally not observed in this way during conventional heat-treatment of sapphires. The conclusion should be made that the HPHT-treatment must be viewed as both, a color and clarity enhance-ment instead of a color enhancement only.

Regarding the disclosure of this treatment in comparison to conven-tional heat-treatment, a few important points must be remembered. The classification aspects of clarity enhancement (‘crack tarnish’) and the issue of material durability. The brittleness of the HPHT-treated sapphires is of particular concern and it must be closely observed if an unusual increase in cases of damages are reported when wearing HPHT-treated sapphire jewelry (see Fig. 10). In regards to the stability of the color, no color fading has yet been observed in HPHT-treated sapphires judging on a 3 year period (tested in 2015 and rechecked in 2018). GRS thus considers it as adequate to apply the GRS-type color grading to this type of sapphires.

The treatment is similar to a conventional heat-treatment and does not have the hallmarks of a diffusion treatment method sofar. The role of Li found on the surface has to be further investigated by means of LA-ICP-MS in profile sections of HPHT-treated sapphires.

The discovery of lithium (Li) by LIBS testing as well as carbonate and rutile (TiO) by Micro-Raman mapping on the surface of the HPHT sapphires suggests that a type of lithium carbonate additive is used in the treatment-process. Lithium carbonate (Li2CO3) flux additives are commonly made into slurry solutions and painted onto the surface of the gemstones as salt flux reagents (Themelis, T. 2018). The pres-ence of surface titanium could be exsolved from the crystal lattice and or from an additional chromophoric additive (TiO2) in slurry solution (Nassau, K. 1984). The presence of Ti-enrichments on the surface (rutile) normally points to Ti-diffusion treatment however, in this situ-ation the explanation needs further investigation.

Based on the research data currently available, GRS has decided not to place this kind of treatment in the same category as conventionally heated sapphires (see Fig. 28). Data has shown that this classification approach may not be shared by other laboratories and peers around the world. Future harmonization is required to internationally posi-tion, place and fully disclose this product in a uniform and transparent matter. GRS advocates its philosophy of full disclosure. At present, GRS categorizes the new material with the treatment code ‘PHT’ (as opposed to ‘H’ which is reserved for conventionally heated sapphires only). We are confident that GRS’ approach with regards to this matter will establish itself as the industry standard*.

AcknowledgmentsSpecial gratitude to the treatment laboratory for supplying us with a large amount of first-hand HPHT-treated samples.

Brittleness of HPHT-treated sapphiresFig. 10. Sketch illustrating the formation of impact cracks within a HPHT-treated sapphire (left) after hammering compared to the expected outcome (right). With the hammering of a sapphire we wanted to obtain samples of the core, mantle and rim. Diffused cracking patterns made the core, mantle to rim separation very challenging after impact.

When we tried to scratch a facet edge of a HPHT-treated sapphire with a paper clip, the facet edges crumbled. A clear indication of reduced durability. When re-polishing the surfaces of HPHT-treated sapphires, the cutter reported that the stones got unusually hot. Further, when sawing the sapphires to produce wafers, we noticed certain samples were breaking off the last quarter of the sawing path.

It is possible that only multi-step HPHT-treated sapphires (conventional heating plus additional HPHT-treatment) show this type of brittleness. However, it is safer to assume a higher degree of brittleness for all stones, once this new treatment method is detected.

* [Update 14 Nov. 2018] After the release of this report in early November 2018, GRS was contacted by another internationally recognized laboratory, announcing the adaptation of a differentiated disclosure policy in regards to this treatment.

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Fig. 14. Synthetic graphitized material formed within a surface-reaching crack. Fig. 15. Pinpoint type fluid inclusion feather with extension cracks marked by thin whitish rims.

Fig. 16. Three-dimensional whitish clouds with triangular geometrical structure (GRS internal designation ‘cloud galaxy’). An inclusion feature that may turn out to be solely found in HPHT-treated sapphires.

Fig. 17. A solid inclusion in a HPHT-treated sapphire, presumably a zircon dis-playing tension cracks extending in three dimensions (GRS internal designation ‘super-nova’) in comparison to unheated or conventionally heated sapphires where the zircon tension cracks seem to be less pronounced.

Fluorescence observations using DiamondView

Fig. 18. DiamondView fluorescence image of the study-case sample which highlights the differences between the sapphire and the amorphous layer due to treatment. Focusing the diamond view spot on the partially polished portion of the sample, one can see the well visible straight parallel and angulated structures of the corundum crystal lattice as well as the reddish core.

Fig. 19. DiamondView image of the study-case sample acquired on the surface. In the amorphous layer are no structures visible, the fluorescence is chalky and color becomes more yellowish where the layer is of greater thickness.

Microscopic observations

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Fig. 20d. A cross section profile of a HPHT-treated study-case sample, obtained applying micro-FTIR technique showing differences of 3309cm-1 OH-line series, normally associated to conventionally heated sapphires and badly structured broad bands. All spectra have been obtained from the same sample showing a big variation of FTIR spectra within a single faceted gemstone.

