The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments

Klomranok, Chunenapa; Sakkaravej, Somruedee; Wongkokua, Wiwat; Saiyasombat, Chatree; Monarumit, Natthapong

doi:10.3390/cryst16070475

Open AccessArticle

The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments

by

Chunenapa Klomranok

¹,

Somruedee Sakkaravej

¹,

Wiwat Wongkokua

²

,

Chatree Saiyasombat

³

and

Natthapong Monarumit

^1,*

¹

Department of Earth Sciences, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand

²

Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand

³

Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima 30000, Thailand

^*

Author to whom correspondence should be addressed.

Crystals 2026, 16(7), 475; https://doi.org/10.3390/cryst16070475

Submission received: 15 June 2026 / Revised: 16 July 2026 / Accepted: 19 July 2026 / Published: 22 July 2026

(This article belongs to the Section Mineralogical Crystallography and Biomineralization)

Download

Browse Figures

Versions Notes

Abstract

The blue color of sapphire is associated with Fe and Ti impurities that replace Al³⁺ in the corundum structure. Typically, sapphire color depends on its geological origin, such as basaltic or metamorphic localities, and the blue hue can change after heating in an oxidizing environment. Nonetheless, previous studies on the blue color mechanism have left some questions unanswered. Therefore, this research examines how the oxidation states of Fe and Ti influence sapphire color and explores the mechanism of blue coloration before and after heating in oxidizing and reducing atmospheres. This study involved collecting sapphire samples from various gem localities, including basalt-related sapphires from Kanchanaburi, Thailand, and metamorphic-related sapphires from Sri Lanka. The samples were heated in an oxidizing environment at 1100 °C, then at either 1300 °C or 1500 °C in both oxidizing and reducing environments. As a result, after heating under an oxidizing environment at either 1300 °C or 1500 °C, the basalt-related sapphires turned from light blue to pale blue, and the metamorphic-related ones turned colorless. The Fe³⁺-Ti⁴⁺ mixed acceptor states decreased because an electron from the valence band recombined with a hole in the color center during heating. On the other hand, the blue color observed in sapphire samples after heating in a reducing environment at 1300 °C could be explained by electrons being depleted from the hole color center associated with Fe³⁺-Ti⁴⁺ mixed acceptor states within the energy band gap, thereby making them ready to receive electrons from the valence band upon optical excitation. Therefore, it can be concluded that the blue color mechanism in sapphires before and after heating under different atmospheric environments can be explained by an energy-band model involving the presence or absence of Fe³⁺-Ti⁴⁺ mixed acceptor states, as well as a hole color center within the energy band gap. Furthermore, after heating at 1300 °C and 1500 °C in a reducing environment, the oxidation state of Fe gradually decreases from 3+ to 2+. The samples turn black upon heating to 1500 °C, indicating that Fe²⁺-Ti⁴⁺ is responsible for the black color rather than the blue observed in sapphires.

Keywords:

color mechanism; atmospheric environments; X-ray absorption spectroscopy; energy band model; heating process

1. Introduction

Corundum, the alumina mineral (Al₂O₃), is colorless when pure but exhibits various colors when Al³⁺ is substituted by transition-metal trace elements in the corundum structure [1,2,3,4,5]. Geologically, corundum deposits are associated with basaltic-related sapphire localities and with metamorphic-related ones. Basaltic sapphires with higher Fe content are generally darker blue than metamorphic ones. The darker blue results in a higher value for gemstones, such as royal blue and cornflower blue sapphires. Basaltic sapphires are found in Thailand, Australia, Cambodia, etc., and metamorphic sapphires are found in Sri Lanka, Madagascar, Myanmar, etc. [6]. In this study, sapphire samples are collected from Kanchanaburi Province, Thailand (KAN), and Sri Lanka (SLK) to represent basaltic and metamorphic origins, respectively.

The blue color of blue sapphire is explained by intervalence charge-transfer (IVCT) theory [2,7,8,9,10,11,12,13,14] between Fe²⁺ and Ti⁴⁺, which substitute for pairs of Al³⁺ sites in the structure [2,14,15,16,17,18]. A theory of defect clustering involving Fe, Ti, and vacancies has also been proposed [19]. After heat treatment under oxidizing atmospheric conditions, blue sapphire turns colorless because Fe²⁺ is oxidized to Fe³⁺. Conversely, after heat treatment under reducing conditions, blue sapphire becomes darker blue because Fe³⁺ can be reduced to Fe²⁺. In addition, changes in Fe’s oxidation state may affect the intensity of the blue color, and atmospheric conditions during heat treatment may also influence it.

However, the oxidation state of Fe in blue sapphire before and after heat treatment in an oxidizing atmosphere at 1100 °C is still reported as Fe³⁺, consistent with Fe³⁺-Ti⁴⁺ mixed acceptor states [20], a model proposed by Wongrawang et al. [21] and confirmed by X-ray absorption spectroscopy (XAS). According to this theory, the cause of the blue color in blue sapphire before and after heat treatment [7,8,9,10,11,22,23,24,25,26,27,28] could conflict with the IVCT theory.

This study aims to investigate the mechanism of blue coloration in sapphires from different localities, both before and after heating in oxidizing [7,8,9,10,22,23,29,30] and reducing [7,9,11,31] atmospheres. These findings can help the gemstone industry improve the quality of natural sapphires through heat treatment, either by lightening overly dark blue sapphires to a lighter blue or by darkening light-blue stones to a deeper blue.

2. Materials

This study examined twenty sapphire samples from two locations: Kanchanaburi province in Thailand (KAN), known for basaltic origin, and Sri Lanka (SLK), which has metamorphic origins. The samples ranged from milky to light blue and were heated at 1100 °C for 1 h in an oxidizing atmosphere [20]. One of the SLK samples was destroyed in this state. The remaining nineteen samples are shown in Figure 1. In this experimental procedure, the samples were divided into two groups based on the heating environment: either 1300 °C or 1500 °C under oxidizing conditions in an alumina crucible, and either 1300 °C or 1500 °C under reducing conditions. Under reducing conditions, the samples were covered with graphite powder in an alumina crucible.

3. Methodology

In this study, sapphire samples were analyzed with gemological tools both before and after exposure to oxidizing and reducing heating environments, as shown in the flowchart in Figure 2. These tools included a gem microscope for observing internal features, a refractometer for measuring the refractive index, a polariscope for examining optical properties, and an electronic balance for weighing the samples and determining specific gravity. A portable GIA gem set was used to identify color, including hue, tone, and saturation, and to assign a color code according to gemological standards. The colorimeter measured the color of the samples as CIELAB indices using the AvaSpec-2048 spectrometer from Avantes of the Netherlands, Apeldoorn, The Netherlands (measurement range 275 nm to 1100 nm), and the results were calculated using AvaSoft 7.5. After that, advanced gemological instruments were applied, including an EDXRF spectrometer (Horiba, Kyoto, Japan, XGT9000: multi-condition 5 filters, 50 s per filter, capillary: 1.2 mm) to analyze chemical composition [32]; SEM-EDS (Oxford instruments, Abingdon, UK, X-Max^N: HV 15.00 kV, magnification 1500×, HFW 99.5 µm, WD 9.6 mm, Z Cont mode) to observe micro inclusions [33]; an FTIR spectrometer (Thermo Fisher Scientific, Madison, WI, USA, Nicolet iS5: DTGS KBr, Optical velocity 0.4747, Aperture: 100, No. of scans: 64, Resolution: 4) to analyze functional groups in the samples [24,25,26]; a UV–Vis-NIR spectrophotometer (PerkinElmer, Shelton, CT, USA, Lambda 1050: Data Interval: 3 nm, Scan Speed: 405.07 nm/min, Detector Settings: PMT, InGaAs 0.40 s) to determine the absorption spectra of trace elements related to the cause of color in the sapphire samples before and after heating environments [34]; and XAS focused on Fe K-edge and Ti K-edge XANES spectra [21,24,25,26,35] for pre-edge features [36,37].

