Investigation of the Gemological Characteristics and Types of Inclusions of Emeralds from Sumbawanga, Tanzania

Abstract

Africa hosts numerous emerald deposits, among which Sumbawanga, located at the junction of Tanzania, Zambia, Congo, and Malawi, stands out as one of the significant localities. This study presents a comprehensive analysis of the gemological, spectroscopic, and inclusion characteristics of Sumbawanga (Tanzania) emerald samples utilizing techniques such as gem microscopy, UV-Vis-NIR spectroscopy, Raman spectroscopy, GEM-3000, and EPMA, etc. These emerald crystals look like rolled pebbles and display a bluish-green coloration. They contain fingerprint-like fluid inclusions, which occasionally encompass a circular bubble (the gas phase is CO₂). Sumbawanga emeralds are characterized by abundant mineral inclusions, including quartz, apatite, anhydrite, diaspore, chrysoberyl, rutile, hematite, and magnetite. Particularly diagnostic are the mineral inclusion of chrysoberyl twins and the assemblages of quartz and diaspore.

1. Introduction

Emerald (Be₃Al₂Si₆O₁₈) fascinates with its unparalleled color; its distinctive vivid green hue is attributed to the internal electron transition of chromium (Cr) and/or vanadium (V). The abundant inclusions within emeralds create a miniature garden, such as healed fingerprint-like fractures, jagged or needle-shaped or rectangular fluid inclusions, flaky brown phlogopite or red hematite, black granular magnetite, colorless quartz, feldspar, calcite, etc. [1,2,3,4,5]. While these common inclusions cannot be directly used to determine the origin of emeralds, they can provide important information about the emerald-forming environment. Among the various emerald mining locations, Colombia, renowned as a traditional source of emerald, remains synonymous with high-quality specimens. Zambia, an emerging player in the emerald market, has rapidly gained prominence and secured a significant market share in recent years. Southern Africa includes several important emerald mining sites in countries such as Zambia, Tanzania, Mozambique, Zimbabwe, Ethiopia, and Madagascar, each contributing gemstones resources in the global emerald market [5,6,7,8,9,10,11,12].

In Tanzania, emerald mining research and development have followed a distinctive trajectory. In 1988, a secondary emerald deposit was discovered in Sumbawanga, which was quickly brought into production [13]. This discovery made this area the second emerald mining location in Tanzania, succeeding the previous Lake Manyara deposit.

Around the 2000s, several researchers investigated inclusions in emeralds from Lake Manyara and Sumbawanga, but these studies notably lacked clear photomicrographs of the inclusions [7,8]. Research on Tanzania emeralds, particularly those from Sumbawanga, has primarily focused on their chemical composition and comparative studies across different geographic origins [14,15,16].

In summary, there is a significant lack of studies on inclusions, particularly those of the mineral type, present in Tanzanian emeralds. As a consequence, the provenance and formation mechanisms of emeralds from Sumbawanga secondary deposit remain largely enigmatic. In this study, through a comprehensive suite of analysis techniques, including standard gemological testing, microscopic observation, fiber optic spectrometer, UV-Vis-NIR spectroscopy, Raman spectroscopy, and electron probe microanalysis (EPMA), the inclusion characteristics and gemological properties of Sumbawanga emeralds have been systematically updated and refined. A deeper understanding of the inclusions in emeralds provides solid evidence to determine the geographic origin of emeralds. Furthermore, it is expected to establish a fundamental basis for investigating the geographic origin of Sumbawanga secondary emeralds.

2. History and Geological Setting of the Sumbawanga Deposit

Since its discovery in 1988, the emerald deposit in Sumbawanga has been rapidly exploited [13]. However, Sumbawanga emeralds are typically more suitable for cutting into cabochons or for carving purposes, as the gem-quality material is exceptionally rare [13]. It was only in 2011 that gem dealer Farooq Hashmi purchased two relatively fine pieces of rough emerald from Sumbawang [17].

The Sumbawanga emerald deposit is located northwest of Lake Rukwa in southwestern Tanzania, tectonically situated within the Ubendian Orogenic Belt (Figure 1). This NW-SE trending orogenic belt extends from the Democratic Republic of Congo to Malawi, connecting the Archean Tanzania Craton with the Paleoproterozoic Bangweulu Block [18]. The inland Lake Rukwa is situated within this orogenic belt.

