Characterization of Red, Pink, Orange, and Purple Gem-Quality Spinel from Four Important Areas

Abstract

Spinel is a precious stone with a long history. In ancient societies spinel was considered to be an imitation of ruby. With the depletion of ruby mineral resources, gem—grade spinel has attracted more and more attention from consumers. In the last decade, as the popularity of spinel in the global colored gem market continues to rise, plenty of domestic and foreign jewelry brands have launched spinel based jewelry. This study takes spinels from Burma, Vietnam, Sri Lanka, and Tanzania as its research objects and performs a series of tests to obtain their gemological characteristics, spectral characteristics, and chemical composition, with the aim of comparing the differences between spinels with different colors from different areas and exploring the chromogenic mechanism of spinels. Only Burmese red spinels have a typical Cr spectrum. The types of inclusions and the contents of trace elements are the main differences between spinels from the four areas. Burmese spinel is characterized by an octahedral negative crystal filled with dolomite or a mixture of dolomite and calcite. Magnesite is present in Sri Lankan spinels, and dolomite is present in Tanzanian spinel. Dislocation systems and the presence of titanite and talc inclusions are strongly indicative features of Vietnamese spinel. Sri Lankan spinel is characterized by abundant gas–liquid inclusions, such as the beaded healing fissure. The trace element contents of the four areas are different. Burmese spinel is poor in Fe and Zn (Fe: 135.68–3925 ppm; Zn: 338.58–1312 ppm), while Burmese red spinel is rich in Cr (up to 7387 ppm). Vietnamese spinel is rich in Fe (3669.63–19,425 ppm) and poor in Ti content (<89 ppm), while Tanzanian spinel is rich in Zn (5129.96–7008 ppm). High content of Cr + V can lead to the red color in spinel, and the contents of Cr and V change obviously with color. Spinels appear red when Cr content is higher than V, while spinels appear orange when V content is higher than Cr. The red, pink, and orange spinels are colored by Cr³⁺ and V³⁺, showing a wide absorption band centered at 400 nm and 550 nm. Fe plays a dominant role in purple spinels. The purple spinel is colored by Fe³⁺ and Fe²⁺.

1. Introduction

Gem-quality spinel (Figure 1) is a kind of precious stone with a long history. In ancient societies, the red gem-quality spinel was confused with ruby for a long time. With the gradual depletion of ruby mineral resources, gem-quality spinel has attracted increased attention from researchers and consumers [1].

Currently, more research has been conducted on spinel from Burma than from other regions [2,3]. Studies of Burmese spinel initially focused on mineralogical, gemological, and chemical features [4,5]. Particularly, many studies are conducted on the inclusions of Burmese spinels [6,7,8]. With the progress of science and technology, the optimized treatment technique and synthetic technique of spinels gradually matured [9,10]. Heat treatment is the most common method of optimized treatments. People have begun to explore a variety of heat treatment processes and synthetic processes [1,11,12,13], as well as the identification of optimized spinel, synthetic spinel, and natural spinel [1,14,15]. Research on Vietnamese spinel focused on the coloring mechanism of blue and purple spinel [16]. Researchers have always focused on how to identify cobalt-diffused blue spinel and natural blue spinel [17]. So far, little research has been conducted on Sri Lankan spinel, as researchers have focused more on corundum rather than spinel. Tanzania is rich in pink spinel, especially in 2007, when huge pink spinel crystals were discovered in the Mahenge area. However, much attention has focused on trade and commerce.

The chemical composition of spinel group minerals is relatively complex, and the chemical formula is RR’₂O₄ or RO·R’₂O₃, where R is a bivalent cation and R’ is a trivalent cation. When R occupies 1/8 of the tetrahedral (T) site and R’ occupies 1/2 of the octahedral (M) site, this spinel is called positive spinel. If R is distributed in the octahedral (M) site, with half of R’ in the tetrahedral (T) site and the other half in octahedral (M) sites, then this class of spinel is called anti-pinel. Mg²⁺, Fe²⁺, Zn²⁺, Mn²⁺, and other bivalent cations can occupy the position of R, Fe³⁺, Cr³⁺, and V³⁺; and other trivalent cations can occupy the position of R’. According to the types of these cations, spinel is divided into many subspecies: magnesium aluminum spinel, zinc spinel, chromium spinel, iron spinel, manganese spinel, and so on [18].

The purpose of this paper is to explore the correlation between the gemological characteristics, spectral characteristics, and chemical composition of the red, pink, orange and purple spinel from Burma, Vietnam, Sri Lanka and Tanzania, summarize the characteristics of spinel from each area, and compare the differences between spinel from each area. At the same time, the paper explores the color mechanism, obtains the effective information of spinel origin identification, in order to supplement the content of different origin characteristics of spinel, and provide a theoretical basis for the initial establishment of origin database.

2. Gemological Settings

Presently, more than 1000 kinds of spinel minerals are found in the world, and their production is distributed all over the world. However, the area of gem-quality spinel is quite rare, and most of the spinels in the market are crystal specimen-grade ores. Spinel production countries include Burma, Vietnam, Thailand, Tanzania, Tajikistan, Sri Lanka, and Afghanistan. Carbonate inclusions have been found in spinel deposits in Burma, Sri Lanka, and Tanzania, which are related to the formation of spinel deposits. Gem-quality spinel is mainly distributed in East Africa, Central Asia, and Southeast Asia. Most of the deposits occur in amphibolite–granulite facies marble.

2.1. Mogok, Burma

Burma is located in Southeast Asia, and Mogok region is the most important spinel producing area of Burma in the twentieth century. In this study, Burmese spinels are from Mogok. Spinels were mainly found in the “Mogok metamorphic belt” (MMB) located in Mandalay Province (Figure 2a) [19,20]. Regional metamorphism of these marbles occurred during the Indosinian and Himalayan orogeny 250–240 million years ago [21,22,23]. The Himalayan mountain belt was formed 45 million years ago during the Tertiary collision of the Indian Plate northward into the Eurasian Plate. This geologic activity resulted in massive marble units containing ruby, spinel, phlogopite, muscovite, scapolite, margarite, titanite, pyrite, and graphite [9,20,24,25]. The marble units consist of discontinuous horizons up to 300-m thick, interbedded with calc-schists or sillimanite-bearing gneisses [26,27]. They are generally coarsely crystallized and typically consist of pure white granite dikes and small syenite. They are usually coarse crystallized and are typically pure white granite dikes and small syenite in color. MMB was formed during the Indo-Eurasian collision and is known for its high-temperature plastic deformation [3,28,29]. The spinel deposits in Mogok were enclosed in the Mogok metamorphic zone and consist of metamorphic platform carbonate sedimentary series [30]. Spinels were found in almost all primary and secondary corundum deposits in Mogok. The negative octahedral crystals filled with dolomite or calcite and dolomite are the diagnostic characteristics of Burmese spinel.