X-ray Absorption Near Edge Structure (XANES) measurements at the Fe and Ti K-edges were conducted at the BL1.1W Multiple X-ray Techniques beamline at the Synchrotron Light Research Institute (SLRI), Thailand. The synchrotron storage ring operated at an electron energy of 1.2 GeV, with a typical beam current of 80 to 150 mA. The primary X-ray beam was monochromatized using a Si (111) double-crystal monochromator (DCM), and precise energy calibration was achieved using the first-derivative inflection points of standard Ti (4966.0 eV) and Fe (7112.0 eV) metallic foils. All sample data were collected in fluorescence mode at room temperature. The incident X-ray beam intensity (I₀) was continuously monitored using a gas-filled ionization chamber, while the secondary fluorescence signal (I_f) was collected with a 13-element Germanium solid-state detector positioned at a 45° angle relative to the incident beam to minimize elastic scattering contributions. XANES spectra were recorded with fine energy steps of 0.2 eV across the immediate edge transitions to ensure high-resolution detection of pre-edge features. Between 2 and 3 individual scans were acquired and averaged per sample to optimize the signal-to-noise ratio. Standard data reduction procedures, including pre-edge background subtraction, post-edge normalization, and energy alignment, were performed using the Athena software within the Demeter package [35]. Subsequent pre-edge fitting analysis was performed using Larix within the open-source Larch 2026.2.0 software package [38]. To ensure quantitative reliability, the precision of the pre-edge fitting was established by rigorously evaluating the fit residuals and parameter uncertainties across the baseline and peak functions, yielding a centroid determination precision of ±0.05 eV (reduced χ² = 7.01 × 10⁻⁶).

For the heating experiment, sapphire samples from Kanchanaburi, Thailand, and Sri Lanka were heated in an electric furnace at either 1300 °C or 1500 °C under oxidizing and reducing conditions. Heating used a 5 °C/min ramp rate for 1 h, with samples in air for the oxidizing environment and covered in graphite powder for the reducing environment [31]. Afterward, the data will be analyzed to understand the mechanisms behind the blue color in sapphires from various gem localities, both before and after heating in different environments.

4. Results

The sapphire samples from both gem localities were examined with basic gemological tools. Their properties included a specific gravity of 3.80 to 4.00, refractive indices of 1.760 and 1.770, and optical features indicating double refraction (uniaxial).

The color codes of sapphire samples heated to 1100 °C in an oxidizing environment, using the GIA gem set, were analyzed. Results showed that most samples from Kanchanaburi province, Thailand, were classified as B3/1. Similarly, Sri Lankan samples also fell into the B3/1 category (light, grayish blue, blue).

After heating at 1300 °C or 1500 °C in an oxidizing environment, samples from both gem localities showed reduced blue intensity. They remained mostly B3/1 (light, grayish blue, blue), except for one sample from Kanchanaburi province, Thailand, which was B2/2 (very light, slightly grayish blue, blue). When heated at 1300 °C in a reducing environment, the sapphire samples from both gem localities were also B3/1 (light, grayish blue, blue), except that only one from Sri Lanka was B4/2 (medium light, slightly grayish blue, blue). However, when heated to 1500 °C in a reducing environment, the samples turned black.

However, naked-eye color observation of GIA gem sets is qualitative; a quantitative method using a spectrophotometer can measure gemstone color [39]. The CIELAB color indices (L*, a*, and b*) of sapphire samples, measured with a colorimeter, are shown in Table 1. After heating to 1100 °C in an oxidizing environment, the L*, a*, and b* values shifted toward the origin, reflecting a reduction in blue intensity. Similarly, heating at 1300 °C under the same oxidizing environment caused the samples from Kanchanaburi province, Thailand, and Sri Lanka to move closer to the origin, as depicted in Figure 3a and Figure 4a. When heated at 1300 °C in a reducing environment, Sri Lankan samples, shown in Figure 4b, shifted toward the -b axis, suggesting a higher blue intensity compared to the Kanchanaburi samples in Figure 3b. Regarding brightness, samples from both localities became brighter after heating at 1300 °C in an oxidizing environment, whereas in a reducing environment, brightness decreased.

The chemical composition of the sapphire samples is detailed in Table 2, including the weight percentages of the primary component (Al₂O₃) and impurities (Fe₂O₃, TiO_2, and Cr₂O₃) after heating at 1100 °C in an oxidizing environment and at 1300 °C under both oxidizing and reducing conditions. As shown in Figure 1, sapphires from Kanchanaburi province, Thailand (originating from basalt) were mostly darker blue and had higher Fe and lower Ti contents than those from Sri Lanka (metamorphic origin), reflecting their geological sources. After post-heating at 1100 °C in an oxidizing environment, the chemical composition remained largely unchanged, as indicated in Figure 5a. Similarly, after heating at 1300 °C in both oxidizing and reducing atmospheres, the composition remained stable, as depicted in Figure 5b.

In this study, microinclusions in the sapphire samples were identified as various mineral types using SEM-EDS (Table 3), providing an estimate of the microinclusion profile. Samples contained microinclusions such as hematite [33] from Kanchanaburi Province, Thailand, and rutile from Sri Lanka. Heating at 1100 °C in an oxidizing environment caused partial melting of these microinclusions, as the temperature was insufficient for complete melting. After heating at 1300 °C in both oxidizing and reducing environments, the microinclusions remained in some samples, indicating that the temperature was still not high enough to fully melt them.

The UV–Vis–NIR absorption spectra of sapphire samples from both localities after heating at 1100 °C in an oxidizing environment are shown in Figure 6 and Figure 7. The absorption peaks at 377 nm and 450 nm correspond to Fe³⁺/Fe³⁺, while the peak at 388 nm is attributed to single Fe³⁺ [30,40,41,42], causing yellow coloration. The Fe-Ti absorption peaks at 580 nm and 710 nm were absent in colorless samples. For samples from Kanchanaburi province, Thailand, three peaks at 580 nm, 710 nm, and 890 nm indicated Fe-Ti pairs linked to blue coloration [21]. The 890 nm peak was a key feature of basaltic sapphire sources [43]. After heating at 1300 °C in an oxidizing environment, samples from Kanchanaburi still displayed peaks at 710 nm and 890 nm, whereas Sri Lankan samples lost only the peaks at 377 nm, 388 nm, and 450 nm. Under reducing conditions at 1300 °C, Fe-Ti pairs persisted in Kanchanaburi samples; however, Sri Lankan samples showed Fe-Ti absorption peaks at 580 nm and 710 nm without the 890 nm peak, suggesting a metamorphic origin in some samples, as the 890 nm peak was absent in metamorphic sources [22].

The optical band gaps (E_opt) of the samples were calculated using a Tauc plot [44], as shown in the insets of Figure 6a,b and Figure 7a,b. Both sapphire samples, after heating at 1100 °C under oxidizing conditions, exhibited smaller E_opt values than those heated at 1300 °C under similar oxidizing conditions. Furthermore, sapphire samples from Kanchanaburi, Thailand, heated at 1100 °C under oxidizing conditions, had smaller E_opt than samples heated at 1300 °C under reducing conditions. In contrast, sapphires from Sri Lanka heated at 1100 °C under oxidizing conditions showed larger E_opt values than those heated at 1300 °C under reducing conditions.

