Gem News International Gems & Gemology, Fall 2017, Vol. 53, No. 3

Yellow, Green, and Blue Sapphires Reportedly from Antang and Gombe, Nigeria

Sapphires from Antang and Gombe, Nigeria.
Figure 1. These sapphires (0.64–8.27 ct) are from Antang and Gombe, in northern Nigeria. Photo by Shunsuke Nagai.

GIA’s Tokyo laboratory borrowed 19 sapphires (figure 1) from gem dealer Ambrose & Co. in Kofu, Japan. The samples consisted of eight blue, one bluish green, eight yellow, and one greenish yellow/blue bicolor sapphires, all faceted. They were reportedly from Antang and Gombe in northern Nigeria (figure 2). Mambilla, in the southeastern part of the country, is the only well-known Nigerian sapphire source (V. Pardieu et al., “New Nigerian source of blue sapphire,” 2014,; Spring 2017 GNI, pp. 134–135), and the two new sources are not well documented.

Map of Nigerian sapphire sources.
Figure 2. Map of Nigerian sapphire sources. Sapphires from the new deposits at Antang and Gombe have potentially high quality.

Sapphires from both Antang and Gombe are basalt related. The mines are located in a basement complex and on Cretaceous/Tertiary sedimentary rocks near volcanic rocks. The sapphires are mined in alluvial deposits or unconsolidated sediments. According to the dealer, the blue sapphires were from Antang and the yellow sapphires and bicolor sapphire from Gombe. Brume Jeroh, the supplier in Nigeria, mentioned that Antang produces blue, green, yellow, and parti-color sapphires, while Gombe produces mainly yellow sapphires and a few blues. He also noted that stones from these sources may have been mixed together at the market in the city of Jos.

The samples’ standard gemological properties are listed in table 1. Refractive index and specific gravity values were all within the range of corundum. Representative inclusions observed in these sapphires are shown in figures 3 and 4. Quantitative analysis of trace elements was carried out with LA-ICP-MS, and the results are summarized in table 2 and figure 5.

TABLE 1. Standard gemological properties of sapphires from northeastern Nigeria.
Samples Color Weight (ct) Refractive index Specific gravity Source
N01 Yellow 2.84 1.763–1.770 4.00 Gombe
N02 Yellow 2.07 1.765–1.773 4.02 Gombe
N03 Yellow 2.06 1.764–1.772 4.00 Gombe
N04 Yellow 1.84 1.764–1.771 4.00 Gombe
N05 Yellow 1.28 1.764–1.772 4.01 Gombe
N06 Yellow 1.21 1.767–1.772 3.99 Gombe
N07 Yellow 1.03 1.762–1.770 4.00 Gombe
N08 Yellow 0.94 1.762–1.770 4.00 Gombe
N09 Yellow 0.78 1.762–1.770 3.99 Gombe
N010 Greenish yellow and blue 2.04 1.762–1.770 4.03 Gombe
N011 Bluish green 3.51 1.764–1.772 4.01 Gombe*
N012 Blue 2.09 1.763–1.772 4.00 Gombe*
N013 Blue 8.27 1.760–1.768 3.99 Antang
N014 Blue 2.15 1.762–1.770 4.00 Antang
N015 Blue 1.82 1.762–1.770 4.00 Antang
N016 Blue 1.16 1.762–1.770 4.00 Antang
N017 Blue 0.97 1.765–1.770 3.99 Antang
N018 Blue 0.88 1.764–1.771 3.99 Antang
N019 Blue 0.64 1.760–1.768 3.98 Antang
*Samples N11 and N12 were supplied as Antang sapphires but could be Gombe sapphires that were mixed at the market in Jos. This study identified their origin as Gombe based on LA-ICP-MS results.
Inclusions in Gombe sapphires.
Figure 3. Inclusions in sapphires from Gombe: healed fissures with hazy edges (A), irregular particles and short needles (B), wavy graining (C), and flake-like particles (D). Photomicrographs in darkfield illumination (A–C) and diffused brightfield illumination (D) by Shunsuke Nagai. Fields of view 1.95 mm (A), 2.15 mm (B), 2.80 mm (C), and 2.80 mm (D).
Inclusions in Antang sapphires.
Figure 4. Inclusions in sapphires from Antang: zircon crystals (A), reflective thin films with hexagonal patterns (B), bands of minute particles and short needles (C), and angular banding and perpendicularly oriented particles (D). Photomicrographs in darkfield illumination by Yusuke Katsurada (A–C) and Shunsuke Nagai (D). Fields of view 2.80 mm (A), 1.00 mm (B), 4.00 mm (C), and 4.30 mm (D).
Trace elements in Nigerian sapphires.
Figure 5. LA-ICP-MS quantitative analysis allowed the separation of blue sapphires from Antang and yellow sapphires in some trace elements (e.g., Fe vs. V and Fe vs. Ga). The two parts (N10Y and N10B) of the bicolor sapphire show a close trend. The bluish green sapphire (N11) and blue sapphire (N12) are considered to be from Gombe. Results are not averaged.
TABLE 2. Trace element concentrations (in ppma) obtained by LA-ICP-MSa.
Sample no. Be Mg Ti V Cr Mn Fe Ga
N01 0.38 11.4 10.9 1.65 bdl 0.18 2610 68.9
N02 bdl 14.2 12.6 2.0 0.28 0.21 2790 60.9
N03 bdl 15.8 12.1 0.47 bdl 0.08 2970 36.7
N04 bdl 22.8 21.4 1.51 2.36 0.25 3860 39.0
N05 bdl 16.0 13.3 1.88 1.05 0.03 2660 71.6
N06 0.45 20.2 17.6 2.27 3.36 0.04 2667 66.1
N07 bdl 14.9 12.4 1.69 bdl 0.13 2900 70.7
N08 bdl 12.3 33.8 1.04 bdl bdl 1930 89.9
N09 bdl 16.2 14.2 1.82 0.61 0.11 2760 71.0
N010Y bdl 15.4 13.7 1.66 bdl 0.10 2740 68.0
N10B bdl 8.98 40.2 1.67 bdl 0.03 2490 66.6
N011 bdl 10.8 14.6 2.04 21.9 0.27 4000 67.9
N012 1.19 6.88 17.5 1.80 bdl bdl 2380 78.9
N013 bdl 2.25 21.2 4.79 20.9 0.06 807 45.6
N014 bdl 12.8 21.4 6.82 0.37 0.06 1290 49.7
N015 bdl 7.37 92.0 8.79 3.27 bdl 924 56.4
N016 bdl 13.4 60.6 7.22 bdl 0.11 1280 61.0
N017 bdl 8.16 18.4 5.20 5.03 0.03 1470 54.1
N018 bdl 5.10 30.0 6.47 18.3 bdl 1190 49.7
N019 bdl 4.27 30.1 5.89 4.81 bdl 1260 52.7
aAverage of three spots
bdl: below detection limit

