Effect of Reduction Annealing on the Coloration Mechanism of Yellow Sapphire with High Iron Content

Abstract

The color of yellow sapphire from Africa characterized by high iron content and low levels of other transition metal elements was changed from yellow to grayish-blue after high-temperature reduction annealing. Before reduction annealing, the optical absorption spectra showed that the outer d–d electron transitions of Fe³⁺ were the main coloring cause of yellow sapphires, but the charge transfer between O²⁻ and Fe³⁺ may have a greater contribution. The change in lattice parameter indicates that Fe³⁺ is reduced to Fe²⁺ during reduction annealing, and adjacent Fe²⁺ and Fe³⁺ form an Fe²⁺-Fe³⁺ ion pair. The absorption caused by intervalence charge transfer of Fe²⁺-Fe³⁺ is the essential reason for the grayish-blue appearance of yellow sapphires after reduction annealing. The charge compensation mechanism of Fe²⁺-Fe³⁺ in natural sapphire is also discussed, and oxygen vacancy is considered to be the most suitable charge compensator for Fe²⁺-Fe³⁺.

1. Introduction

The abundance of iron is highest among transition metal element; iron is also a common impurity in minerals such as corundum, beryl, tourmaline and spinel. Iron substitutes Al³⁺ ions in corundum (α-Al₂O₃), generally showing two valence states of Fe²⁺ and Fe³⁺. Both of these valence states have an important influence on the color of corundum crystals. The isolated Fe²⁺ ion has exhibits no absorption in the visible region and has a minimal effect on the color of the corundum; however, when it forms an ion pair with Ti⁴⁺ or Fe³⁺, the corundum exhibits a remarkable color. The Fe²⁺-Ti⁴⁺ heteronuclear ion pair was first proposed by Townsend [1], producing two broad intervalence charge transfer (IVCT) absorption bands at 17,800 and 14,200 cm⁻¹, respectively, giving the corundum a beautiful blue color. An Fe²⁺-Fe³⁺ homonuclear ion pair is formed by adjacent Fe²⁺ and Fe³⁺. Lehmann and Harder [2] studied natural yellow, green and blue sapphires and found that as the proportion of Fe²⁺/Fe³⁺ increased, the color of corundum gemstones gradually changed from yellow to blue. However, Lehmann and Harder [2] attributed the 11,400 cm⁻¹ (870 nm) absorption band in the optical absorption spectrum to the ⁵T₂→⁵E₂ transition of Fe²⁺. Subsequently, Faye [3] pointed out that the correct assignment of the 11,400 cm⁻¹ absorption band should be the IVCT of Fe²⁺-Fe³⁺. In the same year, Ferguson and Fielding [4] also discussed the assignment of the 11,400 cm⁻¹ absorption band in the process of studying iron-doped corundum synthesized by the flux method and attributed the absorption band to Fe²⁺-O²⁻-Fe³⁺, which was consistent with Faye’s [3] point of view. Ferguson and Fielding [4] speculated that the charge compensation of Fe²⁺-Fe³⁺ in the corundum synthesized by flux method was completed by F⁻, as suggested by Krebs [5]. Ferguson and Fielding [6] also proposed that the charge compensation would be effected by the creation of vacancies in the oxygen sublattice or additional interstitial Al³⁺ ions in the absence of foreign ions in natural corundum gemstones. Therefore, it is still important to study the charge compensation mechanism of Fe²⁺-Fe³⁺ in natural corundum gemstones. In addition to Fe²⁺-Fe³⁺ and isolated Fe³⁺ ions, Fe³⁺ also interacts with adjacent Fe³⁺ and O²⁻ in corundum. Exchange-coupled Fe³⁺ pairs have been shown to have an important effect on the intensity of the ⁴A₁, ⁴E^a and ⁴E^b bands of isolated Fe³⁺ [5]. The charge transfer between O²⁻ and Fe³⁺ causes three absorption bands in the ultraviolet region of the optical absorption spectrum of synthetic corundum doped with low-content iron (0.02 at% Fe³⁺, flux-grown) [7]. The tails of the absorption bands extend even further to the violet region of the visible region as the Fe³⁺ content increases [8]. Therefore, when studying the coloring mechanism of yellow sapphire with high iron content, the charge transfer absorption of O²⁻ and Fe³⁺ should also be considered.