The FTIR spectra of sapphire samples from both localities, shown in Figure 8, were recorded over the range of 1000 cm⁻¹ to 4000 cm⁻¹. After heating to 1100 °C in an oxidizing environment, the spectra exhibited absorption peaks for C-H stretching at 2921 cm⁻¹ and 2850 cm⁻¹, CO₂ at 2350 cm⁻¹, and -Ti-OH stretching at 3309 cm⁻¹. These features were observed in Sri Lankan samples with high Ti content. In contrast, the absence of -Ti-OH stretching in samples from Kanchanaburi province, Thailand, was likely due to their lower Ti content.

Heating at 1300 °C in an oxidizing environment did not alter the C-H stretching and CO₂ peaks. The -Ti-OH peak at 3309 cm⁻¹ on the sapphire structure, along with Ti-bearing mineral inclusions such as rutile, decreased due to weakened Ti-OH bonds, which correlates with the loss of blue color [26]. Conversely, heating at 1300 °C in a reducing environment sharpened the -Ti-OH peak [25,26], as Ti-OH bonding increased and rutile inclusions dissolved, thereby restoring the blue color.

To examine the oxidation states of iron and titanium in sapphire samples after heating at 1100 °C under oxidizing conditions and at 1300 °C under both oxidizing and reducing conditions, X-ray absorption spectroscopy (XAS) was performed at BL1.1 W Multiple X-ray Techniques (MXT) at the Synchrotron Light Research Institute (SLRI), Thailand. The spectra of sapphire samples from Kanchanaburi province, Thailand, and Sri Lanka were acquired in fluorescence mode. To confirm the oxidation states, XAS spectra of Fe and Ti standards with known oxidation states were recorded in transmission mode. The spectra of the sapphire samples and standards were then analyzed using Athena 0.9.26 software [35].

Based on Table 4 and Figure 9, the Fe standards included Fe foil, FeO, and Fe₂O₃, representing Fe⁰, Fe^2+, and Fe³⁺, respectively. After heating at 1100 °C under oxidizing conditions and at 1300 °C under oxidizing and reducing conditions, the Fe oxidation state in the sapphire samples from Kanchanaburi province, Thailand, and Sri Lanka was identified as Fe³⁺, consistent with the Fe standards. The Fe K-edge XANES spectra showed a binding energy (E₀) of about 7124 eV for Fe³⁺ after these heating treatments. Notably, a shoulder appeared post-pre-edge in the spectra of samples from both regions after heating under reducing conditions, suggesting a possible change in oxidation state. Therefore, the pre-edge fit was used to determine the Fe oxidation state by analyzing the pre-edge centroid position with Larch 2026.2.0 software [38].

Table 5 lists the Ti K-edge photon energy (E₀) derived from Ti K-edge XANES spectra. Ti foil and TiO₂ served as reference standards for Ti⁰ and Ti⁴⁺, respectively. Results indicate that the Ti oxidation state in samples from both localities is Ti⁴⁺, with a photon energy of 4979.85 eV, closely matching the TiO₂ standard (Figure 10).

The Fe oxidation state, derived from the pre-edge fit shown in Figure 11 and informed by the characteristics of pre-edge features [37,45], indicates that Fe³⁺ is present in all samples after heating at 1100 °C under oxidizing conditions and at 1300 °C under oxidizing conditions. However, at 1300 °C under reducing conditions, Fe³⁺ shifts toward Fe²⁺, which retains 6-fold coordination, indicating no change in symmetry.

To confirm the pre-edge results, linear combination fitting (LCF) was performed in Athena, using FeO and Fe₂O₃ spectra as fitting standards for Fe²⁺ and Fe³⁺, respectively, over an energy range from 20 eV below to 30 eV above the edge. Derivative LCF results for sapphire samples from Kanchanaburi, Thailand, under oxidizing and reducing environments at 1300 °C and 1500 °C yielded FeO and Fe₂O₃ weights, along with their uncertainties. The LCF results shown in Table 6 are consistent with the pre-edge results, except for those from Kanchanaburi, Thailand, in reducing environments at 1300 °C, which show no change in Fe³⁺ content, in contrast to the pre-edge results. However, at 1500 °C, the oxidation state changes from Fe³⁺ to Fe²⁺. The results indicate that Fe²⁺-Ti⁴⁺ is responsible for the black color rather than the blue observed in sapphires, which is associated with the formation of Fe²⁺-bearing inclusions such as FeO and FeTiO₃ [8,29,46,47].

5. Discussion

In this study, we did not observe a shift from Fe²⁺ to Fe³⁺ under oxidizing conditions, as shown in Figure 11a, either because the oxidation state remained unchanged or because of measurement uncertainty. However, the shift from Fe²⁺ to Fe³⁺, which is not directly related to the reduction in the blue color [8,29], could be explained by electrons from the valence band recombining with the Fe³⁺-Ti⁴⁺ mixed-acceptor states, thereby reducing localized screening in the valence band [48]. Therefore, the reduction in blue color is due to recombination rather than a change in oxidation state.

Under reducing conditions, the pre-edge centroid energies in both sample groups indicate a shift from Fe³⁺ to Fe^2+, as shown in Figure 11b. This is consistent with electron transfer from mixed-acceptor states to the valence band, thereby increasing localized valence-band screening. The phenomenon observed in yellow sapphire heated under reducing conditions, which turns dark [31], could be explained similarly. The Sri Lanka samples show lower Fe content and a greater shift from Fe³⁺ to Fe²⁺ than the samples from Kanchanaburi, Thailand, reflecting greater uncertainty in the pre-edge centroid. The increased blue color in this case is due to electron depletion in the Fe³⁺-Ti⁴⁺ mixed-acceptor states, which enhances light absorption at 580 nm and 710 nm.

The results can be explained by the energy band model, which accounts for the presence or absence of blue color in sapphires after heating at 1100 °C and 1300 °C under oxidizing and reducing conditions. The analysis considers two types of geological gem localities: basaltic sapphires from Kanchanaburi, Thailand, and metamorphic sapphires from Sri Lanka, examined under both oxidizing and reducing conditions.

Sapphire samples from Kanchanaburi, Thailand, with high iron content, show blue coloration due to Fe³⁺ and Ti⁴⁺ mixed acceptor states at 1.39 eV, 1.75 eV, and 2.14 eV—corresponding to absorption peaks at 890 nm, 710 nm, and 580 nm—resulting in energy band gaps of 3.95 eV for KAN2 and 3.99 eV for KAN4 (Figure 12a and Figure 13a). Conversely, sapphires from Sri Lanka, with lower iron levels, display blue coloration mainly from Fe³⁺ and Ti⁴⁺ acceptor states at 1.75 eV and 2.14 eV, with absorption at 710 nm and 580 nm, and band gaps of 4.05 eV for SLK1 and 3.74 eV for SLK4 (Figure 14a and Figure 15a). Heating at 1100 °C and 1300 °C in an oxidizing environment reduces or eliminates the blue color because the heating process fills the color-center holes with electrons from the valence band, preventing electrons from entering these states. Conversely, heating at 1300 °C in a reducing environment enhances the blue color because electrons are depleted from the acceptor states, allowing electrons from the valence band to refill the holes via optical excitation.