2.2. Luc Yen, Vietnam

The spinel deposits in Vietnam are mainly found in the Luc Yen area in the northern Yen Bai Province, around the Cenozoic Day Nui Con Voi metamorphic zone and the Red River shear zone (Figure 2b) [31]. The amphibolite facies metamorphism occurred in this area, with a metamorphic temperature and metamorphic pressure of 780 °C and 7 kbar, respectively [32]. The area contains three types of typical deposits with economic value, namely, metamorphic–metasomatic deposit, magmatic pegmatite deposit, and alluvial placer. Placer deposits are now the main source of all gem minerals [26]. Sapphire, garnet, sillimanite, quartz, and spinel coexist in placers. Spinel deposits are found in marble units that have undergone intense metamorphism [33]. The local metamorphism is conducive to the formation of gemstone deposits with a relatively complex mineral assemblage of dolomite, calcite, clinohumite, forsterite, tschermakite, and clinochlore [34].

2.3. Sri Lanka

Ancient metamorphic rocks have been eroded to form the Sri Lankan gem placers that are rich in gem minerals. Due to physical and chemical weathering, gems were redeposited along hillsides or riverbeds after being released from the parent rock [35]. In addition to the northwest coastal zone covered by Miocene calcareous rocks, Sri Lanka is mainly dominated by Precambrian metamorphic rocks, which can be divided into three parts, namely, the Plateau Group, Vega Yang Group, and Southwest Group (Figure 2c) [36,37]. Most gemstone deposits including spinels are located in the Plateau Group. The Sri Lankan plains are topographically divided into the first, second, and third peneplain from low to high by the height of the sea [37]. Most of the gems are located in the second peneplain [38]. Precambrian rocks in Sri Lanka were metamorphosed under the granulite facies. Spinels are mined in the gem district in the north of Elahera, and the parent rock is a 12 × 35-m-wide skarn body of the Bakamuna Zone in the Plateau Group [38,39,40]. Skarns are formed in the contact zone between a pegmatite dyke and marbles. The inner pegmatite zone is sterile. The intermediate zone contains fine-grained spinel and scapolite aggregates with occasional corundum and phlogopite. Although having a similar composition to the intermediate one, the outer zone is coarse grained and contains porphyroblasts of corundum and spinel [38]. Spinels coexist with sapphire, tourmaline, and zircon in residual deposits [41]. Magnesite inclusions could appear in spinel as a carbonate mineral.

2.4. Tanzania

Tanzania is located in East Africa, and spinel deposits are mainly distributed in the southern region (Figure 2d). In the 1980s, spinel was first discovered in Morogoro Province, near Matombo and Mahenge [42]. In the late 1990s, a large alluvial deposit was discovered in the Tunduru region of southern Tanzania, near Mozambique. The Tunduru region has now become a major source of spinel. In 2000, high-quality spinel crystals were found in marble deposits in the Ipanko region of Mahenge [43,44]. Since then, the number of spinels has been rising. As a result of the Pan-African orogeny 750–450 million years ago, spinels occur in marble and calc-silicate rocks [45,46]. Marble is composed of calcite and dolomite. The dolomite becomes the primary inclusion in the spinel when the spinel crystals capture the dolomite during growth. Spinels in Mahenge were formed through contact metamorphism of magma intruding into limestone or dolomite, and some may also occur in aluminum-rich mafic magmatic rocks [45,47,48].

3. Materials and Methods

3.1. Materials

The spinel samples studied in this study come from four areas, namely, Burma, Vietnam, Sri Lanka, and Tanzania (Figure 3). Shenzhen Guild Technology Co., Ltd. (Shenzhen, China) provided all samples.

A total of 18 Burmese spinel samples were obtained from Mogok, numbered B-1–B-18. The samples include red (B-11–B-18), pink (B-3, B-7, B-8, B-10), orange (B-2, B-5, B-6), and purple (B-1, B-4, B-9). B-6 and B-14 can be recognized as octahedral crystals (Figure 4a). B-13 is a water-eroded pebble with a matte surface. The rest are irregular fragments. All the samples except the red samples have a smooth polished surface. All red samples are subtranslucent and have weak glassy luster, while the rest are transparent and have bright glassy luster. The surfaces of B-13, B-14, B-15, and B-18 were polished, which were convenient for basic gemological and spectroscopy analysis.

A total of eight Vietnamese spinel samples are numbered V-1–V-8. The samples include pink (V-1, V-3, V-4, V-5, V-8), orange (V-6), and purple (V-2, V-7). V-3, V-5, and V-6 are octahedral crystals (Figure 4b), while the rest are irregular fragments with smooth polished surfaces. All samples are transparent and have bright glassy luster.

A total of five Sri Lankan spinel samples are numbered S-1–S-5. The samples include pink (S-2, S-3, S-5) and purple (S-1, S-4), which are transparent and have bright glassy luster. S-1–S-3 are round brilliant cut, while S-4 and S-5 are cushion brilliant cut (Figure 4c).

A total of two Tanzanian spinel samples are numbered T-1 and T-2. The samples are pink, transparent and have bright glassy luster. T-1 is oval brilliant cut (Figure 4d), and T-2 is triangle brilliant cut.

3.2. Methods

The basic gemological analysis, gemstone microscopic observation, infrared spectrum analysis, and UV—visible spectrum analysis of this study were completed in the Gemological Experimental Teaching Center of School of Gemology, China University of Geosciences (Beijing, China). The EPMA analysis was completed in the EPMA Laboratory of the Experimental Center of the Research Institute of China University of Geosciences (Beijing, China). The LA-ICP-MS analysis was performed at the Mineral Laser Microprobe Analysis Laboratory, China University of Geoscience (Beijing, China).

A polariscope, refractometer, Chelsea color filter (CCF), grating spectroscope, ultraviolet (UV) fluorescent lamp, gem microscope, and specific gravity meter were used in basic gemological analysis. The magnification of gem microscope (Baoguang Technologies, Nanjing, China) is 10–40 times, which is mainly used to observe the surface and internal characteristics of the samples.

Infrared spectra were collected using a Bruker Tensor 27 Fourier Infrared spectrometer with scanning time of 32 s, resolution of 4 cm⁻¹, and wavenumber between 400 and 2000 cm⁻¹. The sample was investigated using a reflection method.