Several researchers have reported that the emerald deposit in Sumbawanga occurs within biotite schists, which have undergone weathering processes [8,13]. However, the precise formation mechanism remains unclear [8,15].

3. Materials and Methods

3.1. Sample Description

A total of eighteen emerald samples (EST-01 to EST-18) were examined from Sumbawanga, Tanzania. Fifteen emerald samples are round without the well-formed hexagonal prisms (Figure 2). They range in transparency from translucent to transparent and display colors varying from light green to intense green with a slight blue tinge. In addition, there are three thin sections (EST-16 to EST-18) with a thickness of 0.04 mm.

3.2. Gemological Analysis

Conventional gemological analyses of all samples were performed at the Gemological Research Laboratory of China University of Geosciences, Beijing. Each sample was examined using a refractometer, Chelsea filter, long-wave (365 nm) and short-wave (254 nm) UV lamps, DiamondView™ (London, UK), and an apparatus for hydrostatic-specific gravity assessment. Internal and external features were observed with a GI-MP22 gemological photographic microscope utilizing darkfield and top illumination techniques. The inclusion thin sections were examined using an BX-51 polarizing microscope (manufactured by Olympus corporation, Tokyo, Japan) under both plane-polarized and cross-polarized light conditions.

The UV-Vis-NIR spectra of thirteen samples were recorded using a UV-3600 spectrophotometer manufactured by Shimadzu (Kyoto, Japan) in diffuse reflection mode with a wavelength range from 300 to 900 nm. This instrument is equipped with an ISR-3100 integrating sphere attachment, featuring a sphere diameter of 60 mm and a slit width of 20 nm. And the sampling interval is set at 1.0 nm.

The CIE 1976 LAB uniform color space is often used to quantify and characterize the color of gemstones. The color space is a circle with three axes in a three-dimensional space: L* (lightness) is perpendicular to the plane of the paper; the variable a* and b* lie in the plane and define a two-dimensional Cartesian coordinate system that represents the different colors and color saturations [19]. The color parameters of all emerald samples were collected by the QSPEC GEM-3000 spectrophotometer by Biaoqi (Guangzhou, China) at 6700 K color temperature.

Raman spectra were acquired using a Horiba LabRam HR-Evolution Raman spectrometer (HORIBA, Longjumeau, France) at the Gemological Research Laboratory of China University of Geosciences, Beijing, China, employing an Ar-ion laser with 532 nm excitation. The spectral range covered was from 4000 to 100 cm⁻¹, with an integration time of 3 s and 2 scans accumulated. The Raman shifts were calibrated using monocrystalline silicon prior to testing, with a tolerance of ±0.5 cm⁻¹.

The chemical composition of unknown mineral inclusions especially in thin sections was determined by EPMA using a JXA-iHP200F instrument (JEOL Ltd., Tokyo, Japan) housed at Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Science (Beijing, China). Accelerating voltage and current were 15 kV and 20 nA, respectively. Element peaks and background were measured with counting times of 10 s and 5.0 s, respectively.

4. Results

4.1. Gemological Properties

The Sumbawanga emerald samples utilized in this study display a light to medium saturation of green with blue hues. The color parameters of the experimental samples are as follows: L* ∈ (27.47, 50.43), a* ∈ (−18.40, −6.46), and b* ∈ (0.95, 6.79) (Figure 3).

These emerald samples do not exhibit typical and well-developed hexagonal prismatic crystal forms. Instead, they generally show rounded pebble morphologies with localized surface damage, indicating that the samples have undergone transportation and abrasion processes (Figure 4). These morphological characteristics clearly indicate an origin from secondary deposits.

The two samples exhibit a non-uniform color distribution, with distinct, straight bands of color alternating between medium green and light green hues (Figure 5).

The gemological properties of these samples are summarized in

Table 1. Gemological properties of Sumbawanga emerald samples.