Basaltic sapphire samples from Kanchanaburi, Thailand, are light to pale blue, whereas metamorphic sapphire samples from Sri Lanka become colorless after heating at 1100 °C, 1300 °C, and 1500 °C in an oxidizing environment. Despite these treatments, the optical band gaps of both localities remain unchanged after heating to 1300 °C. Kanchanaburi sapphires display a pale blue hue due to Fe³⁺-Ti⁴⁺ mixed acceptor states at 1.39 eV and 1.75 eV, corresponding to absorptions at 890 nm and 710 nm, with an energy band gap of 3.96 eV for sample KAN2 (Figure 12b). Sri Lankan samples are colorless, with an energy band gap of 4.04 eV for SLK1 (Figure 14b). When heated at 1300 °C in a reducing environment, Kanchanaburi sapphires turn light blue due to Fe³⁺-Ti⁴⁺ acceptor states at 1.39 eV, 1.75 eV, and 2.14 eV, corresponding to absorptions at 890 nm, 710 nm, and 580 nm, and an energy band gap of 3.99 eV for KAN4 (Figure 13b,c). Sri Lankan sapphires show a blue color caused by Fe³⁺-Ti⁴⁺ states at 1.75 eV and 2.14 eV, corresponding to absorptions at 710 nm and 580 nm, with an energy band gap of 4.22 eV for SLK4 (Figure 15b,c), because higher Fe³⁺-Ti⁴⁺ energy states influence the color.

Therefore, the blue coloration in sapphires after heating at 1100 °C and 1300 °C under oxidizing and reducing conditions is attributed to the presence or absence of Fe³⁺-Ti⁴⁺ mixed acceptor states and to a hole color center described by the energy band model.

6. Conclusions

The blue color mechanism in sapphires before and after heating in oxidizing and reducing environments at 1300 °C can be explained by the energy-band model. The presence of Fe³⁺-Ti⁴⁺ mixed acceptor states within the energy band gap results in blue coloration in sapphires. After heating in an oxidizing environment at 1300 °C and 1500 °C, the decrease in blue color in both sapphire localities can be attributed to valence-band electrons recombining with the color center. After heating in a reducing environment at 1300 °C, the increase in blue color in sapphires can be explained by electron depletion from the color center, thereby enabling light absorption that produces the blue color. The blue intensity of sapphires is related to the number of Fe³⁺-Ti⁴⁺ mixed acceptor states, as well as to Fe and Ti concentrations. In addition, after heating at 1300 °C and 1500 °C in a reducing environment, the oxidation state of Fe decreases from 3+ to 2+. The samples turn blue at 1300 °C and finally turn black upon heating to 1500 °C.

The purpose of heat treatment can be categorized into two types: decreasing or increasing the blue color intensity of sapphires to achieve a higher-market-value color. To reduce blue color intensity, basaltic sapphire is more effective than metamorphic sapphire because it is darker to begin with. Heating can be performed in an oxygen-containing environment at temperatures from 1300 °C to 1500 °C. On the other hand, to increase blue intensity, metamorphic sapphire is more effective because it starts lighter blue. They can be heated in an oxygen-free environment at 1300 °C. However, the heating temperature cannot be increased to 1500 °C because Fe³⁺ will be reduced to Fe²⁺, destroying the blue color center. These research findings can help the gemstone and jewelry industry enhance the color of natural sapphires.

Author Contributions

Conceptualization, W.W.; Methodology, C.K. and C.S.; Validation, C.S.; Formal analysis, S.S. and W.W.; Investigation, C.K.; Writing—original draft, C.K.; Writing—review & editing, N.M.; Supervision, N.M.; Project administration, N.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by Kasetsart University through the Graduate School Fellowship Program.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors gratefully acknowledge the Department of Earth Sciences, Faculty of Science, Kasetsart University, for providing basic gemological instruments; the Department of Physics, Faculty of Science, Kasetsart University, for color measurements and the electric furnace used in the heating process; the Synchrotron Light Research Institute (Public Organization) for access to the XAS facility at BL1.1 W Multiple X-ray Techniques (MXT); and the Gem and Jewelry Institute of Thailand (Public Organization) for advanced spectroscopic techniques, including FTIR, UV–Vis-NIR, and EDXRF. During the preparation of this manuscript, the authors used GPT-5.5 and Canva Pro for the purpose of the Graphical Abstract. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflict of interest.

