Gem News International
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Gem News International Contributing Editors Emmanuel Fritsch, University of Nantes, CNRS, Team 6502, Institut des Matériaux Jean Rouxel (IMN), Nantes, France ([email protected]) Gagan Choudhary, Gem Testing Laboratory, Jaipur, India ([email protected]) Christopher M. Breeding, GIA, Carlsbad ([email protected]) I ) I ------------------------------------------------------------ 1_______________________________________________________________ _ COLORED STONES AND ORGANIC MATERIALS Blue gahnite from Nigeria. In recent years, Nigeria has gained considerable attention in the gem trade for the dis- covery of sapphires reportedly from the Mambilla Plateau in Taraba State. In addition to sapphires, spinels have been sporadically mined in Jemaa and recently in Kagoro, both in Kaduna State (figure 1). The spinel group of minerals, with a general chemical formula AB O , has a total of 22 2 4 different species. Four of these species have Al predomi- nantly occupying the octahedral site: spinel (MgAl O ), 2 4 galaxite (MnAl O ), hercynite (FeAl O ), and gahnite 2 4 2 4 (ZnAl O ). The spinels from Kagoro have been identified 2 4 as gahnite, the zinc end member. Gahnite is one of the rarer members of the spinel group of minerals, typically found at zinc deposits. Most crystals are very small, in- Figure 1. Three single octahedra of natural gahnite, ap- cluded, and translucent to opaque. Faceted gems are very proximately 1.5 ct each, from Kaduna State, Nigeria. collectible due to their rarity and color. Photo by Robison McMurtry. The geology of the Nigerian spinels has been previously studied (R. Jacobson and J.S. Webb, “The pegmatites of cen- tral Nigeria,” Geological Survey of Nigeria Bulletin, No. variety of source environments. For example, Zn content 17, 1946, pp. 1–61), and the deposits can be subdivided into is high in spinels occurring in rocks that have experienced three groups based on their mineralogy: (1) microcline- low-grade metamorphism and high oxygen and sulfur fu- quartz pegmatites, which commonly occur within the gacity (fO and fS ), whereas Zn content is lowest in high- 2 2 calc-alkaline granitoids and are rarely mineralized; (2) mi- grade metamorphic rocks (A. Heimann et al., “Zincian crocline-quartz-mica pegmatites, found within metasedi- spinel associated with metamorphosed Proterozoic base- mentary sequences; and (3) quartz-mica veins, which occur metal sulfide occurrences, Colorado: A re-evaluation of in schists and gneisses or marginal to group 2 pegmatites. gahnite composition as a guide in exploration,” Canadian Gahnite occurs in groups 2 and 3. Knowledge of the com- Mineralogist, Vol. 43, No. 2, 2005, pp. 601–622). position of zinc-rich spinels is useful in separating a wide Standard gemological examination of three rough octa- hedra (figure 1) gave the following properties: color—blue; pleochroism—none; refractive index—1.791 to over the Editors’ note: Interested contributors should send information and illustra- limit (flat reading from polished crystal face); hydrostatic tions to Stuart Overlin at [email protected] or GIA, The Robert Mouawad specific gravity—4.180–4.294; fluorescence reaction—inert Campus, 5345 Armada Drive, Carlsbad, CA 92008. to both long- and short-wave UV;and color filter reaction— red. Internal characteristics observed with a gemological GEMS &G EMOLOGY,V OL. 55, NO. 3, pp. 434–455. microscope were colorless unidentified crystals, finger- prints, and a red-brown crystal inclusion identified by © 2019 Gemological Institute of America Raman spectroscopy as sphalerite (figure 2). 434 GEM NEWS INTERNATIONAL G EMS & GEMOLOGY F ALL 2019 VISIBLE SPECTRUM 0.8 ) –1 0.7 0.6 0.5 0.4 0.3 0.2 0.1 ABSORPTION COEFFICIENT (cm COEFFICIENT ABSORPTION 0 Figure 2. Raman identification of this inclusion was 400 450 500 550 600 650 700 consistent with sphalerite. Photomicrograph by WAVELENGTH (nm) Nathan Renfro; field of view 0.91 mm. Figure 3. The visible spectrum of the Nigerian gah- nite from Kagoro. The absorption bands from ~500 to 650 nm are largely due to Co2+, but with modifi- Advanced spectroscopic testing was performed on the cation from iron absorption bands. Absorption three samples. The Raman spectra were typical of gahnite, bands around 462 and 470 nm are related to iron with peaks at 420, 510, and 661 nm. The visible spectrum chromophores. showed significant cobalt absorption bands (figure 3) be- tween 500 and 620 nm, with additional contribution from iron absorption bands modifying the cobalt absorption spec- trum (e.g., A.C. Palke and Z. Sun, “What is cobalt spinel? Unraveling the causes of color in blue spinels,” Fall 2018 Laser ablation–inductively coupled plasma–mass spec- G&G, pp. 262–263). trometry (LA-ICP-MS) analysis was used to obtain the ac- curate chemistry of the three stones. The chemical composition was acquired by a ThermoFisher iCAP Q ICP-MS coupled with an Elemental Scientific Laser Figure 4. A vertical line 1 and a horizontal line 2, con- NWR213 laser ablation system. NIST 610 and 612 were taining a total of 44 spots, were selected to apply LA- used as external standards, and 27Al was used as an inter- ICP-MS analyses across the whole section. Mol.