In the UV-visible spectrum analysis, due to the low transparency of sample B-11–B-18, reflection method was adopted, and a fiber optic spectrometer (GEM-3000) was used for analysis. The test range is 200–1000 nm, the integration time is 210 s, and the smoothing width is 2 nm. The other transparent samples were tested via the transmission method, using Shimadzu UV-3600 UV-visible spectrophotometer. The test range is 200–1000 nm, the conversion wavelength of the light source is 300 nm, the detector conversion wavelength is 850 nm, the grating conversion wavelength is 850 nm, and the time constant is 0.1 s.

Raman spectra were collected using Horiba HR Evolution-type micro-confocal laser Raman spectrometer, which ranged from 200 to 4000 cm⁻¹. The laser source was 532 nm and there were 3 acquisition cycles. The exposure time per scan was 20 s.

ZAF3 quantitative analysis method was used for the EPMA analysis. The acceleration voltage is 15 kV, the beam current is 20 nA, and the beam spot diameter is 5 μm.

In LA-ICP-MS analysis, the laser ablation system was NewWave193 UC and the quadruple rod mass spectrometer was Agilent 7900. Helium was used as carrier gas in ablation system. Spot size was 50 µm, repetition rate was 6 Hz, and ablation time was 90 s. Multiple external standards, including NIST 610, BCR-2G, and internal standard (Ca), were used for quantitative calculation.

4. Results

4.1. Gemstone Microscopic Observation

Burmese spinel B-13 has a long crack on the accessible surface (Figure 5a). B-14 is an octahedral crystal with partially damaged and unsharp edges, with many clear etching marks and irregular growth patterns on the surface (Figure 5b). B-3 has a raised triangular growing base on the surface (Figure 5c). B-16 has a triangular etching on the surface (Figure 5d). Spinel from other areas do not have very obvious surface characteristics.

Scattered octahedral crystal inclusions of different sizes are found in Burmese spinel B-3. The clear octahedral outline (Figure 6a) and colorless transparent short columnar inclusion with relatively complete crystal shape and clear edges (Figure 6b) can be seen.

The presence of dislocation systems is the most striking inclusion feature of spinels from Luc Yen [32,49]. These dislocation systems appear as intersecting bands of iridescent parallel-oriented channels (Figure 7a) in Vietnamese spinel V-2. A colorless transparent columnar crystal shaped like a “finger” is also found in V-3 (Figure 7b). Many round gas–liquid inclusions and round crystals are found in V-5 (Figure 7c). Long beaded inclusions are found in V-7 and V-8, which are composed of gas–liquid and crystal inclusions (Figure 7d). Stress cracks around the crystals can also be seen.

Abundant crystals and gas–liquid inclusions are found in Sri Lankan spinel S-4. Two rows of similar size cuboid colorless transparent crystals can be seen (Figure 8a). Moreover, straight lines connect between each row of crystals. The beaded healing fissure is too small to determine whether it consists of an octahedral negative crystal or a gas–liquid inclusion (Figure 8b).

Two colorless transparent crystals with smooth outlines surrounded by many small colorless transparent crystals are found in Tanzanian spinel T-1 (Figure 8c). The crystal in T-2 is shaped like a “boot,” with white stripes that appear at intervals in the crystal (Figure 8d).

4.2. Basic Gemological Characteristics

The 18 Burmese spinel samples were rotated 360° under crossed polars, showing complete darkness from B-11 to B-18 and abnormal extinction with irregular light and dark changes from B-1 to B-10. Total darkness and abnormal extinction are both homogenous characteristics. The refractive index ranged from 1.715 to 1.717. The red samples (B-11–B-18) show a typical Cr spectrum. There are two strong absorption lines (686 nm and 684 nm) in the red region, and a weak absorption band (656 nm) in the yellow region, partial absorption in the green region, and complete absorption in the purple region [32]. Under CCF, the red samples (B-11–B-18) show medium red, while the other samples do not change color. Spinel samples in different colors show great difference in UV fluorescence test. Red and pink samples show medium to strong red fluorescence under long-wave UV fluorescence and weak red fluorescence under short-wave UV fluorescence. The orange and purple samples are inert under long- and short-wave UV fluorescence. The relative density of the Burmese samples ranged from 3.54 to 3.58, measured by the hydrostatic weighing method.

The eight Vietnamese spinel samples were rotated 360° under crossed polars, showing an abnormal extinction with irregular light and dark changes or total darkness. The refractive index ranged from 1.717 to 1.720. Vietnamese spinel samples have no characteristic spectrum under the spectroscope. Under CCF, the pink samples show weak red color, while the other samples do not change color. The pink samples show weak red under long and short-wave UV fluorescence, while the purple samples are inert under long- and short-wave UV fluorescence. The relative density of the Vietnamese samples ranged from 3.54 to 3.61.

The five Sri Lankan spinel samples were rotated 360° under crossed polars, showing an abnormal extinction with irregular light and dark changes or total darkness. The refractive index ranged from 1.715 to 1.718. Sri Lankan spinel samples have no characteristic spectrum under the spectroscope. Under CCF, the pink samples show weak red, while the purple samples do not change color. The pink samples show weak red color under long- and short-wave UV fluorescence, and the purple samples are inert under long- and short-wave UV fluorescence. The relative density of the Sri Lankan samples ranged from 3.51 to 3.61.

The two Tanzanian spinel samples were rotated 360° under crossed polars, showing an abnormal extinction with irregular light and dark changes or total darkness. The refractive index is 1.718. Tanzanian spinel samples have no characteristic spectrum under the spectroscope. Under CCF, the two pink samples are medium red. The two samples show strong red under long-wave UV fluorescence and medium red under short-wave UV fluorescence. The relative density of the Tanzanian samples is 3.63.

The properties of the studied samples are consistent with those of gem-quality spinels reported in the literature [50].

4.3. Fourier Transform Infrared Spectrum

Gem species can be identified according to the characteristic absorption peaks of Fourier transform infrared spectrum. Although the basic characteristics of the infrared spectrum of gem-quality spinels have been studied, many uncertainties still remain about the attribution of its peak position [26]. The mid-infrared region of 400–2000 cm⁻¹ was selected in this study. Since natural spinels have no characteristic absorption peaks in the range of 1000–2000 cm⁻¹, only absorption peaks in the range of 400–1000 cm⁻¹ are discussed.

The infrared spectrum of the spinels from the four areas in the range of 400–1000 cm⁻¹ is relatively consistent. Wide absorption bands are centered on 728 cm⁻¹, 542 cm⁻¹, and 840 cm⁻¹, with two absorption peaks of 586 cm⁻¹ and 478 cm⁻¹ (Figure 9). The Mg–O stretching vibration caused the absorption peak of 728 cm⁻¹, and the Al–O stretching vibration caused the absorption peaks of 586 cm⁻¹ and 542 cm⁻¹ [2]. The oxygen ion vibration caused the high-frequency absorption band centered on 840 cm⁻¹ [2]. Although the absorption peak of 478 cm⁻¹ is relatively rare, there are also some instances that may be related to metal cation movement [51].