References

Hughes, R.W. Corundum; Butterworth-Heinemann: Oxford, UK, 1990. [Google Scholar]
Nassau, K. The Physics and Chemistry of Color; John Wiley & Sons: New York, NY, USA, 2001. [Google Scholar]
Klein, C.; Dutrow, B. The Manual of Mineral Science; John Wiley & Sons: New York, NY, USA, 2008. [Google Scholar]
Karampelas, S.; Kiefert, L.; Bersani, D.; Vandenabeele, P. Gems and Gemmology; Springer: Berlin/Heidelberg, Germany, 2020. [Google Scholar]
Turner, D.P.; Groat, L.A. Geology and Mineralogy of Gemstones; John Wiley & Sons: New York, NY, USA, 2022. [Google Scholar]
Palke, A.C.; Saeseaw, S.; Renfro, N.D.; McClure, S.F. Geographic Origin Determination of Ruby. Gems Gemol. 2019, 55, 580–613. [Google Scholar] [CrossRef] [Scilit]
Kwansirikul, K.; Thanasuthipitak, P.; Thanasuthipitak, T. Experimental Heat Treating of Trapiche-Like Blue Sapphire of Southern Vietnam. Suranaree J. Sci. Technol. 2016, 23, 409–419. [Google Scholar]
Amphon, R.; Chankhantha, C.; Thammajak, N.; Intarasiri, S.; Shen, A.H. Investigation of Blue Coloration in Sapphires Using X-Ray Absorption Near-Edge Structure Spectroscopy (XANES). Cryst. Res. Technol. 2023, 58, 2200152. [Google Scholar]
Kwansirikul, K.; Thanasuthipitak, P.; Chimnakphant, S.; Saidum, O. Heat Treating Blue Sapphire of Pailin, Cambodia. Chiang Mai J. Sci. 2018, 45, 588–600. [Google Scholar]
Pluthametwisute, T.; Nasdala, L.; Chanmuang, N.C.; Wildner, M.; Libowitzky, E.; Giester, G.; Zoysa, E.G.; Jakkawanvibul, C.; Suwanmanee, W.; Sripoonjan, T. Luminescence and a new approach for detecting heat treatment of geuda sapphire. Solid Earth 2025, 16, 81–96. [Google Scholar] [CrossRef] [Scilit]
Kittiauchawal, T.; Limsuwan, P. The Color Change of Natural Blue Sapphires by Heat Treatment. Agric. Nat. Resour. 2007, 41, 262–266. [Google Scholar]
Bristow, J.K.; Parker, S.C.; Catlow, C.R.A.; Woodley, S.M.; Walsh, A. Microscopic origin of the optical processes in blue sapphire. Chem. Commun. 2013, 49, 5259–5261. [Google Scholar] [CrossRef] [Scilit]
Bristow, J.K.; Tiana, D.; Parker, S.C.; Walsh, A. Defect chemistry of Ti and Fe impurities and aggregates in Al₂O₃. J. Mater. Chem. A 2014, 2, 6198–6208. [Google Scholar] [CrossRef] [Scilit]
Fritsch, E.; Rossman, G.R. An update on color in gems. Part 2: Colors involving multiple atoms and color centers. Gems Gemol. 1988, 24, 3–15. [Google Scholar] [CrossRef] [Scilit]
Fritsch, E.; Rossman, G. An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions. Gems Gemol. 1987, 23, 126–139. [Google Scholar] [CrossRef] [Scilit]
Nassau, K. Heat treating ruby and sapphire: Technical aspects. Gems Gemol. 1981, 17, 121–131. [Google Scholar] [CrossRef] [Scilit]
Sherman, D.M. Molecular orbital (SCF-Xα-SW) theory of metal-metal charge transfer processes in minerals. Phys. Chem. Miner. 1987, 14, 355–363. [Google Scholar] [CrossRef] [Scilit]
Shen, C.; Lu, R. The color origin of gem diaspore: Correlation to corundum. Gems Gemol. 2018, 54, 394–403. [Google Scholar]
Moon, A.R.; Phillips, M.R. Defect Clustering and Color in Fe,Ti: α-Al₂O₃. J. Am. Ceram. Soc. 1994, 77, 356–367. [Google Scholar] [CrossRef] [Scilit]
Thengthong, W.; Sakkaravej, S.; Wongkokua, W.; Saiyasombat, C.; Monarumit, N. The blue color mechanism on sapphires from different gem deposits before and after heating under oxidizing atmosphere. J. Phys. Commun. 2023, 7, 125003. [Google Scholar] [CrossRef] [Scilit]
Wongrawang, P.; Monarumit, N.; Thammajak, N.; Wathanakul, P.; Wongkokua, W. Oxidation states of Fe and Ti in blue sapphire. Mater. Res. Express 2016, 3, 026201. [Google Scholar] [CrossRef] [Scilit]
Hughes, E.; Perkins, R. Madagascar sapphire: Low-temperature heat treatment experiments. Gems Gemol. 2019, 55, 184–197. [Google Scholar] [CrossRef] [Scilit]
Notari, F.; Hainschwang, T.; Caplan, C.; Ho, K. The heat treatment of corundum at moderate temperature. InColor 2019, 42, 14–23. [Google Scholar]
Phlayrahan, A.; Monarumit, N.; Boonmee, C.; Satitkune, S.; Wathanakul, P. Fe oxidation state in heat-treated basaltic blue sapphire samples and its implication to the 3309 cm-1-series peaks in infrared absorption spectra. J. Phys. Conf. Ser. 2018, 1144, 012057. [Google Scholar] [CrossRef] [Scilit]
Phlayrahan, A.; Monarumit, N.; Satitkune, S.; Wathanakul, P. Role of Ti content on the occurrence of the 3309-cm–1 peak in FTIR absorption Spectra of Ruby Samples. J. Appl. Spectrosc. 2018, 85, 385–390. [Google Scholar] [CrossRef] [Scilit]
Phlayrahan, A.; Monarumit, N.; Lhuaamporn, T.; Satitkune, S.; Wathanakul, P. Spectroscopic investigation of properties of blue sapphire samples depending on heating conditions. J. Appl. Spectrosc. 2019, 86, 810–816. [Google Scholar] [CrossRef] [Scilit]
Udomkan, N.; Limsuwan, P.; Winotai, P.; Meejoo, S. Effects of heat treatment on blue sapphires as monitored by ESR spectroscopy. Int. J. Mod. Phys. B 2005, 19, 3273–3284. [Google Scholar] [CrossRef] [Scilit]
Phlayrahan, A.; Homkhajorn, H. The new evidence from the fingerprint region in FT-IR spectra to indicate the heat treatment of blue sapphire sample. J. Phys. Conf. Ser. 2021, 2145, 012023. [Google Scholar] [CrossRef] [Scilit]
Amphon, R.; Chankhantha, C.; Srimuang, C.; Vongsetskul, S.; Intarasiri, S.; Shen, A.H. Changes in blue color of sapphire compared with oxidation state changes. Minerals 2024, 14, 333. [Google Scholar] [CrossRef] [Scilit]
Lu, Q.; Li, X.; Sun, L.; Qin, B. Chemical and Spectral Variations between Untreated and Heat-Treated Rubies from Mozambique and Madagascar. Minerals 2022, 12, 894. [Google Scholar] [CrossRef] [Scilit]
Wu, X.; Han, X.; Kang, Y.; Feng, X.; Guo, S. Effect of Reduction Annealing on the Coloration Mechanism of Yellow Sapphire with High Iron Content. Crystals 2022, 12, 1257. [Google Scholar] [CrossRef] [Scilit]
Bonizzoni, L.; Galli, A.; Spinolo, G.; Palanza, V. EDXRF quantitative analysis of chromophore chemical elements in corundum samples. Anal. Bioanal. Chem. 2009, 395, 2021–2027. [Google Scholar] [CrossRef] [Scilit] [PubMed]
Jaijong, K.; Wathanakul, P.; Kim, Y.C.; Choi, H.M.; Bang, S.Y.; Choi, B.G.; Shim, K.B. Nano inclusions in sapphire samples from Sri Lanka. J. Korean Cryst. Growth Cryst. Technol. 2009, 19, 84–89. [Google Scholar]
Pisutha-Arnond, V.; Häger, T.; Atichat, W.; Wathanakul, P. The role of Be, Mg, Fe and Ti in causing colour in corundum. J. Gemmol. 2006, 30, 131–143. [Google Scholar] [CrossRef] [Scilit]
Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537–541. [Google Scholar] [CrossRef] [Scilit] [PubMed]
Monarumit, N.; Lhuaamporn, T.; Wathanakul, P.; Saiyasombat, C.; Wongkokua, W. The acceptor-donor pair recombination of beryllium-treated sapphires. Radiat. Phys. Chem. 2023, 206, 110756. [Google Scholar] [CrossRef] [Scilit]
Bunnag, N.; Kasri, B.; Setwong, W.; Sirisurawong, E.; Chotsawat, M.; Chirawatkul, P.; Saiyasombat, C. Study of Fe ions in aquamarine and the effect of dichroism as seen using UV–Vis, NIR and x-ray. Radiat. Phys. Chem. 2020, 177, 109107. [Google Scholar] [CrossRef] [Scilit]
Newville, M. Larch: An Analysis Package for XAFS and Related Spectroscopies. J. Phys. Conf. Ser. 2013, 430, 012007. [Google Scholar] [CrossRef] [Scilit]
Tipkanon, N.; Monarumit, N.; Lhuaamporn, T.; Wongkokua, W. Color measurement of yellow sapphire by UV-Vis reflectance spectroscopy. J. Phys. Conf. Ser. 2021, 2145, 012062. [Google Scholar] [CrossRef] [Scilit]
Ferguson, J.; Fielding, P. The origins of the colours of yellow, green and blue sapphires. Chem. Phys. Lett. 1971, 10, 262–265. [Google Scholar] [CrossRef] [Scilit]
Dubinsky, E.V.; Stone-Sundberg, J.; Emmett, J.L. A quantitative description of the causes of color in corundum. Gems Gemol. 2020, 56, 1–27. [Google Scholar] [CrossRef] [Scilit]
Monarumit, N.; Lhuaamporn, T.; Sakkaravej, S.; Wathanakul, P.; Wongkokua, W. The color center of beryllium-treated yellow sapphires. J. Phys. Commun. 2020, 4, 105018. [Google Scholar] [CrossRef] [Scilit]
Wongkokua, W.; Lhuaamporn, T.; Monarumit, N.; Kajornboonsook, P.; Wathanakul, P. Explanation on Yellow, Green and Brown Coloration Series of Basalt Related Bangkaja Sapphires. In Proceedings of the International Gemmological Conference, Tokyo, Japan, 23–27 October 2023; pp. 147–149. [Google Scholar]
Kusuma, H.H.; Saidin, M.K.; Ibrahim, Z. Optical properties of Ti: Al2O3 single crystal. J. Fiz. UTM 2009, 4, 42–49. [Google Scholar]
Wilke, M.; Farges, F.O.; Petit, P.-E.; Brown, G.E., Jr.; Martin, F.O. Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study. Am. Mineral. 2001, 86, 714–730. [Google Scholar] [CrossRef] [Scilit]
Monarumit, N.; Wongkokua, W.; Satitkune, S. Fe²⁺ and Fe³⁺ Oxidation States on Natural Sapphires Probed by X-ray Absorption Spectroscopy. Procedia Comput. Sci. 2016, 86, 180–183. [Google Scholar] [CrossRef] [Scilit]
Monarumit, N.; Satitkune, S.; Wongkokua, W. Role of ilmenite micro-inclusion on Fe oxidation states of natural sapphires. J. Phys. Conf. Ser. 2017, 901, 012074. [Google Scholar] [CrossRef] [Scilit]
Zaanen, J.; Sawatzky, G.A.; Allen, J.W. Band gaps and electronic structure of transition-metal compounds. Phys. Rev. Lett. 1985, 55, 418–421. [Google Scholar] [CrossRef] [Scilit] [PubMed]

Figure 1. Sapphire samples from Kanchanaburi, Thailand (a), and Sri Lanka (b) in this study were heated at 1100 °C for 1 h in an oxidizing environment.