Properties	Results
Color	Medium green to bluish green
Clarity	Medium to heavy included
Refractive indices	No = 1.577–1.581; Ne = 1.571–1.574
Birefringence	0.006–0.007
Specific gravity	2.66–2.72
Pleochroism	Green (o-ray) and bluish green (e-ray)
Fluorescence	Inert
Chelsea filter	Red
External features	Rounded pebble morphologies with localized surface damage, displaying an abraded, frosted texture Parallel color bands of alternating light green and vivid green.
Internal features	Fingerprint-like fluid inclusions Abundant mineral inclusions: quartz, diaspore, chrysoberyl, apatite, hematite, magnetite, rutile, and anhydrite

. The refractive index ranges from 1.571 to 1.574 for the extraordinary direction (Ne) and from 1.577 to 1.581 for the ordinary direction (No), yielding a birefringence of 0.006 to 0.007. The refractive index conforms to the theoretical value of emerald, but is overall lower, while the birefringence is consistent with the report in 2012 [17]. The specific gravity varies between 2.66 and 2.72. All samples exhibit a red appearance when observed under the Chelsea color filter, with EST-14 showing particularly pronounced redness. Furthermore, they remain inert under both short and long-wave ultraviolet radiation.

4.2. Microscopic Characteristics

4.2.1. Associate Minerals

The surface of Sumbawanga emeralds exhibits various associate minerals, including colorless, gray, and yellow quartz, colorless beryl, brown phlogopite, black biotite, orange-pink apatite, yellowish-brown rutile, black hematite and black magnetite, which are formed within the same space (Figure 6).

All these mineral-associated characteristics have been identified through Raman spectroscopy. Figure 7 shows their corresponding Raman spectra, along with comparative reference spectra from the RRUFF database.

4.2.2. Fluid Inclusions

Fluid inclusions in Sumbawanga emeralds are commonly observed within healed fractures, typically exhibiting fingerprint-like patterns (Figure 8A). Individual fluid inclusions are generally small, with sub-rectangular morphologies, measuring approximately 10 μm in size and rarely exceeding 50 μm. Single-phase fluid inclusions are more prevalent, although occasional gas–liquid two-phase inclusions have been identified (Figure 8B).

Raman spectroscopic analysis reveals that the gas-phase primarily consists of CO₂ (Figure 9, 1280 and 1385 cm⁻¹). However, effective Raman spectra of the liquid phase components have not been successfully obtained, probably due to the rapid increase and subsequent saturation of emerald samples’ Raman signal intensity above 2500 cm⁻¹.

4.2.3. Mineral Inclusions

Sumbawanga emeralds contain abundant mineral inclusions, including numerous colorless, transparent minerals, yellowish-green chrysoberyl, elongated tabular transparent minerals and small black pinpoint inclusions (Figure 10). The colorless, transparent inclusions are usually made of quartz and apatite and measure about 50 μm. The elongated tabular minerals often exhibit parallel, densely packed arrangements, sometimes displaying a “bamboo-like” appearance, with widths around 10 μm. The fine black granular minerals are identified as hematite and magnetite, while brownish-red and brown platy minerals are hematite. Additionally, brown quadrangular columnar rutile inclusions measure approximately 200 μm in width.

Quartz inclusions occur either individually or in clusters, exhibiting both prismatic and hexagonal platy morphologies (Figure 11A). These colorless minerals have been identified through Raman spectroscopy, displaying characteristic Raman shifts at 126, 204, and 463 cm⁻¹ (Figure 11B). Some quartz inclusions contain brown hematite within them (Figure 11C). Hematite was identified by Raman with characteristic Raman shifts at 224, 291, and 409 cm⁻¹ (Figure 11D). Rutile inclusions are predominantly brown and opaque, ranging in size from 5 to 200 μm, with Raman shifts at 441 and 609 cm⁻¹ (Figure 10D and Figure 11E,F). Chrysoberyl inclusions typically exhibit a yellowish-green coloration and also have been identified through Raman spectroscopy, displaying characteristic Raman shifts at 364, 452, 514, 634, 774, and 924 cm⁻¹ (Figure 11G,H).

Furthermore, during Raman spectroscopic analysis, anhydrite inclusions were identified, exhibiting characteristic Raman peaks at 416, 497, 620, 1016, and 1127 cm⁻¹ (Figure 12). These anhydrite inclusions are occasionally found coexisting with quartz (Figure 12C,D).