4.4. UV–Visible Spectrum

The UV–visible spectra of red, pink, and orange spinel samples from the four areas all show absorption bands of 400 nm and 550 nm and absorption peaks of 658 nm (Figure 10a,b). Cr³⁺ and V³⁺ formed the two absorption bands [52,53]. The absorption bands are near to each other and generally merge into a wide absorption band centered on 400 nm and 550 nm. V³⁺ forms absorption bands at 394 nm and 541 nm, which are attributed to the electron d-d transitions ³T_1g(F)→³T_1g(P) and ³T₁(F)→³T₂(F) permitted by the spin of V³⁺ at the octahedral position (M) [54]. Cr³⁺ forms two strong and wide absorption bands at 388 nm and 532 nm, which are attributed to the electron d-d transitions ⁴A_2g→⁴T_1g(F) and ⁴A_2g→⁴T_2g(F) permitted by the spin of Cr³⁺ at the octahedral position (M) [55]. The exchange of Fe²⁺-Fe³⁺ generates the absorption peaks of 658 nm (Figure 10b) [55], but the absorption intensity is much lower than the two absorption bands of Cr³⁺ and V³⁺. Therefore, they do not play a leading role in the coloration of spinels.

The slightly different positions of 400 nm absorption band shown in Figure 10a,b may be because the red color of the red samples is darker than that of the pink and orange samples or because the transparency of the red samples is poor. The absorption intensity of V-8, V-5, and T-1 is stronger than that of S-5, S-3, and B-7, possibly due to their different pink tones. V-8, V-5, and T-1 are pink with reddish tones and darker than S-5, S-3, and B-7, which are also distinguished by the curves of one dark pink and the other light pink shown in Figure 10b.

The UV–visible spectrum of purple samples from Burma, Vietnam, and Sri Lanka are basically same (Figure 10c). The absorption band are mainly caused by the spin-forbidden transitions of Fe²⁺ and Fe³⁺ [56,57]. The absorption of S-1 is the strongest, followed by V-2, B-1, S-4, and B-9, which is suspected to be related to the concentration of purple spinel tone. S-1 has the deepest purple hue, and V-2 has a lighter purple hue, followed by B-1, S-4, and B-9. B-9 has the lightest purple hue, which is very lavender.

4.5. Microscopic Laser Raman Spectrum

The Raman spectrum excited by the 532-nm light source shows an obvious fluorescence package at 469 cm⁻¹, resulting in severe deformation of the Raman spectrum. All absorption peaks will be affected by this peak and be elevated. To obtain an ideal Raman spectrum, Origin Mapping software was used for baseline correction of all laser Raman spectra to eliminate the influence of fluorescence package.

The Raman spectra of spinel samples from the four areas are relatively consistent, showing four characteristic absorption peaks of 312 cm⁻¹, 408 cm⁻¹, 663 cm⁻¹, and 764 cm⁻¹ (Figure 11). The Mg symmetric bending vibration generates the 408 cm⁻¹ absorption peak at the tetrahedral position [18]. The transition of Mg in the tetrahedral position generates the absorption peak of 312 cm⁻¹ [18]. The symmetric stretching vibration of Mg–O generates the absorption peak of 764 cm⁻¹ in the tetrahedral element [18]. The absorption peak at 663 cm⁻¹ may be generated by internal vibrations of the AlO₆ octahedron and/or the A²⁺O₄ tetrahedron [18].

The spinel deposits in Burma belong to the carbonate type [20,35]; therefore, carbonate-type inclusions are one of the most common inclusions, such as calcite, magnesite, and dolomite. In carbonate mineral crystal, there is a wide range of isomorphic substitutions among metal cations Zn²⁺, Ca²⁺, Mn²⁺, Mg²⁺, and Fe²⁺, among others, and complete or incomplete isomorphic substitutions can be carried out [58]. The [CO₃]²⁻ group of carbonate minerals contains four kinds of Raman-active vibration mode: ν_ab (out-of-plane bending vibration of carbon and oxygen), ν_ib (in-plane bending vibration), ν_s (symmetric stretching vibration), and ν_as (antisymmetric stretching vibration) [59]. According to the different Raman peaks they represent, the Raman displacement and peak position assignment of calcite, dolomite, and magnesite can be obtained (

Table 1. Raman displacement and assignment of carbonate minerals [39].

Raman Displacement			Vibration Mode Assignment
Calcite	Dolomite	Magnesite
274	292, 331	324	νab
706	719	731	νib
1080	1092	1091	νs
1430	1437	/	νas
1747	1756	1757	νas+νib

).

Two octahedral crystal inclusions in Burmese spinel B-3 (Figure 6a) were selected for Raman analysis (Figure 12). Figure 12a shows not only the absorption peaks of spinel at 407 cm⁻¹, 665 cm⁻¹ and 762 cm⁻¹ but also four dolomite absorption peaks of 177 cm⁻¹, 302 cm⁻¹, 719 cm⁻¹, and 1096 cm⁻¹ in the smaller octahedral crystal inclusion in B-3. Therefore, it can be known that the smaller octahedral inclusion of B-3 is composed of dolomite (Zaw, 2017). Figure 12b shows not only the absorption peaks of spinel at 407 cm⁻¹, 663 cm⁻¹, and 762 cm⁻¹ but also the dolomite absorption peaks of 177 cm⁻¹, 302 cm⁻¹, and 1098 cm⁻¹ as well as the calcite absorption peaks of 153 cm⁻¹, 280 cm⁻¹, 711 cm⁻¹, and 1084 cm⁻¹ in the bigger octahedral crystal inclusion in B-3. Therefore, the bigger octahedral crystal inclusion of B-3 is composed of a mixture of calcite and dolomite [30]. This result is consistent with the conclusion obtained by Gübelin et al., who proposed that the crystal part with clear octahedral inclusions is dolomite, while the one with linear distribution near the crystal is calcite by EPMA [7].

Spinel usually occurs together with minerals containing magnesia, such as clinohumite and humite (Mg–Fe–silicate), gold mica (K–Mg mica), and isochrite (Na–CaMg–Fe amphibole), which are related to the genesis of carbonate-type deposits [35]. The short columnar inclusion in Burmese spinel B-5 (Figure 6b) was selected for Raman analysis (Figure 12). As shown in Figure 12c, B-5 has humite absorption peaks of 845 cm⁻¹, 865 cm⁻¹, 935 cm⁻¹, and 970 cm⁻¹. The Raman spectral data of humite (RRUFF ID: R040071) can be searched in the RRUFF database, and the absorption peaks of 845 cm⁻¹, 865 cm⁻¹, 935 cm⁻¹, and 970 cm⁻¹ are basically consistent with the data in the database. Therefore, it can be determined that humite is the short columnar inclusion of B-5.

