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For years, gem-quality yellow danburite was known from Myanmar and Madagascar (e.g., W. Wight, "Danburite," Canadian Gemmologist, Vol. 6, No. 4, 1985, pp. 110–113), and also reported from Sri Lanka (see, e.g., Spring 1986 Lab Notes, p. 47) and California (e.g., Fall 1998 Gem News, p. 220). Most recently, yellow danburite gemstones were described from a new locality in Tanzania ("Nuovi ritrovamenti: Danburite gialla di qualità gemma e gatteggiante dal Madagascar e dalla Tanzania," Rivista Gemmologica Italiana, Vol. 2, No. 3, 2007, pp. 228–229).
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Figure 1. This pegmatite in Tanzania produced significant quantities of yellow danburite. Courtesy of W. Radl.
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In January 2008, we learned more about this new Tanzanian danburite from Mark Kaufman (Kaufman Enterprises, San Diego, California) and Werner Radl (Mawingu Gems, Niederwörresbach, Germany). Mr. Radl subsequently visited the deposit, which is located in the Morogoro region, in April 2008. The danburite is mined from at least two steeply dipping granitic pegmatites (e.g., figure 1) using hand tools and explosives. The pegmatites are hosted by marble, and locally contain coarse-grained pale blue-green K-feldspar (amazonite) and black tourmaline (identified as dravite-schorl by Dr. William "Skip" Simmons at the University of New Orleans by electron-microprobe analysis). The same pegmatites have reportedly produced smoky quartz and a translucent bluish violet quartz that appears to be colored by abundant micro-inclusions.
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Figure 2. Yellow danburite from Tanzania commonly contains abundant growth tubes, as shown by this chatoyant cabochon (8.22 ct) and twinned crystal (12.68 g). Photo by Robert Weldon.
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Mr. Kaufman loaned GIA, for examination, two rough specimens (a 1.61 g fragment and a 12.68 g crystal) as well as two faceted (1.88 and 7.14 ct) and two cabochon-cut (8.22 and 19.41 ct) examples of the yellow danburite (e.g., figure 2). Mr. Radl showed one of us (BML) some additional faceted stones (e.g., figure 3) at the February 2008 Tucson gem shows, including a 22.48 ct oval cut, and donated a 4.37 g crystal to the Institute. Both of the crystals were tabular and twinned, resulting in a leaf-like appearance (again, see figure 2).
Examination of the cut stones gave the following properties (where they differed, values for the cabochons are noted in parentheses): color—light-to-medium yellow to orangy yellow; pleochroism—none; RI—nα = 1.629–1.631 and nγ = 1.638 (spot reading of 1.62); birefringence—0.007–0.009; hydrostatic SG—3.01 (2.97 and 3.00); inert to both long- and short-wave UV radiation; and no spectrum observed with the desk-model spectroscope. Most of these properties are consistent with those given by M. O’Donoghue, Ed. (Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, p. 403): nα = 1.627–1.633 and nγ = 1.633–1.639, birefringence—0.006–0.008, and SG—3.00. However, O’Donoghue reports blue to blue-green fluorescence to long-wave UV radiation, and in some stones an absorption spectrum consisting of fine lines that correspond to rare-earth elements (REEs). Wight (1985) also noted blue (and violet-blue) to blue-green fluorescence in danburite. The absence of fluorescence in the Tanzanian samples we examined—which were confirmed as danburite by Raman spectroscopy—is therefore somewhat unusual, though consistent with the Sri Lankan stones described in the Spring 1986 Lab Note.
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Figure 3. Transparent yellow danburite from Tanzania has been faceted into attractive gemstones (here, 9.92 and 8.67 ct). The pear shape shows distinct color zoning (yellow and near colorless). Courtesy of Mawingu Gems; photo by Robert Weldon.
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Microscopic examination of the two faceted samples revealed long, thin, curved growth tubes (figure 4, top), as well as "fingerprints" containing angular liquid inclusions and some two-phase (liquid and gas) inclusions. The presence of growth tubes and fingerprints is consistent with internal features reported in yellow danburite from Mogok, Myanmar (Summer 2007 GNI, pp. 167–168). While the two cabochons contained similar inclusions, plus a few fractures, a much denser concentration of parallel growth tubes in some stones (figure 4, bottom) resulted in chatoyancy (again, see figure 2).
The cause of color in yellow danburite is as yet undetermined. EDXRF analysis of five samples detected the presence of Si and Ca in major amounts, minor Sr, and traces of Fe and REEs (Ce and possibly La or Pr). Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS)—performed on the two cabochons and the larger twinned crystal by GIA Laboratory research scientist Dr. Andy H. Shen—verified the presence of Sr (~450 ppm), Fe (~30 ppm), and the light rare-earth elements La (~350 ppm), Ce (~500 ppm), Pr (~30 ppm), and Nd (~60 ppm). Heavier REEs (Sm through Lu) were present in very low concentrations (<5 ppm) or were below the detection limit of the instrument. A UV-Vis-NIR spectrum collected from the larger faceted stone showed (in addition to a transmission window encompassing the yellow region) a minor absorption feature extending from ~564 to 589 nm. This feature peaked at 585 nm, which correlates with an absorption line attributed to REEs in yellow danburites from Sri Lanka (again, see the Spring 1986 Lab Note).
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Figure 4. Growth tubes in the Tanzanian danburite may exhibit a curved, almost hooked, appearance (top). Chatoyant material contains dense concentrations of parallel growth tubes (bottom). Photomicrographs by K. M. Chadwick; image width approximately 4.4 mm.
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According to Mr. Radl, the Tanzanian danburite was first discovered in late 2006, but only small amounts were produced until late 2007. As of the February 2008 Tucson shows, he had obtained approximately 1 tonne of mixed-grade material, but less than 5 kg was of gem quality. From this small amount he had faceted ~200 carats, in clean stones weighing up to 27.8 ct (but typically <10 ct). In late February 2008, gem dealer Syed Iftikhar Husain (Syed Trading Co., Peshawar, Pakistan) reported seeing a parcel of rough Tanzanian danburite in Bangkok consisting of 2.4 kg of chatoyant material and 1.6 kg of faceted rough, as well as a clean faceted stone weighing about 50 ct. He estimated that larger included stones could be cut in weights approaching 150 ct.
Karen M. Chadwick and Brendan M. Laurs
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