Yellow is an important color type of natural corundum gemstones, and natural yellow sapphires are produced in various mining areas around the world, such as Myanmar, Sri Lanka, Madagascar, Tanzania, Mozambique, etc. [9,10,11,12]. It is well-known that pure corundum (α-Al₂O₃) is inherently colorless and exhibits different colors when various transition metal ions replace Al³⁺ ions [7,13]. The coloration of iron in corundum is relatively complex, and it also easily interacts with other kinds of transition metal ions in natural gemstones. In our research, we obtained a class of yellow sapphires from unknown mining areas in Africa characterized by high iron content and very low levels of other transition metal elements (Figure 1). This type of yellow sapphire is suitable for studying the coloration of iron ions in corundum. In 1993, Fritsch [14] reported two grayish-blue sapphires from Rwanda. The color of these two sapphires was mainly caused by the optical absorption of IVCT of Fe²⁺-Fe³⁺, so he proposed that Fe²⁺-Fe³⁺ is the second coloring mechanism of blue sapphire, in addition to Fe²⁺-Ti⁴⁺. In this paper, the use of reduction annealing to form new Fe²⁺-Fe³⁺ ion pairs in yellow sapphire with high iron content is further investigated with respect to Fritsch’s viewpoint. To the best of our knowledge, research on the formation of Fe²⁺-Fe³⁺ in natural sapphire has not been reported to date. Furthermore, the effect of reduction annealing on the coloration mechanism of yellow sapphire is discussed in this paper.

2. Materials and Methods

Yellow sapphire samples were provided by Lin Shaobo (Thailand, Bangkok) from an unknown mining area in Africa and characterized by high iron content and low levels of other transition metal elements. Three oriented yellow sapphire samples were cut, ground and polished on two sides parallel to the c axis of the crystal with a thickness of 1.80, 1.93 and 0.95 mm, respectively. Annealing was carried out twice in a silicon molybdenum rod electric furnace using a reducing atmosphere produced by carbon powder, and the temperature was kept at 1450 °C for 4 h. The sapphire slices were brownish-yellow before reduction annealing and grayish-blue after reduction annealing.

Elemental analysis was performed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on a Resolution M50 deep ultraviolet 193 nm ArF excimer laser ablation instrument from Resonetics, USA, and an Agilent 7900 ICP-MS. Three points were selected for each sample for testing. The diameter of the ablated beam spot was 60 μm, and the signal acquisition time was 60 s. The element content is reported as the average of three measured points.

The optical absorption spectrum and polarized absorption spectrum were scanned with step Δλ = 1 nm in the range of 300–1600 nm with a PerkinElmer Lambda 950 UV-vis-NIR spectrometer with a 150 mm integrating sphere and small spot-focusing attachments. Polarized spectra were acquired using a PE Model B0505284 polarizer matched Lambda 950. The transmission method was used to test the absorbance of the sapphire slices. The slit width of the instrument was set to 2 nm across the entire studied range. Peakfit v4.12 software was used for spectral decomposition, and nonlinear Gaussian fitting was applied. The XRD pattern was recorded on a Philips X’pert PRO X-ray diffractometer equipped with Cu K_α radiation. The diffraction data of 2θ were collected from 10° to 90° with a scan step of 0.026°. The crystal structure of sapphire samples was refined in terms of Rietveld analysis.

3. Results and Discussion

3.1. Transition Metal Elements

The analysis results of transition metal elements of yellow sapphires are shown in

Table 1. Test results of transition metal elements of yellow sapphire (ppm wt).

Oxide		TiO2	V2O3	Cr2O3	MnO	T-Fe2O3	CoO	NiO	CuO	Ga2O3
Content	YS-1	9.2	2.3	27.1	<1	10,425	<1	<1	<1	11.5
	YS-2	11.3	1.9	21.6	<1	11,018	<1	<1	<1	12.3
	YS-3	8.8	2.6	16.4	<1	10,017	<1	<1	<1	11.2

. All transition metal elements are represented in the form of oxides, and T-Fe₂O₃ represents the total content of FeO and Fe₂O₃. The content of iron oxide was the highest, at approximately 1 wt.% (10,017 ppm); titanium dioxide, chromium oxide and vanadium trioxide were present at a concentration of 10 ppm or lower; and the content of manganese, nickel, cobalt and other elements was lower than the detection limit of the instrument.