The columnar crystal shaped like a “finger” in Vietnamese spinel V-3 (Figure 7b) was selected for Raman analysis (Figure 13). As shown in Figure 13a, V-3 has talc absorption peaks of 206 cm⁻¹, 280 cm⁻¹, 358 cm⁻¹, and 677 cm⁻¹. The Raman spectral data of talc (RRUFF ID: R050087) can be searched in the RRUFF database, and the talc absorption peaks of 206 cm⁻¹, 280 cm⁻¹, 358 cm⁻¹, and 677 cm⁻¹ are basically consistent with the data in the database. Therefore, it can be determined that talc is the columnar crystal inclusion of V-3.

A round crystal in Vietnamese spinel V-5 (Figure 7c) was selected for Raman analysis (Figure 13). As shown in Figure 13b, V-5 has titanite absorption peaks of 163 cm⁻¹, 254 cm⁻¹, 318 cm⁻¹, 335 cm⁻¹, 468 cm⁻¹, 552 cm⁻¹, 612 cm⁻¹, 912 cm⁻¹, and 1086 cm⁻¹. The Raman spectral data of titanite (RRUFF ID: R040033) can be searched in the RRUFF database, and the titanite absorption peaks of 163 cm⁻¹, 254 cm⁻¹, 552 cm⁻¹, and 612 cm⁻¹ are basically consistent with the data in the database. Therefore, it can be determined that titanite is the round crystal inclusion of V-5.

The cuboid crystal inclusion in Sri Lankan spinel S-4 (Figure 8a) was selected for Raman analysis (Figure 14). As shown in Figure 14, S-4 has absorption peaks of 328 cm⁻¹, 737 cm⁻¹, 1092 cm⁻¹, 1444 cm⁻¹, and 1760 cm⁻¹, representing magnesite [39]. Therefore, it can be determined that magnesite is the cuboid crystal inclusion of S-4.

The “boot” crystal in Tanzanian spinel T-2 (Figure 8d) analysis was selected for Raman analysis (Figure 15). As shown in Figure 15, T-2 has absorption peaks of 173 cm⁻¹, 298 cm⁻¹, 340 cm⁻¹, 726 cm⁻¹, 1096 cm⁻¹, 1440 cm⁻¹, and 1754 cm⁻¹, representing dolomite. Therefore, the colorless and transparent part of the “boot” crystal in T-2 is dolomite, but the specific cause of the white stripe part is not known. The two crystals shown in Figure 7c are also identified as dolomite (A7) by Raman analysis.

4.6. Chemical Composition

The main chemical components of spinels from the four areas are MgO and Al₂O₃ as well as oxides such as Cr₂O₃, Na₂O, SiO₂, TiO₂, FeO, MnO, NiO, K₂O, ZnO, and CaO (

Table 2. EPMA analysis of four areas spinel samples (wt.%).

	V-2	V-1	V-6	V-4	S-2	S-1
Cr2O3	0.124	0.112	0.096	0.111	0.013	0.044
Na2O	0.088	0.054	0.05	0.053	0.06	0.078
SiO2	0.007	0.075	0.059	0.049	0.098	0.007
TiO2	0.105	0	0	0.033	0	0
MgO	30.873	30.679	29.537	31.226	31.223	32.031
FeO	0.904	0.685	2.212	1.008	1.472	1.692
MnO	0.009	0.041	0.081	0.007	0.012	0.095
Al2O3	67.096	68.039	67.71	66.589	65.389	65.89
NiO	0.011	0.035	0	0	0	0
K2O	0.018	0.022	0.027	0.026	0.028	0.032
ZnO	0.001	0.086	0.122	0	0.224	0.241
CaO	0.022	0	0.001	0.006	0	0.006
Total	99.26	99.827	99.896	99.108	98.518	100.116
	B-2	B-1	B-15	B-8	T-1
Cr2O3	0.063	0	0.992	0.044	0.096
Na2O	0.03	0.073	0.037	0.036	0.045
SiO2	0.034	0.087	0	0.023	0.103
TiO2	0.017	0.094	0.02	0.011	0.056
MgO	31.21	30.325	31.893	30.893	30.573
FeO	0	0.361	0.202	0.027	0.147
MnO	0.016	0.004	0	0.036	0
Al2O3	69.206	68.79	65.616	69.951	67.89
NiO	0.055	0.024	0	0	0
K2O	0.001	0.037	0.011	0.009	0.017
ZnO	0.068	0.218	0.106	0.099	0.324
CaO	0	0.036	0	0	0.005
Total	100.701	100.049	98.878	101.129	99.256

). The MgO and Al₂O₃ contents in the four areas are basically the same. The MgO content ranges from 29.537 wt.% to 32.031 wt.%, with an average content of 30.95 wt.%. The Al₂O₃ content ranges from 65.389 wt.% to 69.951 wt.%, with an average content of 67.47 wt.%. The content of other oxides is low, and there is little difference among the four areas. CaO and NiO are almost 0 wt.%.

The elements V, Cr, Fe, and Zn are commonly present in highest concentrations, whereas concentrations of Ti, Mn, and Li are generally low. Traces of Be, Co, Ni, and Cu are detected, and other investigated elements are below the detection limits (

Table 3. LA-ICP-MS trace element analyses of spinel samples (ppm).