% end nal standard. LA-ICP-MS results (see appendix 1 at member vs. position profiles revealed that the stone https://www.gia.edu/doc/FA19-GNI-Appendix1.xls) was predominantly composed of gahnite with minor showed that the three stones were predominantly com- hercynite, spinel, and galaxite. Detailed information posed of more than 90 mol.% gahnite (ZnAl O ), with on major element chemistry, site distribution, and 2 4 minor other end members of Al-spinel species. They species for each spot can be found in appendix 1 on- should thus be classified as gahnites. line. Spot spacing is 200 microns. To better understand the composition of this type of spinel, we prepared a cross-section cut from the middle of Line 1 one octahedral crystal, sample NBS3 (figure 4). A vertical 1 line of 22 spots (figure 4, line 1) and a horizontal line of 22 2 3 spots (figure 4, line 2) were selected to cross the whole sec- 4 5 tion from outer rims to the opposite outer rims for LA- 6 1 ICP-MS analyses. All the spots showed more than 90 2 3 4 7 5 6 7 8 8 9 10 mol.% gahnite with hercynite as the second most abun- 11 13 14 1516 dant species (figure 5). Spots near the outer rims contained 171819 11 202122 12 Line 2 more hercynite than spots on the inner rims and core. The 13 mol.% end member distribution was very consistent 14 15 throughout the whole section. Nine trace elements for 16 17 each spot in lines 1 and 2 were plotted in figure 5. In gen- 18 19 eral, the outer rims had higher concentrations of V and Co 20 but lower concentration of Ni than the inner rims and 21 22 core. Maxwell Hain and Ziyin Sun GIA, Carlsbad GEM NEWS INTERNATIONAL G EMS & GEMOLOGY F ALL 2019 435 CROSS SECTION LINE 1 Li 200 50000 --Be --V --Co --Ni --Ga PPMW 20 5000 --Mg PPMW --Mn --Fe 2 500 1 3 5 7 9 11 13 15 17 19 21 1 3 5 7 9 11 13 15 17 19 21 Analytical Spots Analytical Spots CROSS SECTION LINE 2 200 50000 PPMW 20 5000 PPMW 2 500 1 3 5 7 9 11 13 15 17 19 21 1 3 5 7 9 11 13 15 17 19 21 Analytical Spots Analytical Spots Figure 5. Trace element vs. position profiles revealed that the outer rims had higher concentrations of V and Co, but lower concentration of Ni than the inner rims and core. Detailed information on trace elements for each spot in lines 1 and 2 can be found in appendix 2 online (https://www.gia.edu/doc/FA19-GNI-Appendix2.xls). Emerald update with Arthur Groom. At the 2019 AGTA president, Arthur Groom, noted that the Afghanistan GemFair in Tucson, Eternity Natural Emerald (Ridgewood, emerald melee sets (figure 6) had drawn the most interest. New Jersey) had emeralds up to 50 ct on display. Eternity’s This rough melee is particularly attractive, he said, because of its higher brilliance than stones from other sources. “The Afghanistan melee is the brightest in its natural state Figure 6. Melee goods from Eternity Natural Emerald. when it’s cut properly,” he said. “Afghanistan emerald will Photo by Kevin Schumacher. rival Colombian any day of the week.” Eternity has experimented with melee from different sources but finds Afghanistan melee the most plentiful and consistent. The company currently cuts Afghanistan emeralds in sizes from 1 mm to 70 ct, disproving the per- ception in the trade that the country produces only small emeralds. Groom pointed out that vibrant greens are not as easy to find in gems as other colors, and tsavorite is often used for green, but Eternity’s emeralds offer the op- tion to use one of the “Big Three” gemstones. Eternity also sources emerald rough from Ethiopia, Colombia, Brazil, and Zambia. Cutting and mounting emerald melee has traditionally been challenging because of emerald’s tendency for inclu- sions and fractures, and its typical enhancements. Most 436 GEM NEWS INTERNATIONAL G EMS & GEMOLOGY F ALL 2019 Figure 8. A set of calibrated emeralds (approximately 2 to 4 mm each) by Eternity Natural Emerald. Photo by Kevin Schumacher. Figure 7. A set of 15 of Eternity Natural Emerald’s cal- New find of deep blue aquamarine from Nasarawa State in ibrated emeralds ranging from 1 to 5 mm. Photo by Nigeria. The most unusual and exquisite things are often Albert Salvato. found in unexpected places. This has been seen in the ex- plosion in gem discoveries in East Africa since the latter part of the twentieth century.