3.2. Reduction Annealing

The three polished slices of yellow sapphires were annealed twice in a reducing atmosphere, each annealing at a temperature of 1450 °C for 4 h. The color of the yellow sapphire slices changed from yellow to grayish-blue after reduction annealing (Figure 2). The thickness of YS-1, YS-2 and YS-3 slices was 1.80, 1.93 and 0.95 mm, respectively. The color of the slice labeled YS-3 was the best after reduction annealing twice. Thickness and polishing quality may have an effect on the color rendering of sapphire slices. One scale of the straightedge placed below the samples in Figure 2 as a reference represents 1 mm.

3.3. Optical Spectroscopy

The analysis of optical absorption spectra of the three yellow sapphire slices before and after reduction annealing is exemplified by slice YS-3 with the best color effect.

Before reduction annealing, the optical absorption spectrum of yellow sapphire mainly has three relatively sharp absorption peaks at 377, 387 and 453 nm (Figure 3a). These three absorption peaks are assigned to the d–d electron transitions: ⁶A₁→⁴E(⁴D), ⁶A₁→⁴T₂(D) and ⁶A₁→⁴E₁⁴A₁(⁴G) of Fe³⁺ [12,15]. The d–d electron transitions, ⁶A₁→⁴E(⁴D) and ⁶A₁→⁴E₁⁴A₁(⁴G), are temperature- and concentration-dependent, and their absorption is enhanced by exchange-coupled Fe³⁺-Fe³⁺ ion pairs [5]. These absorption peaks have a strong absorption background as a result of charge transfer absorption between O²⁻ and Fe³⁺ [7], similar to the decomposition spectrum shown in Figure 3b. Although the peak of charge transfer absorption between O²⁻ and Fe³⁺ is located in the ultraviolet region, its tail extends to the visible light region in the absorption spectrum of the yellow sapphire sample, with an important influence on the coloration of sapphire. Therefore, the color of yellow sapphire with high iron content is mainly contributed by the charge transfer absorption of O²⁻ and Fe³⁺ and the absorption of the d–d electron transition of Fe³⁺.

In addition to the three absorption peaks of Fe³⁺, a new strong, broad absorption band with a peak at ~855 nm appears in the near-infrared region in the optical absorption spectrum (Figure 3a) after reduction annealing, which is similar to the absorption caused by the IVCT of Fe²⁺-Fe³⁺ [14,16]. The optical absorption spectrum of YS-3 after reduction annealing was fitted, as shown in Figure 3b. The intensity and full width at half maximum (FWHM) of each absorption band obtained by fitting are shown in

Table 2. Intensity and FWHM of absorption bands obtained by fitting.

Band center / cm−1	26,550	25,840	22,170	11,690
Absorption coefficient / cm−1	1.780	2.887	3.223	3.249
FWHM / cm−1	683	1,077	1,492	6,153

. An absorption band at 855 nm (11,690 cm⁻¹) observed in blue sapphire from Thailand was mentioned as early as Lehmann and Harder’s article [2] but was assigned to Fe²⁺. Iron-doped corundum crystals grown by the flux method showed uneven blue color [5], and the optical absorption spectra of these crystals also had a strong absorption band (11,400 cm⁻¹) in the near-infrared region but was assigned to Fe²⁺ by the author, which is consistent with the viewpoint of Lehmann and Harder [2]. In the same year, Ferguson and Fielding [4] pointed out that this band should be attributed to Fe²⁺-O²⁻-Fe³⁺ instead of Fe²⁺. Then, in 1993, Fritsch [14] observed this absorption band in two grayish-blue sapphires from Rwanda and proposed that Fe²⁺-Fe³⁺ is the second coloring mechanism, in addition to Fe²⁺-Ti⁴⁺, that makes sapphire blue. In the polarized absorption spectra (Figure 3c), the intensity of the absorption band at 855 nm is greater in the E⊥C direction than in the E∥C direction, indicating that it is polarized along the vector between the interacting cations. Large band half-width, intense polarization dependence along the metal-to-metal bond direction and high absorbance are features of the 855 nm absorption band, consistent with the identifying characteristics of the IVCT absorption band proposed by Mattson and Rossman [17]. Therefore, it can be inferred that the 855 nm absorption band is caused by the intervalence charge transfer of Fe²⁺-Fe³⁺, meaning that Fe²⁺-Fe³⁺ is newly formed in sapphire samples during the reduction annealing process and causes the color of sapphire to change from yellow to grayish-blue.