	Li	Be	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
B15-1 (red)	0	0	497	1274	7387	7.6	2148	1.45	23.8	0.29	546
B15-2	0	0	497	1295	7830	9.6	2141	1.58	23.3	0.56	549
B15-3	5.1	1.03	420.89	1041.77	5878.24	8.84	2633.31	1.48	15.71	0.8	371.58
B15-4	7.11	1.56	414.47	1060.79	5864.25	5.83	3080.58	1.01	15.48	<0.86	338.58
B15-5	6.26	0	408.21	1108.44	6091.4	7.51	2733.39	1.28	18.7	<1.03	358.06
B15-6	<5.65	2.85	451.79	1072.98	5998	8.96	1929.07	1.39	17.22	<0.89	386.5
V6-1 (orange)	28.3	9.5	88.1	912.4	440	94.6	8551	10.03	14.33	20.4	192.54
V6-2	29.4	7.4	89	1069	607	94.3	8721	11.98	14.49	24.1	194.8
V6-3	14.33	5.32	77.24	1084.27	486.86	87.59	4089.77	13.01	23.16	<1.04	193.45
V6-4	50.32	4.9	86.57	1051.05	700.79	84.98	3432.36	12.48	19.87	<1.04	198.68
V6-5	27.35	1.63	82.86	1039.38	753.62	85.73	4118.73	13.16	18.94	1.08	196.97
V6-6	19.84	7.48	84.43	1069.89	764.33	89.47	3669.63	12.8	23.98	<0.94	197.07
B2-1 (orange)	7.9	2.51	45.7	2280	6.5	6	293.6	0.074	4.62	0.25	574
B2-2	4.66	3.03	46.7	2518	5.2	5.3	276.4	0.099	4.4	0.68	557.5
B2-3	6.61	3.11	49.38	1928.52	30.58	2.44	481.52	0.108	3.6	<1.08	413.13
B2-4	8.58	1.68	45.99	1932.61	30.8	<2.62	360.56	0.143	<2.74	<1.14	393.7
B2-5	8.04	3.7	45.42	1955.11	18.65	3.77	371.95	0.177	<3.17	<1.16	414.01
B2-6	6.2	2.41	50.97	1983.7	13.64	6.72	405.93	0.324	3.94	<1.01	385.17
T1-1 (pink)	26	25.9	25.6	1079.1	1151	14.1	1682.1	3.85	15.9	0.44	7008
T1-2	24.7	24.9	24.3	1059	1141	14.3	1658.1	3.94	17	0.35	6879
T1-3	31.61	20.8	26.39	976.51	1044.62	12.6	1077.06	3.18	12.83	<0.84	5434.86
T1-4	19.14	15.35	20.02	960.67	1023.98	9.95	1156.71	2.63	12.95	<0.73	5130.75
T1-5	23.22	14.09	29.03	968.68	1026.78	11.53	1050.14	3.22	12.69	<1.12	5129.56
T1-6	17.46	17.2	27.46	977.21	1036.14	11.51	1188.37	3.02	11.09	<0.94	5211.23
V4-1 (pink)	773	16.1	32.9	68.7	933	226.1	19,425	1.4	3.15	2.37	1395.4
V4-2	765	14.9	34.4	68.5	850	222.4	19,320	2.08	3.08	0	1246.98
V4-3	710.2	10.3	41.13	61.16	812.49	209.34	8094.04	2.8	4.35	<0.94	1435.33
V4-4	696.1	9.57	22.83	60.72	817.92	201.81	8413.21	3.3	3.93	<0.93	1359.96
V4-5	735.43	15.1	27.94	61.64	808.29	201.79	8161.55	2.73	<2.43	1.01	1282.97
V4-6	715.07	8.65	31.28	61.24	772.05	191.53	8031.34	2.56	<2.43	<0.90	1314.1
B8-1 (pink)	16.6	20.6	0	506.7	568	6.8	180.3	0.313	39.2	0.74	908
B8-2	16.5	16.37	0.26	505.5	574	7.6	185.7	0.292	43.2	0.59	926
B8-3	18.74	19.46	<2.81	461.8	514.76	5.34	164.81	0.24	30.5	<1.18	703.85
B8-4	20.19	15.06	<3.34	456.68	526.5	7.64	157.13	0.229	31.02	0.88	684.21
B8-5	8.67	14.1	<3.85	460.83	525.7	6.25	192.75	0.388	32.88	<0.91	657.44
B8-6	11.44	17.09	3.05	449.02	513.4	6.09	135.68	0.351	34.65	<1.00	679.89
V1-1 (pink)	179	8.4	6.4	12.44	175.6	134	13,770	2.66	2.86	0.83	1522
V1-2	178.6	8.12	7.6	11.41	169.6	129.7	13,861	2.63	3.9	0.67	1507
V1-3	166.23	5.09	8.31	11.08	150.95	114.28	11,915.66	2.44	4.27	<0.98	1313.17
V1-4	180.06	3.8	<3.96	10.6	166.31	120.64	11,941.2	2.45	<2.56	<1.03	1276
V1-5	175.03	4.63	9.42	10.57	145.08	113.58	11,688.84	2.26	<2.76	<1.02	1223.09
V1-6	151.6	7.3	3.11	10	148.11	119.35	12,063.13	2.13	<2.85	<1.16	1258.5
S2-1 (pink)	193.3	2.3	37	76.3	842	659.7	5557	1.54	1.68	0	1482.5
S2-2	191.1	2.03	34.9	76.6	832	644.9	5525	1.62	1.82	0	1529.6
S2-3	188.14	1.98	27.55	66.73	739.63	585.92	2415.21	1.98	3.93	<1.08	1560.36
S2-4	177.63	0.35	26.61	69.57	737.17	563	2549.84	1.39	4.45	<1.17	1467.53
S2-5	159.08	2.1	28.41	66.64	749.55	575.91	2393.63	1.51	5.05	<1.02	1456.59
S2-6	158.47	1.63	31.14	67.02	716.88	558.55	2103.37	1.3	2.4	<1.00	1371.09
B1-1 (purple)	8.19	4.9	199.1	212.5	191	20.2	3884	2.95	7.9	0.56	1312
B1-2	12.9	4.1	250	214.4	237.8	21.4	3925	2.96	7.2	0.54	1306
B1-3	<7.11	3.69	193.07	188.38	75.77	19.31	2538.87	2.25	8.51	<1.17	936.37
B1-4	12.59	2.05	196.31	186.21	58.83	17.62	2199.57	2.03	3.65	<1.04	859.07
B1-5	8.15	2.1	199.7	188.4	49.32	22.21	1704.23	2.09	3.35	1.17	853.19
B1-6	9.22	1.05	208.35	187.06	52.36	18.18	1364.84	2.22	5.69	<1.09	920.05
V2-1 (purple)	179.5	5.64	3.1	8.98	100.8	174.9	14,237	4.6	4.8	0.46	1564
V2-2	185.8	6	3	9.33	102.8	175.4	14,120	4.46	4.25	0.58	1557
V2-3	142.41	1.43	3.4	8.22	94.06	160.55	6405.98	4.84	3.73	<0.96	1492.53
V2-4	161.35	2.79	8.85	7.76	95.14	157.91	6784.71	4.51	<2.73	<0.93	1307.97
V2-5	183.76	1.03	3.96	8.54	107.44	158.88	6460.82	4.04	4.24	<0.97	1340.96
V2-6	162.71	3.5	6.74	8.76	104.93	157.29	6932.67	4.28	2.89	<0.92	1258.47
S1-1 (purple)	12.5	9	172.6	191.6	758	229	9414	2.16	3.05	0	420.6
S1-2	12.4	8.6	170.6	191.3	813	228.1	9397	1.59	3.38	2.12	449.13
S1-3	11.52	5.39	143.62	169.69	694.07	199.58	4058.49	3.25	5.19	<1.05	450.21
S1-4	<9.45	4.1	136.17	168.1	696.4	196.79	3657.9	2.3	<2.40	<0.95	453.22
S1-5	17.54	7.24	138.61	170.07	664.16	192.88	3218.03	3.21	<2.55	<0.85	428.6
S1-6	10	6.01	146.17	167.57	677.23	203.2	3170.28	2.62	3.33	<0.85	427.1