The yellow sapphires and the greenish yellow part of the bicolor sapphire showed a typically high Fe, and their Mg concentration is greater than Ti except for one sample (N08; again, see table 2). As shown in figure 5, the high Fe (approximately 2000 ppma) and medium V (approximately 0.50–2.00 ppma) of all the yellow sapphires (N01 to N09) and the greenish yellow part of the bicolor sapphire (N10Y) showed the same trend as those of a bluish green sapphire (N11) and one blue sapphire (N12). The greenish yellow part of the bicolor sapphire (N10Y) matches other yellow sapphires in all elements, and the blue part of the bicolor (N10B) overlaps with N11 and N12. The titanium levels are significantly different between the two parts of N10. This matters, as the titanium is necessary for blue coloration in sapphire with iron. N11 and N12 show different trends from other blue sapphires (from N13 to N19) for V, Fe, and Ga. This suggests that these two samples originated from a different locality. Given that these two samples’ trace element composition is close to that of the blue part of the bicolor sapphire (N10B) and different from the rest of the blue sapphires, we considered them to be from Gombe (again, see table 1). In addition, inclusions exhibited in figure 4 in N13–N19 were not seen in N11 and N12, and these two samples had similar features to each other, as shown in figure 3. Greenish blue and blue sapphires (N11 and N12) and the blue part of the bicolor sapphire (N10B) from Gombe show high Fe (approximately 2000 ppma) and medium V (approximately 1.50–2.00 ppma). Blue sapphires from Antang (from N13 to N19) show lower Fe (approximately 800–1500 ppma) and high V (approximately 4.50 ppma). 

Although sapphires from these two new Nigerian sources are limited in quantity and color range, they are potentially high quality. The importance of the sources is not yet known, and more field data and advanced testing data such as quantitative analyses of trace elements are needed. Despite the different trends in some trace elements between the localities, as this study revealed, they still overlap with trace elements of different magmatic origins including Mambilla (data not shown). Since characteristic inclusions are not always present in basalt-related sapphires and trace elements cannot always be separated, origin determination of these sapphires remains challenging. Interpretation of trace element data such as discriminant analyses and the building of a rich database are necessary.

Yusuke Katsurada is a scientist and staff gemologist at GIA in Tokyo.