TOURMALINE AS an INCLUSION in ZAMBIAN EMERALDS by John I
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TOURMALINE AS AN INCLUSION IN ZAMBIAN EMERALDS By John I. Koivula This article reports the identification of tourmaline The two deposits are geologically similar. The crystals as inclusions in emeralds from Zambia, in emeralds are found in biotite-phlog'opite schists south-central Africa. To understand the paragenesis in which dark brown to blaclz tourmaline also responsible for this unusual association, the geology occurs. Other roclzs intimately associated with of the area is reviewed. The gemological properties of the emerald- and tourmaline-bearing mica schists these emeralds are also noted. are talc-magnetite schists and quartz-amphibo- lite-chlorite schists with secondary quartz veining (Bank, 1974; Sinkankas, 1981). It is thought that the chromium necessary to provide the trace- elemental coloration of the emerald was derived To date, a variety of interesting mineral inclu- from magnetite in the talc-magnetite schists, as sions have been reported in the emeralds from the the magnetite in these roclzs has been shown to deposits at Milzu-Kafubu, in Zambia (Sinlzanlzas, contain a small percentage of chromium (Bank, 1981). These include the micas biotite and phlo- 1974). gopite, rutile, and apatite. Recently, the author had the opportunity to study three rough emer- TOURMALINE AS AN INCLUSION alds and one faceted stone from this locality with Because tourmaline is found throughout the schist inclusions that appeared to be tourmaline, a min- host rock in direct association with the emeralds, eral that' has been reported as inclusions in em- it is not surprising to find tourmaline crystals as eralds from the Urals in the Soviet Union and inclusions in the emeralds. The inclusions ob- from the Habachtal mine in Austria (Sinlzanlzas, served in the four specimens studied by the au- 1981; Gubelin, 1974)but has not previously been thor are prismatic and, as is typical of tourma- noted as occurring in Zambian emeralds. lines, striated parallel to the c-axis (see figure 1). If the inclusions the author observed are in They are transparent dark orangy brown in a fact tourmaline, then a new paragenetic relation- strong transmitted light or on very thin edges, but ship could be established and a new species added generally they appear blaclz and opaque. They oc- to the list of known inclusions in Zambian em- cur up to 10 mm in length and 2 mm in diameter. eralds. To this end, the geology of the area is re- Many are easily seen with the naked eye. viewed and the inclusions themselves described Surrounding the tourmalines are numerous in detail. Also, the gemological properties of these tiny, whitish-appearing tension fractures that ex- I stones are compared to those previously reported tend into the host emerald. Some of these, as il- for Zambian emeralds. lustrated in figure l, are decorated with brownish GENERAL GEOLOGY OF THE ZAMBIAN EMERALD AREA ABOUT THE AUTHOR The main emerald-producing area in Zambia Mr. Koivula is the senior staff gemologist in the Gem Identification Department of the Gem Trade Laboratory, Inc., comprises two deposits, Milzu and Kafubu, which Santa Monica, California. lie within a few kilometers of each other. This emerald field is located in the Kitwe district of Acknowledgments: The author would like to express his sincere thanks to Julius Petsch for donating the emeralds with northern Zambia, approximately 32 lzm south- tourmaline inclusions for this study, and to Peter Keller for west of Kitwe and 40 lzm west-northwest of Luan- arranging the donation. A special thanks goes to Chuck Fryer shya, near the entrance of the Miku River into the for the X-ray diffraction work, to Roger Kuchek for his assistance in obtaining the ColorMaster notations, to Stephen Kafubu River (Bank, 1974).Both Miku and Kafubu Hofer for his consultation on the absorption spectra, and to produce excellent, gem-quality emeralds; some William Kerr for faceting the large stone used in this study. crystals weigh well over 100 ct. ''1983 Gemological Institute of America Notes and New Techniques GEMS & GEMOLOGY Winter 1982 225 Figure 2. Cross-sectional view of one of the dravite tourmaline inclusions breaking the surface of a rough crystal of Zambian emerald, Oblique illumination, magnified 50x. GEMOLOGICAL PROPERTIES One faceted and three rough emeralds were ex- amined. The faceted stone was an emerald cut that weighed 0.91 ct, and the largest rough crystal section weighed 3.48 ct. The emeralds varied in color from a bluish green of medium intensity to a dark, intense bluish green. Using the GEM ColorMaster, the color of the 0.91-ct faceted stone was dialed in