). Burmese spinel has high Cr content (up to 7387 ppm) and high Ti content (up to 497 ppm). The investigated samples from Vietnam stand out by having the highest Fe content (3669.63–19,425 ppm), as well as low Ti concentrations (<89 ppm). Mn content is high in Sri Lankan spinels (192.88–659.7 ppm). The contents of Ti, Mn, and Li in the sample from Tanzania are very low. Samples from Tanzania show the lowest values for Ti (20.02–29.03 ppm), Mn (9.95–14.3 ppm), and Li (17.46–31.61 ppm). The Co content is very low, ranging from 0.074 to 13.01 ppm, which basically fails to reach the color limit of 10 ppm and is generally not the main reason for spinel to produce blue and purple tones [17]. Mn and Ti do not play a decisive role in the formation of color in spinel.

The Cr content of red spinel is the highest, reaching 7387 ppm (Figure 16a). The V content of orange spinel is the highest, reaching 2280 ppm, followed by red spinel with 1000–1400 ppm (Figure 16b). The origin characteristics of Cr and V elements are not obvious. The Fe content of Vietnamese spinel is generally high, especially Vietnamese pink and purple spinels. The Fe content of Vietnamese pink spinel reaches nearly 20,000 ppm, while the Fe content of Burmese spinel is generally low (Figure 16c). The variation of Zn content with the change of color is not obvious. However, Tanzanian spinels showed the highest Zn contents (5129–7008 ppm), which could be considered characteristic of spinel from Tanzania [32].

5. Discussion

5.1. Chromogenic Mechanism of Spinel

The UV–visible spectrum results can be used to analyze the chromatic mechanism of gemstones. The reason for the selective absorption and color of the vast majority of gemstones can be either the presence of major chemical components or the presence of trace elements, called chromogenic elements [18]. In RR’₂O₄ spinel structure, the spinel structure is deformed as the R and R’ cation radius changes, resulting in different colors [58]. Taking magnesium aluminum spinel as an example, when the composition of magnesium aluminum spinel is pure MgAl₂O₄, it is colorless and transparent. The cation radius changes when Mg²⁺ and Al³⁺ are replaced by ions such as Fe²⁺, Cr³⁺, V³⁺, and Co²⁺ and other chromogenic elements. Thus, the tetrahedron and octahedron are distorted, and the spinel shows colorful features [60] Cr, V, and Fe are not only the trace elements with high content but also the most important chromogenic elements in spinel. Especially, because the inclusions in spinel are small, it has no effect on the overall color of spinel.

Cr + V can lead to the red color in spinel. The higher the Cr + V content, the more obvious the red color and the higher the saturation. The red spinels have the highest Cr + V content (7000–8600 ppm) (Figure 17b), with the obvious red tone, bright color, and high saturation. The Cr content of red spinel is the highest among all colored spinels (6000–8000 ppm), while the V content is only 1000–1500 ppm (Figure 17a). Therefore, Cr plays a dominant role in red spinel. The Cr + V content of orange spinels is significantly lower than that of red spinel (2000–3000 ppm) (Figure 17b), with almost no red tone and low saturation. Orange spinel has the highest V content of all colored spinels (1000–2500 ppm), while the Cr content is only 5.2–764.33 ppm (Figure 17a). Therefore, V plays a leading role in orange spinel. The Cr + V content of pink spinels is lower than that of red spinels (<2200 ppm). The red tone is obviously lighter than that of red spinels, and the saturation is not high. Tanzanian pink spinel has the highest Cr + V content among pink spinels, and its pink tone is the deepest and brightest.

Combined with the analysis of the chemical composition and UV–visible spectrum, red, pink, and orange spinels are classified as Cr- and V-rich spinels due to their similar UV–visible spectral characteristics and high Cr and V content.

B-15, T-1, and V-6 were regarded as red, pink, and orange spinel representatives, and their UV–visible spectra were used to analyze the chromogenic mechanism. Figure 18 shows the absorption peaks and attributions. By referring to the absorption peak of the red spinel numbered 890292D from Burma in the previous literature and its attributions [18], the UV–visible spectrum data obtained in this study were compared and analyzed and summarized in

Table 4. Absorption bands and attributions of UV-visible spectrum of red, pink, and orange spinels (nm).

	890292d (Red)	B-15 (Red)	V-6 (Orange)	T-1 (Pink)	Attributions
a	387	361	387	390	MCr3+: Spin-allowed transition 4A2g→4T1g(F)
b	393	371	402	407	MV3+: Spin-allowed transition 3T1g(F)→3T1g(P)
c	422	377	425	412	MCr3+: Spin-allowed transition 4A2g→4T1g(F)
d	529	527	531	528	MCr3+: Spin-allowed transition 4A2g→4T2g(F)
e	533	538	536	538	MV3+: Spin-allowed transition 3T1(F)→3T2(F)
f	562	571	569	586	MCr3+: Spin-allowed transition 4A2g→4T2g(F)
g	668	678	656	670	Fe2+-Fe3+: Exchange interaction

.

Seven absorption bands (a–g) with relatively fixed positions appear in the range of 300–800 nm, indicating that the spin-allowed transitions of Cr³⁺ and V³⁺ are the main reason for the red, pink, and orange spinels. The absorption bands of red, pink, and orange samples are located at 400 nm and 550 nm. The region with an absorption band of 400 nm in visible light is blue and purple, and the region with an absorption band of 550 nm in visible light is yellow and green. Therefore, when light passes through red, pink, and orange spinels, they selectively absorb blue-violet light and yellow-green light of visible light, presenting a complementary color of blue-violet light and yellow-green light, that is, a mixture of red and yellow. In addition, the absorption region with an absorption band of 658 nm in visible light is red, and the spinel also selectively absorbs red light and appears purple. However, the absorption intensity of 658 nm is much lower than the absorption band of 400 nm and 550 nm in red, pink, and orange samples, so the mixed color of red and yellow is mainly presented.

The results of trace element analysis show that the Cr content in the red sample B-15 is higher than that of V. Therefore, the absorption bands of a, c, d, and f caused by Cr³⁺ are more dominant than those of b and e caused by V³⁺ (Figure 18a), making the blue-violet and yellow-green light in visible light more absorbed and pass through more red light. As a result, spinels appear red.