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Fig. 21a-d Non-polarized UV-VIS-NIR spectra of HPHT-treated sapphires shown in absorption mode. Fig. 21a. Spectrum resembling those of conventionally heated sapphires with UV absorption edge at approx. 300nm and a maximum transmission at around 350nm. Note the strong absorption band around 600nm that is commonly referred to the Fe2+- Ti4+ intervalence charge transfer (ICT), see also Lit 2. Fig. 21b. Spectrum of a sapphire with complete absence of a 450nm and 388nm absorption band, but replaced by a broad absorption band at 400nm. Fig. 21c. Spectrum with pronounced 388nm and 450nm absorption lines. Fig. 21d. The same type of spectrum with shift of the absorption in the UV-range towards higher absorption levels in comparison to Fig. 21c.

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Fig. 22a. UV-VIS-NIR polarized absorption spectra of a HPHT- treated sapphire resembling unheated sapphires known from Kashmir or Madagascar, beside the absorption edge is different than their untreated counterpart in this case at 300nm. Absorption characteristics are corresponding to the known intervalence charge transfer containing Fe2+ and Ti4+ and absorption lines related to Fe3+ (388nm).

Fig. 22b. UV-VIS-NIR polarized absorption spectra of a HPHT- treated sapphire reminiscent of conventionally heated sapphires. The absorption characteristics are corresponding to the know intervalence charge transfer containing Fe2+ and Ti4+ and absorption lines related to Fe3+ (388nm). A minimum of absorption in the UV area around 330nm.

Fig. 22c. UV-VIS-NIR polarized absorption spectra of a HPHT treated sapphire reminiscent of heated or unheated Burmese or Sri Lankan sapphires (besides the minimum of absorption). Absorption characteristics are corresponding to the known intervalence charge transfer containing Fe2+ and Ti4+ and absorption lines related to Fe3+ (388nm). A minimum of absorption is in the UV area around 330nm. The prominent fluorescence line at 694nm is due to chromium concentrations.

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Fig. 23. (a) Microscopic image of the study sample surface analyzed by Raman in reflected light, 50x mag. (b) Raman spectrum obtained from the surface. The absence of bands could be correlated to an amorphous layer probably due to treatment residues.

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Fig. 25. (a) Microscopic image of the sample, transmitted light, 100x mag. The white square of dots indicates the area where the Raman mapping process was applied to identify the different components, in particular:

(b) “Cor” = corundum, (c) “Rut” = rutile that is visible in the form of very small needle inclusions, (d) “Cal” = calcite, (e) “Gr” = carbon graphitization. From (b) to (e) see the corresponding Raman spectra.

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Fig. 24. GRS’s Senior Research Gemologist Dr. Maya Musa performing micro-Raman analysis at the GRS headquarter in Meggen, Switzerland.

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Fig. 26. (a) Microscopic image (in transmitted light with 100x mag.) of pinpoint inclusions and (b) corresponding Raman spectrum. According to the literature, the bands at 379, 418, 431, 449, 578 and 749 cm-1 can be assigned to corundum vibrational modes hosting the inclusions while 1283 and 1386 cm-1 correspond to CO2 vibrational modes. On the basis of the Raman analyses and according to literature these represent small crystal inclusions melted during the treatment.

Fig. 27. Microscopic image (in transmitted light 100x mag.) of small rutile needles characterizing a different aspect as found in the sample (a) well formed and oriented and (b) non-oriented.

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References1 Song, J. Noh, Y., and Song, O., 2015. Color Enhancement of Natural Sapphires by High Pressure High-Temperature Process, Journal of the Korean Ceramic Society, Vol.52, No. 2, pp.165-170

2 Choi H., Kim S., Kim Y., Leelawatanasuk T., Lhuaumporn T., Atsawatanapirom N., Ounorn P., 2018. Sri Lankan Sapphire Enhanced by Heat with Pressure. The Journal of The Gemmological Association of Hong Kong, 2018 Volume XXXIX, 16-25.

3 Themelis, T., 2018. The Heat Treatment of Ruby and Sapphire, 3rd Edition, Volume 1, pp.185-186

4 Nassau, K., 1984. Gemstone Enhancements, History Science and State of the Art, 2nd Edition, 39-41

5 Dobrinets, I., Vins, V., Zaitsev, A., 2013. HPHT-Treated Diamonds. Diamonds Forever, V. 181, Springer Series in Materials Science.

6 Deljanin, B., Alessandri, M., Peretti, A., Åström M., 2015. NDT breaking the 10 carat barrier: World record faceted synthetic diamonds investigated. www.gemresearch.ch, [Online]. 1, 10. Available at: http://gemresearch.ch/ndt-breaking-the-10-carat-barrier-world-record-faceted-and-synthetic-diamonds-investigated/ [Accessed 10 November 2018].

Fig. 28. Current sample report of GRS disclosing HPHT-treated sapphires. GRS does not put this treatment into the ‘H’ code category that is reserved exclusively for conventionally heated sapphires without application of pressure. Special care has to be given to the brittleness of HPHT-treated sapphires, hence the separation in disclosure policy. GRS Terms & Conditions (on report reverse) state the following comment: ‘HPHT - enhanced by heat at elevated pressure’.

Fig. 29. Relative price ranges of ‘heated’, royal blue sapphires for different years and size categories (2, 5 and 10 ct). The graph shows the possible deviations in price for each size due to quality variations. The data for this relative price graph is based on several direct communications with vendors specializing in buying and selling blue sapphires.

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