as 00 red, 43 Figure 1. A black-appearing crystal of dravite green, and 22 blue using the E modifier. This read- tourmaline in a Zambian emerald is decorated ing calculated as 0.230 x, 0.586 y, and 5.72 z for by numerous small tension fractures, some the coordinates on the CIE color chart, giving a of which are stained by limonite. This very slight bluish green by CIE standards. photomicrograph was taken using a combination of low-intensity transmitted light Refractive index and optic character were de- and oblique illumination, magnified 20~. termined using a Duplex I1 refractometer and a sodium vapor light source. Refractive indices of = 1.581 and M = 1.588 were noted, which gives orange limonite. In cross section, as in figure 2, an optic character and sign of uniaxial negative the tourmalines display typical trigonal or trian- and a birefringence of 0.007. gular outlines. No crystallograpl~icallydictated The emeralds were studied for their visible alignment was noted between the tourmaline in- light absorption characteristics using a Beck clusions and their host. wavelength prism spectroscope and the GEM Noted gemologist Chuck Fryer scraped one of spectroscope unit. In the direction of the ordinary the inclusions that reached the surface and ob- ray, parallel to the c-axis, sharp absorption lines tained enough powder for X-ray diffraction anal- could be seen at approximately 479 nin, 610 nm, ysis. The inclusion was found to correspond to 638 nm, and 670 nm, with a single, somewhat tourmaline in the schorl-dravite series, schorl thicker band at approxin~ately682 nnl. having the chemical composition Na(Fe,Mn).jAl,jBs A general absorption band was also noted be- Si,A7(OH,F),,and dravite, NaMg3~BB.jSi,j027(OH,F).i.tween 580 nin and 610 nm. At 90Âfrom the c-axis, The diffraction pattern obtained matched dravite only four distinct absorption lines were observed: more closely than schorl, but a detailed chemical 638 nm, 650 nnl, 670 nin, and 690 nm. All of the analysis would be necessary to conclusively place absorption points in these emeralds can be cor- these tourmalines in their proper position in the related to known chromium absorption peaks schorl-dravite series. (Sinl<ankas, 198 1). Notes and New Techniques GEMS & GEMOLOGY Winter 1982 The emeralds showed no reaction to long- not yet been found included in Zambian tour- wave or short-wave ultraviolet radiation, or to malines, it can be inferred that the emeralds may X-radiation. be geologically younger than the tourmalines. The specific gravity was hydrostatically deter- In reviewing the suite of inclusions now known mined for two of the emerald specimens. One to occur in Zambian emeralds, we find striking specimen, devoid of tourmaline and containing similarities to inclusions in emeralds from other only a small number of mica platelets, showed an schist-type deposits. In fact, the emeralds from average specific gravity reading of 2.71. The sec- the mica schist in Habachtal, Austria (Gubelin, ond specimen, shot through with numerous tour- 1956, 1974), are known to play host to all of the maline crystals as well as with mica and other minerals so far found as inclusions in Zambian assorted inclusions, revealed a specific gravity of emeralds. Such comparable parageneses support 2.82. This higher S.G. could be due in part to the the view that schist-type emeralds all undergo presence of the schorl-dravite tourn~aline,which similar stages of genesis. has a specific gravity (3.03-3.25) that is much higher than that of emerald. The gemological properties obtained by the author for refractive Editor's note: An attractive example of a faceted Za~nbicinemerald if, illustrated in the Gem Trade index and specific gravity (on the less included Lab Notes section of this issue. stone) were in close agreement with those pub- lished by Bank (1974) for emeralds from Miku, Zambia. REFERENCES CONCLUSION Bank H. (19741 The emerald occurrence of Miku, Zambia. journal of Gemmology, Vol. 14, No. 1, pp. 8-15. Although tourmaline has been reported as inclu- Gubelin EJ. (1956) The emerald from Habachtal. Gems es) sions in.erneralds from the Ural Mountains in the Gemology, Vol. 8, No. 10, pp. 295-309. Soviet Union and from the Habachtal emerald Gubelin E.J. (1974) Internal World of Gemstones. ABC edi- tion, Zurich, Switzerland. mine in Austria, this is the first report of such Sinkankas J. (19811 Emerald and Other Beryls. Chilton Book inclusions in African emeralds. Since emerald has Co., Radnor, PA. $9 SECOND ANNUAL @ GEMS & GEMOLOGY MOST VALUABLE ARTICLE AWARD This issue marks the end of the 1982 the postcard ballot and drop it in the mail volume year of Gems & Gemology. 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