The center position of the IVCT absorption band of Fe²⁺-Fe³⁺ differs depending on the reference: for example, 11,400 cm⁻¹ (877 nm) [2], 11,300 cm⁻¹ (885 nm) [5] and 890 nm [14]. In α-Al₂O₃ crystals, adjacent [AlO₆] octahedrons have two relative positional relationships: a face-sharing relationship in the direction parallel to the c axis and an edge-sharing in relationship in the direction perpendicular to the c axis. The distance between Al³⁺ and Al³⁺ ions is not equal in all directions. The calculation results show that the IVCT energies of Fe²⁺-Fe³⁺ are 1.43 ev (867 nm) for face sharing and 1.30 ev (953 nm) for edge sharing [18]. Therefore, the IVCT absorption band of Fe²⁺-Fe³⁺ should have two different center positions, similar to the IVCT absorption band of Fe²⁺-Ti⁴⁺. We hypothesize that this is the reason why the center position of the IVCT absorption band of Fe²⁺-Fe³⁺ is differs depending on the reference.

In addition to the absorption caused by IVCT of Fe²⁺-Fe³⁺ and charge transfer between O²⁻ and Fe³⁺, there are two regions in the fitted spectrum with no further fitting, represented by the dashed line in Figure 3b. First, yellow sapphire contains a small amount of titanium, some of which can be attributed to the IVCT absorption band of Fe²⁺-Ti⁴⁺. This means that Fe²⁺ formed during reduction annealing interacts with both Fe³⁺ and Ti⁴⁺. Second, yellow sapphire also contains a minute amount of Cr³⁺ and Ti³⁺, which may form during the reduction annealing process. Therefore, the unfitting absorption of the area marked by the dashed line in Figure 3b may be caused by Fe²⁺-Ti⁴⁺, Cr³⁺ and Ti³⁺ [1,19,20]. Third, these unfit absorption regions may also be co-contributed by F, F⁺, F₂, ${\mathrm{F}}_2^{+}$ and ${\mathrm{F}}_2^{2+}$[21]—a possibility that cannot be ruled out.

3.4. Analysis of Lattice Parameter

The space symmetry group of corundum crystal (α-Al₂O₃) is R$\bar{\mathit{3}}$c (No.167), and the lattice parameter is a(Å) = 4.76 095, c(Å) = 12.9 962. Figure 4 shows the results of the crystal structure refined in terms of Rietveld analysis of YS-3 before and after reduction annealing, where △ represents the difference between the measured and calculated intensities of XRD patterns. All of the diffraction peaks of sapphire YS-3 are in agreement with those of pure α-Al₂O₃ (ICSD #10-0173), that is, the structure of yellow sapphire did not change before and after reduction annealing (

Table 3. Diffraction data of yellow sapphire from Rietveld refinement results.

(h k l)	Before Reduction Annealing		After Reduction Annealing
	d/nm	2θ/o	d/nm	2θ/o
(0 1 12)	0.34906	25.497	0.34891	25.508
(1 0 4)	0.25553	35.088	0.25547	35.098
(1 1 0)	0.23833	37.713	0.23829	37.72
(1 1 3)	0.20829	43.409	0.20907	43.239
(0 2 4)	0.17414	52.505	0.17411	52.516
(1 1 6)	0.16041	57.398	0.16019	57.481
(2 1 1)	0.15497	59.611	0.15486	59.658
(0 1 8)	0.15118	61.264	0.15127	61.221
(2 1 4)	0.14073	66.369	0.14062	66.429
(3 0 0)	0.13755	68.111	0.13746	68.164
(1 2 5)	0.13353	70.462	0.13373	70.338
(1 0 10)	0.12406	76.766	0.12402	76.796
(1 1 9)	0.12343	77.23	0.12358	77.116
(2 2 0)	0.11914	80.564	0.11908	80.611
(3 0 6)	0.11611	83.119	0.11598	83.236
(2 2 3)	0.11486	84.228	0.1148	84.289
(0 2 10)	0.11001	88.88	0.10996	88.934

). The average fitted lattice parameters of YS-3 are a(Å) = 4.76 502, c(Å) = 13.0 038 before reduction annealing and a(Å) = 4.76 245, c(Å) = 13.0 017 after reduction annealing. Compared with those of pure α-Al₂O₃, the average lattice parameters of YS-3 are larger as a result of the substitution of Al³⁺ by Fe²⁺ and Fe³⁺ having a larger ionic radius (

Table 4. The state and ionic radius of iron ions in corundum (α-Al₂O₃).