Aforementioned mineral inclusions have been identified through Raman spectroscopic analysis. Notably, the abundant elongated platy transparent mineral inclusions in Sumbawanga emeralds remain unidentified, as their Raman spectra only show consistency with either quartz or emerald (Figure 13). However, these spectra correspondence contradict the bamboo-like morphology of these inclusions. Therefore, further investigations using polarized light microscopy and EPMA are warranted.

The slender platy inclusions consist of central light green columnar mineral segments and marginal colorless platy minerals (Figure 14).

The former exhibits positive high relief, distinct surface roughness, and birefringence colors reaching second-order yellow or purple, consistent with the optical characteristics of diaspore. The latter shows no cleavage and displays first-order gray-white to yellowish-white interference colors. Compositional analysis identified the former as quartz and the latter as diaspore. The analytical results for diaspore are presented in

Table 2. Chemical composition of diaspore in emerald (EST-17) by EPMA (wt.%).

Points	Na2O	MgO	Al2O3	SiO2	TiO2	Cr2O3	MnO	FeO	Total	Al	Si	Ti	Cr	Mn	Fe
17-1	0.016	0.004	72.595	0.096	- 1	- 1	0.003	4.045	76.774	0.959	0.001	0	0	0.002	0.038
17-2	- 1	- 1	72.751	0.063	0.175	0.03	- 1	5.404	78.455	0.835	0.001	0.102	0.018	0	0.044
17-3	0.014	0.035	71.556	0.262	0.054	0.02	0.011	4.718	76.721	0.900	0.003	0.035	0.013	0.007	0.042

¹ The data is below the detection limit and will not be calculated.

.

Additionally, well-developed chrysoberyl twin inclusions have been identified within the emeralds, including heart-shaped contact twins and radial wheel twins (Figure 15).

4.3. Spectral Characteristics

UV-Vis-NIR Spectroscopy

As a complement to gemological characteristics, UV-Vis-NIR spectroscopy is frequently employed to reflect the color mechanism of emeralds. After eliminating two samples with abundant associate minerals, the UV-Vis-NIR spectra of 13 emerald samples were collected, and they were quite similar to each other. The representative UV-Vis-NIR spectra of Sumbawanga emeralds (EST-09 and EST-10) are shown in Figure 16. There are four characteristic absorption bands: a sharp band at around 370 nm, a narrow band at 426 nm, and two broad bands at around 550–680 and 750–880 nm. Two absorption maxima at around 426 and 606 nm are assigned to the d-d electronic transition of Cr³⁺ [12,20]. The absorption maximum at around 835 nm is caused by the ⁵T₂→⁴E electronic transition of Fe²⁺ at the octahedral Y-site, and the sharp peak at 370 nm is caused by the ⁶A₁→⁴E(D) electronic transition of Fe³⁺ [12,21,22,23]. The bluish-green color of Tanzanian emeralds is jointly induced by Cr and Fe. Among them, Cr makes it present a green hue, while Fe gives it a bluish tone [2,6,12].

4.4. Chemical Composition Characteristics

Representative microprobe analyses of the investigated emerald are provided in

Table 3. Chemical composition of emerald from Sumbawanga by EPMA (wt.%).

