Multi-Color Sapphires Reportedly from the Garba Tula District, Isiolo County, Kenya
The Garba Tula sapphire deposit, located in Isiolo County in central Kenya, is a unique igneous gem-corundum producing area. It is a productive source of blue, green, and yellow sapphires (C. Simonet et al., “The Dusi (Garba Tula) sapphire deposit, Central Kenya–A unique Pan-African corundum-bearing monzonite,” Journal of African Earth Sciences, Vol. 38, 2004, pp. 401–410). Figure 1 shows the overall color appearance of 28 Garba Tula sapphires recently examined in this study. They were purchased from a reliable dealer at the Tucson gem shows in 2019. It was noticed that their color varied from dark blue to yellow through various shades of blue and green. Some showed strong yellow and blue zoning, while samples with bluish and greenish color banding were also observed. Interestingly, one of the sapphires exhibited a color-change appearance (indicated by the black arrow in figure 1). Windows were polished into the samples in order to study their internal features and collect chemical data. For some samples, windows were polished parallel to the c-axis for spectroscopic measurements.
The studied materials revealed a slightly higher refractive index (1.761–1.773) than typical metamorphic sapphires. The other standard gemological properties were typical of natural corundum. The vast majority of these Garba Tula sapphires (75%) displayed no fluorescence under long-wave UV radiation, while 18% fluoresced red and the remainder showed zoned orange fluorescence (with very weak red fluorescence or none). The fluorescence intensity to long-wave UV of Garba Tula sapphires is generally very weak to weak. However, they were totally inert to short-wave UV light.
The characteristic inclusions observed from these 28 Garba Tula sapphire samples are presented in figure 2. More than 90% were included with irregular brownish needles and platelets identified by Raman as hematite/ilmenite (Y. Katsurada et al., “Golden sheen sapphire and syenite/monzonite–hosted sapphire from Kenya,” Fall 2018 G&G, pp. 322–323), formed either as planes along the color banding or as a group or cloud. These multiple twinning planes, commonly associated with parallel and/or intersecting growth tubes, were found in more than 80% of the stones. A few samples contained numerous single and scattered clusters identified by Raman spectroscopy as zircon crystals (figure 2, right), sometimes with fine rutile needles and reflective particle clouds, but with fewer irregular brownish platelets. Fingerprint inclusions as well as the other crystal inclusions identified using Raman spectroscopy and the RRUFF reference database, such as apatite and mica, were occasionally observed in the Garba Tula sapphires studied. Moreover, crystals of carbonate minerals were also previously reported in Garba Tula sapphire (Y. Katsurada et al., 2018). However, they typically lack sharp growth structure and milky clouds, as frequently seen in Madagascar and Sri Lanka.
FTIR spectra obtained from the sapphires often showed one or more diagnostic features: a single 3309 cm–1 peak as well as OH-related mineral features such as boehmite, kaolinite, and gibbsite. It was noticed that every sample exhibited a single 3309 cm–1 peak without any subordinate peaks, while the boehmite and kaolinite were observed often (>70%) with a small percentage (18%) showing the gibbsite mineral feature.
Interestingly, the UV-Vis-NIR spectrum of greenish blue to blue sapphires from Garba Tula exhibited a metamorphic-type sapphire spectrum, with strong Fe3+-related absorption features at 377, 388, and 450 nm and also an Fe2+-Ti4+ intervalence charge transfer (A.C. Palke et al., “Geographic origin determination of blue sapphire,” Winter 2019 G&G, pp. 536–579). A broad band centered at 880 nm, which is typically used to indicate basalt-hosted sapphire, was not observed.
LA-ICP-MS showed comparable trace element chemistry profiles between the different color areas of the stones, as presented in table 1. The Garba Tula corundum contains significant amounts of Fe and small amounts of Ti. High Fe content ranging from 1555–3177 ppma and 2121–2943 ppma were detected in greenish blue to blue and yellowish to greenish zones, respectively, while the Ti concentration ranged from 3–41 ppma. The high amounts of Fe were also higher than those of Mogok (Myanmar) and Tunduru (Tanzania) sapphires, which have been previously reported as high-Fe metamorphic sources (W. Soonthorntantikul et al., “An in-depth gemological study of blue sapphires from the Baw Mar mine (Mogok, Myanmar),” GIA Research News, 2017, https://www.gia.edu/gia-news-research/blue-sapphires-baw-mar-mine-mogok-myanmar). Therefore, the chemical composition of the Garba Tula sapphires, particularly their high Fe, is not common for metamorphic sapphire deposits.
The discovery of this source for yellow, green, and blue sapphires could be interesting for the gem trade. The material represents a true challenge for gemologists working on origin determination, as their formation is quite complicated. Although some properties and chemistry of the Garba Tula sapphires are comparable to the corundum xenocrysts originating from alkali basalt fields, their UV-Vis-NIR and FTIR spectra and their internal features are not consistent with typical basalt-related sapphires. The inclusion scene might, in some aspects, overlap between high-Fe metamorphic and basalt-related sapphires. The reflective/iridescent brownish platelets can be seen in metamorphic sapphires from Mogok, Myanmar, as well as in basalt-hosted sapphires from Thailand and Ethiopia. But none of the glassy melt inclusions usually observed in basalt-related sapphires were noticed in the Garba Tula sapphires. One hypothesis is that they formed in the same way as the basalt-related stones but just were not brought to the surface by the basalt. Therefore, a combination of multiple techniques can be useful in the origin determination of sapphires from Garba Tula.