Figure 2. Flowchart of the methodology.

Figure 3. Color plot in CIELAB space of sapphire samples from Kanchanaburi, Thailand, illustrating brightness levels before and after heating in oxidizing (a) and reducing (b) environments.

Figure 4. Color plot in CIELAB space of sapphire samples from Sri Lanka, illustrating brightness levels before and after heating in oxidizing (a) and reducing (b) environments.

Figure 5. Chemical concentrations (%TiO₂ versus %Fe₂O₃) of sapphire samples from Kanchanaburi, Thailand, and Sri Lanka, analyzed after heating at 1100 °C in oxidizing conditions (a), and after heating at 1300 °C under both oxidizing and reducing conditions (b).

Figure 6. UV−Vis−NIR spectra and Tauc plots of sapphire samples from Kanchanaburi, Thailand, are shown after heat treatment in oxidizing (a) and reducing (b) environments.

Figure 7. UV−Vis−NIR spectra and Tauc plots of sapphire samples from Sri Lanka are shown after heat treatment in oxidizing (a) and reducing (b) environments.

Figure 8. Representative FTIR spectra of sapphire samples from Kanchanaburi, Thailand, heated at 1100 °C and 1300 °C in oxidizing (a) and reducing (b) environments, compared with sapphire samples from Sri Lanka heated at 1100 °C and 1300 °C in oxidizing (c) and reducing (d) environments.

Figure 9. Fe K-edge XANES spectra of sapphire samples from Kanchanaburi, Thailand (a), and Sri Lanka (b), heated at 1100 °C and 1300 °C under oxidizing and reducing conditions, are compared with Fe reference standards.

Figure 10. Ti K-edge XANES spectra of sapphire samples from Kanchanaburi, Thailand (a), and Sri Lanka (b), heated at 1100 °C and 1300 °C under oxidizing and reducing conditions, are compared with Ti reference standards. The dashed circle indicates the pre-edge position.

Figure 11. The relationship between the pre-edge centroid energy position and the integrated pre-edge intensity of sapphire samples from Kanchanaburi, Thailand, and Sri Lanka is shown after heating at 1100 °C and 1300 °C under oxidizing (a) and reducing (b) conditions. The uncertainties are 0.40 eV for the centroid position and 0.04 for the integrated intensity.

Figure 12. An energy band model illustrating the Fe³⁺−Ti⁴⁺ states in basalt-related sapphires from Kanchanaburi province, Thailand, following heating at 1100 °C under oxidizing conditions (a) and at 1300 °C under oxidizing conditions (b).

Figure 13. An energy band model illustrating the Fe³⁺−Ti⁴⁺ states in basalt-related sapphires from Kanchanaburi province, Thailand, after heating at 1100 °C under an oxidizing environment (a), electron depletion at 1300 °C under a reducing environment (b), and after heating at 1300 °C under a reducing environment (c).

Figure 14. An energy band model illustrating the Fe³⁺−Ti⁴⁺ states in metamorphic-related sapphires from Sri Lanka, following heating at 1100 °C under oxidizing conditions (a) and at 1300 °C under oxidizing conditions (b).

Figure 15. An energy band model illustrating the Fe³⁺−Ti⁴⁺ states in metamorphic-related sapphires from Sri Lanka, after heating at 1100 °C under an oxidizing environment (a), electron depletion at 1300 °C under a reducing environment (b), and after heating at 1300 °C under a reducing environment (c).

Table 1. Color indices on the CIELAB scale for sapphire samples before and after heating under various atmospheric conditions.

Samples	Photo	L*	a*	b*	h	C
KAN1 (after oxidizing environment at 1100 °C)		88.11	5.61	0.02	0.21	5.61
KAN1 (after oxidizing environment at 1300 °C)		82.98	4.30	−2.41	330.69	4.93
KAN2 (after oxidizing environment at 1100 °C)		81.08	−0.50	−2.60	259.08	2.65
KAN2 (after oxidizing environment at 1300 °C)		81.66	−1.81	−3.95	245.41	4.34
KAN3 (after oxidizing environment at 1100 °C)		83.30	0.17	0.23	54.61	0.29
KAN3 (after oxidizing environment at 1300 °C)		88.97	−1.28	−0.92	215.49	1.58
KAN4 (after oxidizing environment at 1100 °C)		79.76	2.33	−5.53	292.84	6.00
KAN4 (after reducing environment at 1300 °C)		73.96	0.14	−4.80	271.73	4.80
KAN5 (after oxidizing environment at 1100 °C)		88.24	−0.69	−1.09	237.65	1.29
KAN5 (after oxidizing environment at 1500 °C)		85.30	−2.73	−3.88	234.86	4.74
KAN6 (after oxidizing environment at 1100 °C)		81.59	1.25	−0.18	351.69	1.26
KAN6 (after oxidizing environment at 1300 °C)		83.15	−0.38	−0.53	234.33	0.65
KAN7 (after oxidizing environment at 1100 °C)		96.36	−0.58	0.19	161.57	0.61
KAN7 (after reducing environment at 1500 °C)		55.92	0.89	−4.53	281.08	4.62
KAN8 (after oxidizing environment at 1100 °C)		90.34	1.23	−0.43	340.57	1.30
KAN8 (after reducing environment at 1300 °C)		81.52	−0.30	−1.02	253.42	1.06
KAN9 (after oxidizing environment at 1100 °C)		96.66	−0.72	−2.00	250.26	2.13
KAN9 (after reducing environment at 1300 °C)		89.34	−0.57	−0.69	230.44	0.90
KAN10 (after oxidizing environment at 1100 °C)		94.69	−0.86	−1.51	240.42	1.74
KAN10 (after reducing environment at 1300 °C)		91.39	−0.29	−0.10	199.71	0.31
SLK1 (after oxidizing environment at 1100 °C)		90.33	2.55	0.66	14.62	2.63
SLK1 (after oxidizing environment at 1300 °C)		92.37	0.93	−1.02	312.39	1.38
SLK2 (after oxidizing environment at 1100 °C)		96.65	1.45	0.67	24.98	1.60
SLK2 (after reducing environment at 1300 °C)		80.47	1.01	−2.53	291.81	2.72
SLK3 (after oxidizing environment at 1100 °C)		98.41	0.60	−0.78	307.44	0.98
SLK3 (after reducing environment at 1500 °C)		50.07	2.20	−2.46	311.79	3.30
SLK4 (after oxidizing environment at 1100 °C)		94.42	0.60	0.68	48.51	0.91
SLK4 (after reducing environment at 1300 °C)		79.81	0.41	−6.94	273.34	6.95
SLK5 (after oxidizing environment at 1100 °C)	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
SLK6 (after oxidizing environment at 1100 °C)		93.04	0.02	−2.60	270.52	2.60
SLK6 (after reducing environment at 1300 °C)		79.87	0.71	−4.13	279.68	4.19
SLK7 (after oxidizing environment at 1100 °C)		96.01	0.79	−0.31	338.62	0.85
SLK7 (after oxidizing environment at 1500 °C)		100.37	−0.13	−2.12	266.43	2.12
SLK8 (after oxidizing environment at 1100 °C)		100.12	0.47	0.42	41.46	0.63
SLK8 (after oxidizing environment at 1300 °C)		98.68	−0.20	−0.24	230.20	0.31
SLK9 (after oxidizing environment at 1100 °C)		93.13	1.00	−0.79	321.63	1.27
SLK9 (after oxidizing environment at 1300 °C)		95.80	−0.01	−0.33	268.33	0.33
SLK10 (after oxidizing environment at 1100 °C)		92.65	0.97	1.45	56.25	1.74
SLK10 (after oxidizing environment at 1300 °C)		96.74	−0.57	1.16	116.32	1.29

Remarks: n.d. indicates that the samples were not detected because they were destroyed during the heating process.