Valence State	Coordination Number	Crystal Field State	Spin State	Ionic Radius (nm)
Fe3+(3d5)	6	weak-field	high-spin	0.065
Fe2+(3d6)	6	strong-field	low-spin	0.061

).

The ionic radius of Fe³⁺ in a six-coordinate octahedral crystal field is 0.055 nm, corresponding to the LS state, whereas a radius of 0.0645 nm corresponds to the HS state [22]. In the octahedral crystal field, D_q/B represents the intensity of the crystal field. D_q is one-tenth of splitting energy, and B is the Racah parameter in the crystal field theory. D_q/B > 2.5 is considered a strong field, and D_q/B < 2.5 is a weak field [23]. The optical absorption spectrum of α-Al₂O₃:Fe³⁺ crystal shows that D_q = 1510cm⁻¹, B = 660 cm⁻¹ and D_q/B = 2.32 < 2.5, so Fe³⁺ is in a weak field and takes a high-spin state [5]. Therefore, the ionic radius of Fe³⁺ should be 0.0645 nm instead of 0.055 nm [22]. Fe²⁺ is generally in a strong field and a low-spin state, and its ionic radius is 0.61 nm [22,23] (Table 4). Refinement results of single-crystal X-ray diffraction data of YS-3 show that lattice distortion is reduced after reduction annealing, so it can be inferred that a portion of Fe³⁺ is reduced to Fe²⁺ during the reduction annealing process. Furthermore, the ionic radius of iron is changed from 0.065nm (Fe³⁺) to 0.061nm (Fe²⁺). Then, Fe²⁺-Fe³⁺ and Fe²⁺-Ti⁴⁺ are generated by adjacent Fe²⁺ and Fe³⁺ and by Fe²⁺ and Ti⁴⁺, respectively, which is consistent with the test results of the above-mentioned optical absorption spectra.

3.5. Charge Compensation

The positive and negative charges in corundum crystal must be balanced. In the Fe²⁺-Fe³⁺ ion pairs, a charge imbalance occurs after Fe²⁺ replaces Fe³⁺. Therefore, the formation of Fe²⁺-Fe³⁺ is inevitably accompanied by the generation of a new charge compensator. Ferguson and Fielding [4] proposed that the charge compensation mechanism for corundum crystals synthesized by flux method with PbF₂ and B₂O₃ as fluxes is F⁻ substitution of O²⁻; however, this explanation does not apply to synthetic corundum with PbO and B₂O₃ as fluxes. Nikolskaya [24] discussed the charge compensation mechanism for Fe²⁺ at the Al³⁺ site in natural sapphire: (i) oxygen vacancy (V_O), (ii) H⁺ or Ti⁴⁺ and (iii) interstitial ion ${\mathrm{Fe}}_{\mathrm{i}}^{2+}$. The H in corundum may be present when the crystal is formed, or it may be introduced through the heat treatment process [25]. H forms OH groups with neighboring O, which can sometimes be observed via IR spectroscopy, and the introduction and escape of OH groups are reversible [26,27]. The IR spectra of OH in corundum are very complex as a result of many factors, such as the lattice occupancy of H and its adjacent transition metal ions [26,27,28,29]. Some scholars have proposed that the critical absorption peak of OH at 3310 cm⁻¹ is associated with redox reactions involving iron, whereas more studies have shown that this infrared absorption peak is mainly related to titanium ions or defect clusters containing titanium ions [27,28,30,31,32]. The infrared absorption peaks of OH are not obvious with lower concentrations of titanium is [32]. The titanium content was also very low in the African yellow sapphire samples we tested. Furthermore, only trace amounts of H+ are present in the form of OH- in natural corundum gemstones [30]. In addition, the crystal lattice structure of the corundum crystal is tight, and Fe²⁺ cannot easily exist in the form of interstitial ions. Our heat treatment experiments were carried out in a reducing atmosphere created with carbon powder without the use of hydrogen. Therefore, we propose that the oxygen vacancies should be the most important charge compensator for Fe²⁺ in high-iron-content African yellow sapphire after reduction annealing. Xiang [33] studied the stability of each defect in corundum under oxygen-poor conditions and reported the order as VO > VAl > Ali > Oi > AlO > OAl, supporting our inference. The reactions that occur during reduction annealing are shown in Formulas (1) and (2):

$${\mathrm{O}}_{\mathrm{O}}^{\mathrm{X}}\leftrightharpoons \mathrm{V}\mathsf{ö}+2\mathrm{e}+\frac{1}{2}{\mathrm{O}}_2\uparrow$$