Points	EST-1-1	EST-4-1	EST-5-1	EST-5-2	EST-6-1	EST-9-2	EST-9-4	EST-9-6	EST-9-8	EST-11-1	EST-14-1	EST-15-1
SiO2	64.44	67.38	66.69	65.80	68.02	65.82	65.18	65.24	66.33	66.32	62.82	66.07
Al2O3	16.37	17.87	18.23	16.86	17.28	17.19	17.39	17.55	17.84	16.25	16.36	16.64
FeO	1.25	0.84	0.93	1.01	0.84	1.17	1.00	0.90	0.79	1.06	1.04	0.77
MnO	0.01	0.01	- 1	- 1	- 1	0.04	- 1	0.05	- 1	0.01	0.03	- 1
MgO	1.18	0.53	0.51	1.02	0.63	1.02	0.87	0.63	0.49	1.12	1.04	0.67
Cr2O3	0.64	0.07	0.03	0.31	0.02	0.17	0.27	0.25	0.09	0.45	0.26	0.24
V2O5	0.02	0.02	0.03	0.03	- 1	0.01	0.02	0.02	0.03	- 1	0.02	- 1
TiO2	- 1	- 1	0.03	- 1	0.04	0.03	0.02	0.02	- 1	0.06	0.02	0.02
CaO	- 1	0.01	- 1	- 1	- 1	0.01	- 1	0.02	- 1	0.01	0.01	- 1
Na2O	0.48	0.24	0.20	0.26	0.18	0.33	0.30	0.21	0.16	0.59	0.40	0.32
K2O	0.16	0.11	0.16	0.29	0.08	0.24	0.22	0.17	0.15	0.23	0.34	0.20
BeO 2	13.61	14.15	13.95	13.88	14.36	13.85	13.68	13.67	13.90	14.07	13.22	13.97
Total	98.16	101.23	100.76	99.46	101.45	99.88	98.94	98.73	99.78	100.17	95.54	98.90
Si 3	5.86	5.95	5.92	5.91	5.98	5.89	5.89	5.91	5.95	5.90	5.87	5.96
Al	1.76	1.86	1.91	1.78	1.79	1.81	1.85	1.87	1.88	1.70	1.80	1.77
Fe2+	0.09	0.06	0.07	0.08	0.06	0.09	0.08	0.07	0.06	0.08	0.08	0.06
Mg	0.16	0.07	0.07	0.14	0.08	0.14	0.12	0.19	0.07	0.15	0.14	0.09
Cr	0.05	0.01	0.00	0.02	0.00	0.01	0.02	0.02	0.01	0.03	0.02	0.02
Na	0.09	0.04	0.03	0.04	0.03	0.06	0.05	0.04	0.03	0.10	0.07	0.02
K	0.02	0.01	0.02	0.03	0.01	0.03	0.03	0.02	0.02	0.03	0.04	0.02
Be	2.97	3.00	2.97	2.99	3.04	2.98	2.97	2.98	2.99	3.01	2.97	3.03

¹ The data is below the detection limit and will not be calculated. ² The data is calculated based on the standard formula of beryl (Be₃Al₂Si₆O₁₈). ³ Atoms per formula units.

, which are expressed in oxides and formula. Samples with distinct color bands (EST-5 and EST-9) were tested by selecting points in different color areas, respectively, while uniform samples were tested by randomly selecting points. The studied beryl is essentially composed of SiO₂ (62.82–68.02 wt.%) and Al₂O₃ (16.25–18.23 wt.%). The BeO (12.1–14.76 wt%) content of the analyzed beryl was calculated based on the standard formula of beryl (Be₃Al₂Si₆O₁₈). The chemical formula of the samples can be obtained from the calculated atoms per formula units. Taking EST-1-1 as an example, its chemical formula is Be_2.97(Al_1.76Fe²⁺_0.09Mg_0.16Cr_0.09)_2.1Si_5.86O_17.71.

5. Discussion

5.1. The Unique Inclusion Assemblage in Sumbawanga Emeralds

Parallel, elongated platy mineral inclusions are commonly observed in Sumbawanga emeralds (Figure 10B). Although Raman spectroscopy only provided spectra consistent with emerald or host quartz inclusions, comprehensive analysis combining optical properties under polarized light microscopy and EPMA chemical composition testing confirms that these distinctive inclusions consist of central diaspore surrounded by marginal quartz [24]. Furthermore, gemological microscopy and Raman spectroscopic analysis also reveal that these inclusions are composed of two morphologically distinct mineral phases (Figure 13). Despite the current inability to obtain effective Raman spectra for diaspore, it is still possible to distinguish it from similar tubular fluid inclusions and fibrous amphibole inclusions through careful examination [1,9].

Overall, these diaspore inclusions are predominantly oriented parallel to the c-axis. However, a minor proportion of same mineral inclusions exhibit perpendicular orientations (Figure 14). This unique assemblage of mineral inclusions, along with their distinctive morphologies and distribution patterns, may serve as diagnostic features for the identification of Sumbawanga emeralds.