Table 2. Semi-quantitative analysis of major and trace elements in sapphire samples using EDXRF.

Samples	Concentration/%
Samples	Al₂O₃	Fe₂O₃	TiO₂	Cr₂O₃
KAN1 (Before heating)	99.36	0.54	0.03	0.02
KAN1 (after oxidizing environment at 1100 °C)	99.47	0.48	0.01	0.02
KAN1 (after oxidizing environment at 1300 °C)	99.40	0.50	0.08	BDL
KAN2 (Before heating)	99.27	0.66	0.02	0.01
KAN2 (after oxidizing environment at 1100 °C)	99.26	0.66	0.02	0.01
KAN2 (after oxidizing environment at 1300 °C)	99.37	0.58	0.02	BDL
KAN3 (Before heating)	99.27	0.66	0.02	0.01
KAN3 (after oxidizing environment at 1100 °C)	99.40	0.54	0.01	0.01
KAN3 (after oxidizing environment at 1300 °C)	99.35	0.59	0.03	BDL
KAN4 (Before heating)	99.46	0.47	0.01	0.02
KAN4 (after oxidizing environment at 1100 °C)	99.49	0.42	0.02	0.02
KAN4 (after reducing environment at 1300 °C)	99.49	0.47	0.02	0.01
KAN5 (Before heating)	99.34	0.59	0.02	0.02
KAN5 (after oxidizing environment at 1100 °C)	99.38	0.57	0.01	0.01
KAN5 (after oxidizing environment at 1500 °C)	99.49	0.47	0.03	BDL
KAN6 (Before heating)	99.37	0.57	0.02	0.01
KAN6 (after oxidizing environment at 1100 °C)	99.40	0.54	0.02	0.02
KAN6 (after oxidizing environment at 1300 °C)	99.38	0.57	0.02	BDL
KAN7 (Before heating)	99.37	0.56	0.02	0.01
KAN7 (after oxidizing environment at 1100 °C)	99.40	0.54	0.01	0.01
KAN7 (after reducing environment at 1500 °C)	99.45	0.52	0.02	BDL
KAN8 (after reducing environment at 1300 °C)	99.43	0.52	0.03	BDL
KAN9 (Before heating)	99.42	0.52	0.02	0.01
KAN9 (after oxidizing environment at 1100 °C)	99.40	0.54	0.01	0.01
KAN9 (after reducing environment at 1300 °C)	99.39	0.57	0.02	BDL
KAN10(Before heating)	99.34	0.61	0.01	0.01
KAN10 (after oxidizing environment at 1100 °C)	99.35	0.59	0.01	0.01
KAN10 (after reducing environment at 1300 °C)	99.35	0.61	0.01	BDL
SLK1 (Before heating)	99.75	0.16	0.04	0.01
SLK1 (after oxidizing environment at 1100 °C)	99.71	0.18	0.06	0.02
SLK1 (after oxidizing environment at 1300 °C)	99.79	0.15	0.04	0.01
SLK2 (Before heating)	99.73	0.16	0.06	BDL
SLK2 (after oxidizing environment at 1100 °C)	99.85	0.09	0.03	BDL
SLK2 (after reducing environment at 1300 °C)	99.72	0.16	0.09	BDL
SLK3 (Before heating)	99.77	0.14	0.04	0.01
SLK3 (after oxidizing environment at 1100 °C)	99.77	0.15	0.03	0.01
SLK3 (after reducing environment at 1500 °C)	99.78	0.17	0.04	BDL
SLK4 (Before heating)	99.85	0.08	0.05	0.01
SLK4 (after oxidizing environment at 1100 °C)	99.79	0.09	0.08	0.01
SLK4 (after reducing environment at 1300 °C)	99.79	0.09	0.10	BDL
SLK5 (Before heating)	99.76	0.09	0.11	BDL
SLK5 (after oxidizing environment at 1100 °C)	n.d.	n.d.	n.d.	n.d.
SLK6 (Before heating)	99.79	0.13	0.03	0.01
SLK6 (after oxidizing environment at 1100 °C)	99.78	0.13	0.04	0.01
SLK6 (after reducing environment at 1300 °C)	99.82	0.11	0.06	BDL
SLK7 (Before heating)	99.82	0.11	0.05	0.01
SLK7 (after oxidizing environment at 1100 °C)	99.82	0.11	0.03	0.01
SLK7 (after oxidizing environment at 1500 °C)	99.80	0.14	0.04	0.01
SLK8 (Before heating)	99.78	0.11	0.058	0.01
SLK8 (after oxidizing environment at 1100 °C)	99.78	0.12	0.04	0.01
SLK8 (after oxidizing environment at 1300 °C)	99.88	0.08	0.03	BDL
SLK9 (Before heating)	99.82	0.11	0.04	BDL
SLK9 (after oxidizing environment at 1100 °C)	99.81	0.11	0.04	0.01
SLK9 (after oxidizing environment at 1300 °C)	99.83	0.10	0.04	BDL
SLK10 (Before heating)	99.74	0.14	0.07	BDL
SLK10 (after oxidizing environment at 1100 °C)	99.78	0.14	0.02	0.01
SLK10 (after oxidizing environment at 1300 °C)	99.73	0.16	0.08	BDL

Remarks: BDL is below the detection limit.

Table 3. Microinclusions identified in sapphire samples through SEM-EDS analysis.

KAN2 (after oxidizing environment at 1300 °C)		Hematite (Fe₂O₃)
KAN4 (after reducing environment at 1300 °C)		Hematite (Fe₂O₃)
SLK1 (after oxidizing environment at 1300 °C)		Hematite (Fe₂O₃)
SLK4 (after reducing environment at 1300 °C)		Hematite (Fe₂O₃)
SLK4 (after reducing environment at 1300 °C)		Rutile (TiO₂)

Table 4. Fe K-edge centroid position and photon energy position (E₀).

Sample	Oxidation State	Centroid Position/eV	E₀/eV
Fe foil	Fe⁰	-
FeO standard	Fe²⁺	7112.02	7119.26
Fe₂O₃ standard	Fe³⁺	7113.28	7123.39
KAN1 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.21	7123.85
KAN1 (after oxidizing environment at 1300 °C)	Fe³⁺	7112.98	7123.75
KAN2 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.29	7123.83
KAN2 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.07	7123.88
KAN3 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.56	7123.97
KAN3 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.42	7123.83
KAN6 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.47	7123.93
KAN6 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.26	7123.91
KAN4 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.11	7123.81
KAN4 (after reducing environment at 1300 °C)	Fe³⁺	7112.80	7123.86
KAN8 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.36	7123.84
KAN8 (after reducing environment at 1300 °C)	Fe³⁺	7112.91	7123.94
KAN9 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.46	7123.84
KAN9 (after reducing environment at 1300 °C)	Fe³⁺	7112.87	7123.93
KAN10 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.34	7123.83
KAN10 (after reducing environment at 1300 °C)	Fe³⁺	7112.92	7123.85
SLK1 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.29	7123.91
SLK1 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.17	7123.99
SLK8 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.31	7123.96
SLK8 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.09	7123.94
SLK9 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.45	7123.99
SLK9 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.28	7123.98
SLK10 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.45	7123.92
SLK10 (after oxidizing environment at 1300 °C)	Fe³⁺	7113.33	7123.88
SLK2 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.16	7123.82
SLK2 (after reducing environment at 1300 °C)	Fe³⁺	7112.67	7123.91
SLK4 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.54	7123.89
SLK4 (after reducing environment at 1300 °C)	Fe³⁺	7112.66	7123.64
SLK6 (after oxidizing environment at 1100 °C)	Fe³⁺	7113.55	7123.87
SLK6 (after reducing environment at 1300 °C)	Fe³⁺	7112.89	7123.96

Table 5. Ti K-edge photon energy position (E₀).