(1)

$$2({\mathrm{Fe}}_{\mathrm{Al}}^{3+}-{\mathrm{Fe}}_{\mathrm{Al}}^{3+})+\mathrm{V}\mathsf{ö}+2\mathrm{e}\text{⇋}({\mathrm{Fe}}_{\mathrm{Al}}^{2+}-{\mathrm{Fe}}_{\mathrm{Al}}^{3+}{)}^{\prime }-\mathrm{V}\mathsf{ö}-({\mathrm{Fe}}_{\mathrm{Al}}^{2+}-{\mathrm{Fe}}_{\mathrm{Al}}^{3+}{)}^{\prime }$$

(2)

Under reducing conditions, oxygen vacancies V_O are the most suitable charge compensators formed during the reduction annealing process, possibly forming a (Fe²⁺-Fe³⁺)$-$V_O$-$(Fe²⁺-Fe³⁺) defect cluster with Fe²⁺-Fe³⁺ ion pairs; therefore, it is beneficial for the Fe²⁺-Fe³⁺ ion pair to remain stable.

4. Conclusions

In African yellow sapphires with high iron content, the absorption of charge transfer between O²⁻ and Fe³⁺ and d–d electron transitions of Fe³⁺ are the two main causes of the sapphire’s yellow appearance. After high-temperature reduction annealing, a portion of Fe³⁺ in yellow sapphire is reduced to Fe²⁺; then, Fe²⁺-Fe³⁺ ion pairs are formed by the adjacent Fe²⁺ and Fe³⁺. The oxygen vacancy (V_O_ is a charge compensator of Fe²⁺-Fe³⁺ ion pairs in natural sapphire, forming a (Fe²⁺-Fe³⁺)$-$V_O$-$(Fe²⁺-Fe³⁺) defect cluster with Fe²⁺-Fe³⁺, which helps to maintain the stability of Fe²⁺-Fe³⁺. The IVCT absorption band of Fe²⁺-Fe³⁺ is centered at 855 nm in the near-infrared region and is characterized by a large band half-width, intense polarization dependence along the metal-to-metal bond direction and high absorbance. The absorption of IVCT of Fe²⁺-Fe³⁺ covering the entire visible region and the absorption of charge transfer between O²⁻ and Fe³⁺ are the two main causes of the grayish-blue appearance of African yellow sapphires after reduction annealing.

Although the 855 nm band has been studied for decades, scholars have differing views. In the early studies, some scholars held the viewpoint that the absorption band at 810 nm in beryl was generated by the octahedral site Fe²⁺ and once believed that the band near 855 nm in corundum was also generated by the spin-allowed transition of Fe²⁺. Recently, some scholars used this viewpoint to explain the coloring mechanism of green sapphire. In recent years, few articles have been published focusing on the study of Fe²⁺-Fe³⁺ ion pairs in sapphire, and the misassignment of the 855 nm band is still an issue. Through our heat treatment experiment of natural sapphire with high iron content, we observed the color change of sapphire before and after heat treatment and the appearance of a strong absorption band at 855 nm, effectively supporting the viewpoint proposed by Fritsch in 1993 from the perspective of experimental data and actual phenomena. In addition, Fe³⁺ in corundum is in a weak field and takes a high-spin state, so its ion radius should be 0.065 nm, not 0.055 as commonly cited.

We know that dark-blue sapphires in many mines, such as Changle in Shandong Province, Thailand, and Australia, have a very high iron content, and their color is mainly caused by Fe²⁺-Fe³⁺. Over the years, the value of dark-blue sapphire has not been fully developed due to the lack of superior color improvement processes. Our study of the reduction annealing of yellow sapphire with high iron content may provide a useful reference with respect to color improvement of these dark-blue sapphires.