A Pink Sapphire with Questionable Heat Treatment
A pink sapphire weighing over 20 ct was recently submitted to Guild Gem Laboratories, and it presented a particularly challenging case for the determination of heat treatment. According to the client, the stone had previously received four reports from three different laboratories, with inconsistent conclusions. One laboratory initially classified the stone as unheated four years ago but later revised their assessment to heated. Subsequently, two additional laboratories issued contradictory opinions—one concluding heated and the other not heated. These conflicting assessments prompted the client to seek an in-depth re-evaluation, with the aim of resolving the dispute based on further analytical evidence.
The authors carried out a comprehensive gemmological examination following standard protocols, including FTIR spectroscopy and microscopic observation. FTIR reflectance spectroscopy confirmed the specimen as corundum, and transmission spectra revealed absorption bands at 3309 and 3232 cm-¹ (Figure 1), which are often cited as diagnostic of heat treatment in sapphires, particularly those of metamorphic origin (Hughes & Perkins 2019; Saeseaw & Khowpong 2019). However, microscopic examination revealed an inclusion scene inconsistent with high-temperature heating. The observed features included rutile needles, fluid inclusions and colourless platy crystals (Figure 2), all of which showed no signs of heat-induced deformation, melting or decrepitation.
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Figure 2: (a) Intact needles and (b) a colourless platy phlogopite inclusion in the pink sapphire show no evidence of heat-induced deformation, melting or decrepitation. Photomicrographs by Tiantian Huang; image widths (a) 5.32 mm and (b) 2.35 mm.
To further investigate the apparent contradiction, we obtained Raman spectra of the inclusions. The long needles were confirmed as rutile, displaying sharp and characteristic vibrational modes. One platy inclusion was identified as phlogopite, a hydrous mica that begins to undergo structural breakdown at around 365 ± 15°C (Tutti & Lazor 2008). The Raman spectrum of the phlogopite exhibited well-defined, sharp peaks (Figure 3), with no evidence of peak shift or broadening, features typically associated with heating-induced structural disorder.
The presence of OH-related bands at 3309 and 3232 cm-¹ in the FTIR spectrum of sapphire is a useful criterion for identifying heat treatment, but the authors' experience—and supporting literature (Soonthorntantikul & Palke 2025)—indicate that these features may occasionally be observed in untreated metamorphic sapphires due to their complex geological history. As such, these bands should not be interpreted in isolation. In this case, the well-preserved state of the phlogopite (confirmed by Raman spectroscopy) and other inclusions provided strong evidence against heat treatment. Taken together, the combination of microscopic observations and FTIR and Raman spectral features led the authors to conclude that the sapphire had not undergone thermal enhancement. This instance highlights the importance of multi-modal analytical approaches in resolving questionable heat/no-heat determinations, particularly in cases where individual techniques offer conflicting evidence.
Yujie Gao, Tiantian Huang(candice.huang@guildgemlab.com)and Andrew LucasGuild Gem LaboratoriesShenzhen, China
References
Hughes, E.B. & Perkins, R. 2019. Madagascar sapphire: Low-temperature heat treatment experiments. Gems & Gemology, 55(2), 184-197, https://doi.org/10.5741/gems.55.2.184.
Saeseaw, S. & Khowpong, C. 2019. Gem News International: The effect of low-temperature heat treatment on pink sapphire. Gems & Gemology, 55(2), 290-291.
Soonthorntantikul, W. & Palke, A.C. 2025. The 3309 cm–1 series in sapphire and ruby: A focus on FTIR peak position variation. Gems & Gemology, 61(1), 2-15, https://doi.org/10.5741/gems.61.1.2.
Tutti, F. & Lazor, P. 2008. Temperature-induced phase transition in phlogopite revealed by Raman spectroscopy. Journal of Physics and Chemistry of Solids, 69(10), 2535-2539, https://doi.org/10.1016/j.jpcs.2008.05.009.