Sample	Oxidation State	E₀/eV
Ti foil	Ti⁰	4966.00
TiO₂ standard	Ti⁴⁺	4979.95
KAN2 (after oxidizing environment at 1100 °C)	Ti⁴⁺	4979.85
KAN2 (after oxidizing environment at 1300 °C)	Ti⁴⁺	4979.85
KAN4 (after oxidizing environment at 1100 °C)	Ti⁴⁺	4979.85
KAN4 (after reducing environment at 1300 °C)	Ti⁴⁺	4979.85
SLK1 (after oxidizing environment at 1100 °C)	Ti⁴⁺	4979.85
SLK1 (after oxidizing environment at 1300 °C)	Ti⁴⁺	4979.85
SLK10 (after oxidizing environment at 1100 °C)	Ti⁴⁺	4979.85
SLK10 (after oxidizing environment at 1300 °C)	Ti⁴⁺	4979.85
SLK4 (after oxidizing environment at 1100 °C)	Ti⁴⁺	4979.85
SLK4 (after reducing environment at 1300 °C)	Ti⁴⁺	4979.85

Table 6. Derivative linear combination fitting (LCF) of all spectra from Kanchanaburi province, Thailand, and Sri Lanka, after heating at 1100 °C, 1300 °C and 1500 °C under oxidizing and reducing conditions, with references to Fe²⁺ and Fe³⁺.

Sample	FeO/%	Fe₂O₃/%
KAN1 (after oxidizing environment at 1100 °C)	0.0 ± 2.3	100.0 ± 2.3
KAN1 (after oxidizing environment at 1300 °C)	0.0 ± 3.3	100.0 ± 3.3
KAN2 (after oxidizing environment at 1100 °C)	0.0 ± 2.3	100.0 ± 2.3
KAN2 (after oxidizing environment at 1300 °C)	0.0 ± 2.2	100.0 ± 2.2
KAN3 (after oxidizing environment at 1100 °C)	0.0 ± 2.1	100.0 ± 2.1
KAN3 (after oxidizing environment at 1300 °C)	0.0 ± 2.0	100.0 ± 2.0
KAN5 (after oxidizing environment at 1100 °C)	0.0 ± 2.3	100.0 ± 2.3
KAN5 (after oxidizing environment at 1500 °C)	0.0 ± 2.4	100.0 ± 2.4
KAN6 (after oxidizing environment at 1100 °C)	0.0 ± 2.5	100.0 ± 2.5
KAN6 (after oxidizing environment at 1300 °C)	0.0 ± 2.5	100.0 ± 2.5
KAN4 (after oxidizing environment at 1100 °C)	0.0 ± 2.4	100.0 ± 2.4
KAN4 (after reducing environment at 1300 °C)	0.0 ± 1.9	100.0 ± 1.9
KAN7 (after oxidizing environment at 1100 °C)	0.0 ± 2.9	100.0 ± 2.9
KAN7 (after reducing environment at 1500 °C)	55.7 ± 2.2	44.3 ± 2.2
KAN8 (after oxidizing environment at 1100 °C)	0.0 ± 2.4	100.0 ± 2.4
KAN8 (after reducing environment at 1300 °C)	0.0 ± 2.0	100.0 ± 2.0
KAN9 (after oxidizing environment at 1100 °C)	0.0 ± 2.7	100.0 ± 2.7
KAN9 (after reducing environment at 1300 °C)	0.0 ± 2.1	100.0 ± 2.1
KAN10 (after oxidizing environment at 1100 °C)	0.0 ± 2.6	100.0 ± 2.6
KAN10 (after reducing environment at 1300 °C)	0.0 ± 2.1	100.0 ± 2.1
SLK1 (after oxidizing environment at 1100 °C)	0.0 ± 4.5	100.0 ± 4.5
SLK1 (after oxidizing environment at 1300 °C)	0.3 ± 4.6	99.7 ± 4.6
SLK7 (after oxidizing environment at 1100 °C)	0.0 ± 1.7	100.0 ± 1.7
SLK7 (after oxidizing environment at 1500 °C)	6.8 ± 1.5	93.2 ± 1.5
SLK8 (after oxidizing environment at 1100 °C)	0.0 ± 1.9	100.0 ± 1.9
SLK8 (after oxidizing environment at 1300 °C)	0.0 ± 1.9	100.0 ± 1.9
SLK9 (after oxidizing environment at 1100 °C)	0.0 ± 1.9	100.0 ± 1.9
SLK9 (after oxidizing environment at 1300 °C)	0.0 ± 1.7	100.0 ± 1.7
SLK10 (after oxidizing environment at 1100 °C)	0.0 ± 1.9	100.0 ± 1.9
SLK10 (after oxidizing environment at 1300 °C)	0.0 ± 2.2	100.0 ± 2.2
SLK2 (after oxidizing environment at 1100 °C)	3.1 ± 4.4	96.9 ± 4.4
SLK2 (after reducing environment at 1300 °C)	23.3 ± 3.7	76.7 ± 3.7
SLK3 (after oxidizing environment at 1100 °C)	0.0 ± 2.8	100.0 ± 2.8
SLK3 (after reducing environment at 1500 °C)	54.4 ± 2.0	45.6 ± 2.0
SLK4 (after oxidizing environment at 1100 °C)	0.0 ± 2.4	100.0 ± 2.4
SLK4 (after reducing environment at 1300 °C)	18.1 ± 1.2	81.9 ± 1.2
SLK6 (after oxidizing environment at 1100 °C)	0.0 ± 1.9	100.0 ± 1.9
SLK6 (after reducing environment at 1300 °C)	9.4 ± 1.3	90.6 ± 1.3

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Klomranok, C.; Sakkaravej, S.; Wongkokua, W.; Saiyasombat, C.; Monarumit, N. The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments. Crystals 2026, 16, 475. https://doi.org/10.3390/cryst16070475

AMA Style

Klomranok C, Sakkaravej S, Wongkokua W, Saiyasombat C, Monarumit N. The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments. Crystals. 2026; 16(7):475. https://doi.org/10.3390/cryst16070475

Chicago/Turabian Style

Klomranok, Chunenapa, Somruedee Sakkaravej, Wiwat Wongkokua, Chatree Saiyasombat, and Natthapong Monarumit. 2026. "The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments" Crystals 16, no. 7: 475. https://doi.org/10.3390/cryst16070475

APA Style

Klomranok, C., Sakkaravej, S., Wongkokua, W., Saiyasombat, C., & Monarumit, N. (2026). The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments. Crystals, 16(7), 475. https://doi.org/10.3390/cryst16070475

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Menu

The Blue Coloration of Natural Sapphires After Heating in Oxidizing and Reducing Environments

Abstract

1. Introduction

2. Materials

3. Methodology

4. Results

5. Discussion

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Acknowledgments

Conflicts of Interest

References

Share and Cite

Article Metrics

Article Access Statistics

Further Information

Guidelines

MDPI Initiatives

Follow MDPI