The V content in the orange sample V-6 is higher than that of Cr. The absorption bands of b and e caused by V³⁺ are more dominant than those of a, c, d, and f caused by Cr³⁺ (Figure 18b), making the blue-violet and green light in visible light more absorbed and pass through more orange light. As a result, spinels appear orange.

The Cr content in the pink sample T-1 is slightly higher than that of V, and the Cr content is between B-15 and V-6. The absorption intensity of absorption bands a, c, d, and f caused by Cr³⁺ is between B-15 and V-6 (Figure 18c), and the degree of red and yellow light through is also between B-15 and V-6. As a result, mixed spinels appear pink.

The Fe content of purple spinel is obviously higher than that of Cr + V (Figure 17b), and Fe plays a dominant role in purple spinels. Belley et al. (2021) found that the relatively low Fe content in Vietnamese purple spinels (<2437 ppm) may be the key to create attractive saturated colors [61]. The Fe content of purple spinel ranges from 1364.84 to 14,237 ppm, with an average value of 5748 ppm. Thus, the purple saturation is not high, and the color is light. Purple spinels are classified as Fe-rich spinel due to their high Fe but low Cr and V content.

S-1, V-2, B-1 are regarded as the representative of purple spinel, and its UV–visible spectrum was used to analyze the chromogenic mechanism. Figure 19 shows the absorption peaks and attributions. By referring to the absorption peaks of the purple spinel numbered SX in the previous literature and its attributions [18], the UV–visible spectrum data obtained in this study were compared and analyzed and summarized in

Table 5. Absorption bands and attributions of UV-visible spectrums of purple spinels (nm).

	SX	S-1	V-2	B-1	S-4	B-9	Attributions
a	373	371	372	364	368	368	TFe2+: Spin-forbidden transition 5E→3E
b	387	385	384	386	387	387	TFe2+: Spin-forbidden transition 5E→3T2, 3T1
c	411	406	409	406	412	415	TFe2+: Spin-forbidden transition 5E→3T1 (May be enhanced by ECP conversions)
d	436	/	/	/	/	/	MMn2+: Spin-forbidden transition 6A1(S)→4E, 4A1(G)
e	457	460	460	460	471	/	MFe3+: 6A1g→4A1g, 4Eg
f	478	483	482	481	/	/	MFe3+: Spin-forbidden transition 5E→3T2, 3T1
g	/	532	533	532	529	531	/
h	552	551	555	559	555	558	TFe2+: Spin-forbidden transition 5E→3T2
i	/	581	582	/	588	/	TFe2+: Spin-forbidden transition 5E→3T1
j	665	655	657	657	665	/	MFe2+: MFe2+—MFe3+IVCT
k	798	770	/	769	/	773	TFe2+: Spin-forbidden transition 5E→3T1
l	903	/	/	/	/	/	MFe2+: Spin-allowed transition 5T2g→5Eg

.

A total of 11 relatively fixed absorption bands (a–k) appear in the range of 300–800 nm, indicating that the spin-forbidden transition of Fe³⁺ and Fe²⁺ is the main reason for the purple color of spinels. As shown in Figure 19 and Figure 10c, purple spinel has a wide absorption band centered at 550 nm in the yellow-green zone, a 385-nm absorption band in the purple zone, a 460-nm absorption band in the blue zone, and a 655-nm absorption band in the red zone. The absorption intensity of the 551-nm absorption band h of each sample is the strongest, playing a dominant role in purple spinels. The 551-nm absorption band is caused by the spin-forbidden transition ⁵E(D)→³T₂(H) [56,57]. The absorption intensity of the absorption bands f (483 nm) and g (532 nm) is positively correlated with the Fe content [62]. The average Fe content of spinel S-1, V-2, and B-1 is 9512.15 ppm, 9156.86 ppm, and 2602.75 ppm, respectively. Therefore, the absorption intensity of the absorption bands f and g of S-1 are the highest, followed by V-2 and B-1 (Figure 19). The absorption intensity of the 655-nm absorption band j also increases with the increase in Fe content, which is mainly caused by Fe_Tot, followed by Fe³⁺ [57]. At the 300–330-nm wavelength, the purple and blue light in visible light are strongly absorbed due to the charge transition of O²⁻→Fe²⁺ and O²⁻→Fe³⁺, showing strong UV edge absorption. The UV edge absorption bands a (371 nm), b (385 nm), and c (406 nm) caused by the spin-forbidden transition of TFe²⁺ appear in almost all purple spinels, and their positions are basically the same. As presented in Table 5, the purple color of spinels from Burma, Vietnam, and Sri Lanka is caused by Fe²⁺ and Fe³⁺, and no absorption peak d is caused by Mn²⁺.

5.2. Origin Determination

Burmese red spinel is rich in Cr, while Vietnamese orange spinel is rich in V. The comparison of Zn and Fe contents reflects two chemical element trends, namely, zinc-rich spinel and iron-rich spinel. Fe-rich spinel is distributed along the vertical axis, while Zn-rich spinel is distributed along the horizontal axis (Figure 20). The Fe content of Vietnamese spinel is generally high (3669.63–19,425 ppm), the Zn content of Tanzanian spinel is high (5129.96–7008 ppm), and the Fe and Zn contents of Burmese spinel are low (Fe: 135.68–3925 ppm, Zn: 338.58–1312 ppm). Zn-rich spinels require a Zn content above 3000 ppm [4]. The results show that Tanzanian pink spinels belong to Zn-rich spinels.

6. Conclusions

The types of inclusions and the content of trace elements are the main differences between spinels from Burma, Vietnam, Sri Lanka, and Tanzania. Carbonate minerals exist in the spinels of Burma, Sri Lankan, and Tanzania. Burmese spinel is characterized by an octahedral negative crystal filled with dolomite or a mixture of dolomite and calcite. Magnesite is present in Sri Lankan spinel, and dolomite is present in Tanzanian spinel. Dislocation systems and the presence of titanite and talc inclusions are strongly indicative features for Vietnamese spinel. Sri Lankan spinel is characterized by abundant gas–liquid inclusions, such as the beaded healing fissure. Burmese spinel is poor in Fe and Zn contents, but Burmese red spinel is rich in Cr content. Vietnamese spinel is rich in Fe content and poor in Ti content, while Tanzanian spinel is rich in Zn content.

The high Cr + V content can lead to the red color in spinel, and the Cr and V contents change obviously with color. Spinels appear red when the Cr content is higher than V. Meanwhile, spinels appear orange when the V content is higher than Cr. Fe plays a dominant role in purple spinels.

Red, pink, and orange spinels are colored by the spin-allowed transition of Cr³⁺ and V³⁺, showing a wide absorption band centered at 400 nm and 550 nm. Purple spinel is colored by the spin-forbidden transition of Fe³⁺ and Fe²⁺, and the 551-nm absorption band plays a dominant role.