5.2. Updated Features of Sumbawanga Emeralds

Sumbawanga emeralds exhibit a green hue with a slight bluish tint, displaying the characteristic rounded pebble crystal morphologies typical of secondary deposit gemstones. The birefringence is relatively low, ranging from 0.006 to 0.007, which is consistent with the findings reported in 2012 [17]. Their surfaces often host associated minerals, including colorless, gray, and yellow quartz; pink apatite; brown to black mica; brown hematite; and black magnetite.

The fluid inclusions in Sumbawanga emeralds are mainly composed of CO₂ and are often very small [7]. Mineral inclusions of quartz and mica are very common and in addition, there are also anhydrite and chrysoberyl [7,8]. These features were widely observed in the present study, in line with the findings of previous research.

Internal fluid inclusions in these emeralds are typically around 10 μm in size, exhibiting fingerprint-like patterns, with occasional gas–liquid inclusions containing circular bubbles, whose gas-phase consists primarily of CO₂. Mineral inclusions within Sumbawanga emeralds are abundant and diverse, comprising colorless quartz, apatite, and anhydrite; yellowish-green chrysoberyl; red hematite; brown rutile; and black magnetite. Particularly noteworthy are the parallel-oriented diaspore inclusions surrounded by quartz. The inclusions of chrysoberyl include not only individual crystals but also distinctive twin crystals.

Furthermore, to investigate the coloration mechanism of sample EST-09, which exhibits distinct color banding, variations in the concentrations of iron, chromium, and vanadium were analyzed across regions with differing hues. As illustrated in Figure 17, the chromium content is significantly elevated in the more intensely green areas, whereas the iron content does not show a clear positive correlation with the intensity of green coloration. When interpreted in conjunction with the UV-Vis-NIR spectroscopy, these findings indicate that chromium is the primary chromophore responsible for the green color in emeralds, while iron may contribute to the bluish hue characteristic of Sumbawanga emeralds.

5.3. Identification of the Geograohic Originof Sumbawanga Emeralds

The main substituents for Al at octahedral site of emeralds are plotted as oxides in Figure 18. Mg and Fe are the main substituent in most African emeralds, as most of points is close to the FeO-MgO line. The data in this study are largely consistent with those reported in previous studies [8,14,16]. The plots of emeralds from Sumbawanga deposit in Tanzania are closer to the iron terminal than those from Manyara deposit, Kamakanga in Zambia, Sandawana in Zimbabwe, Kentieha in Ethiopia, and Alto Ligonha in Mozambique except for Jos in Nigeria, which can be used to distinguish between different African origins.

As a secondary deposit, the primary geological origin of Sumbawanga emeralds has not yet been definitively determined. The initial discovery of mineral inclusions—specifically diaspore and quartz—may provide insights into the formation environment of these emeralds. Emeralds are likely to form in the contact zone between beryllium-rich hydrothermal fluids and surrounding rocks containing bauxite minerals, such as diaspore [1,7,8,17]. Therefore, detailed investigation of local bauxite occurrences could play a crucial role in identifying primary emerald deposits.

6. Conclusions

The color parameters of Sumbawanga emeralds are as follows: L* ∈ (27.47, 50.43), a* ∈ (−18.40, −6.46), and b* ∈ (0.95, 6.79). Cr makes it green, while Fe gives it a bluish tone. The bluish-green Sumbawanga emeralds typically exhibit pebble-like morphologies and are associated with minerals such as quartz, phlogopite, biotite, hematite, magnetite, rutile, and apatite. These emeralds contain fingerprint-like fluid inclusions and an abundance of diverse mineral inclusions, including quartz, apatite, anhydrite, diaspore, hematite, magnetite, and rutile.

Chrysoberyl, quartz + anhydrite and quartz + diaspore constitute characteristic inclusion assemblages in Sumbawanga emeralds. The distinctive feature of Sumbawanga emeralds is the presence of two mutually perpendicular sets of elongated platy mineral inclusion assemblages of quartz and diaspore, which serve as characteristic diagnostic features.

Comprehensive inclusion and chemical composition analyses can be utilized for the determination of the geographic origin of African emeralds.