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Hong Kong, March 2010 Spinel: A Gemstone on the Rise Dr. Michael S. Krzemnicki Swiss Gemmological Institute SSEF Switzerland Spinel of 110 ct from Tajikistan © Swiss Gemmological Institute SSEF 1 What is Spinel? Spinel as a gemstone is mainly magnesium-aluminium-oxide MgAl2O4 More general: the spinel group 2+ 3+ (Me ) (Me )2 O4 Me2+ tetrahedral coordination Me3+ octahedral coordination Cubic © Swiss Gemmological Institute SSEF Colours of Spinel Pure MgAl2O4 is colourless. Trace elements such as chromium, vanadium, iron, cobalt produce many different shades of colours such as red, pink, orange, purple, blue, black... © Swiss Gemmological Institute SSEF 2 Crystal Shape of Spinel Most spinel crystals are octahedral, but also other cubic forms exist such as rhombic dodecahedra... © Swiss Gemmological Institute SSEF Crystal Shape of Spinel Spinel octahedron But remind: Rubies may also have a pseudo-octahedral “shape” © Swiss Gemmological Institute SSEF 3 Crystal Shape of Spinel Spinel octahedron Spinel crystals are often twinned. © Swiss Gemmological Institute SSEF Formation of spinel Many gemquality spinels are formed in marbles, similar to rubies. - Spinel in dolomite marble (Mg-Ca-carbonate) - Ruby in calcite marble (Ca-cabonate) © Swiss Gemmological Institute SSEF 4 Formation of spinel Chondrodite © H.A. Hänni. SSEF This 6.76 ct clinohumite is a fine example of the "sunflower stone" from Kuh-i-Lal. © Wimon Manorotkul / Pala International Pargasite © C.Hauzenberger Often occurring together with magnesium minerals such as clinohumite and chondrodite (Mg-Fe-silicates), phlogopite (K-Mg mica), pargasite (Na-Ca- Mg-Fe amphibole). © Swiss Gemmological Institute SSEF History of spinel In ancient times, spinel was not known as a mineral and many of the large historic „balass rubies“ from Central Asia and Persia were later identified mineralogically as spinels. One of the finest examples of a Badakhshan spinel is the Black Prince “ruby“ (~ 140 ct) which is mounted in the front of the Imperial State Crown of Great Britain. These large and often just slightly polished and engraved spinels went through the hands of the kings and shas of central Asia, Persia and India and were only described by a few adventurous European travellers such as Jean-Baptiste Tavernier and others at that time. © Swiss Gemmological Institute SSEF 5 Crystal Shape of Spinel © Swiss Gemmological Institute SSEF Exceptional historic spinel necklace certified at SSEF © Swiss Gemmological Institute SSEF 6 Properties of Spinel Spinel is cubic, RI for natural gem quality spinels usually approx. 1.718 SG 3.60 Lustre: High vitreous Hardness: 8 (Mohs scale) Clarity: Often clean stones, no cleavage Colour: by trace elements in many shades from red, pink, orange, purple, to blue and black no pleochroism, usually no colour zoning red spinels often with Cr-luminescence, resulting in bright red colour © Swiss Gemmological Institute SSEF Properties of Spinel © M.S. Krzemnicki, SSEF Most spinel crystals are octahedral, bFaucte atelds ore do tshpeinre lc (u2.b2i4c c fto) wrmiths s epixniesl tc rsyustcahl (15 ct), both from Burma. a© sPh ortho: oMmia Dbixiocn, Pdaolagdeemsc Iantehrneatdionral ... Octahedral shape perfect fitting for cutting proportions © Swiss Gemmological Institute SSEF 7 Properties of Spinel Pure spinel from Pamir (Tajikistan) of approx. 40 cts. Rubies from Vatomandry, Madagascar (above) and Mong Hsu, Burma (below) Spinels are generally rather pure gemstones without distinct colour zones, in contrast to rubies, which are often quite included and fractured and colour zoned ! © Swiss Gemmological Institute SSEF Spinel is usually not treated! A 1.17 ct spinel from Tanzania before (left) and after heating for 5 hrs at 1400 °C, in oxidizing conditions (right). Ruby heated and with © S. Saeseaw et al., GIA (2009) glassy residues in healed fissures. © H.A. Hänni, SSEF Heating experiments on spinels show no or only limited enhancement, apart from clarifying some turbidity in spinel. Heated spinels are detected by microscopy and Raman-microspectrometry (Photoluminescence) at SSEF. © Swiss Gemmological Institute SSEF 8 Synthetic spinel Most are produced by a flame-fusion process (Verneuil). Visit at Djevahirdian SA, Monthey, Switzerland © Swiss Gemmological Institute SSEF Verneuil synthetic spinel Large quantities are produced for technical purposes (e.g. scratch resistant glasses for watches). Usually these synthetic spinels are highly pure, and often show a colour range from blue - light blue, green, and colourless. Verneuil synthetic spinel. Experiences gemmologists generally can separate them easily from natural spinels based on microscopy, UV fluorescent reactions and slightly increased values for RI (1.725 instead of 1.718) and SG ( 3.65 instead of 3.60). The difference in these physical properties is due to a non-stochiometric chemical composition of these Verneuil synthetic spinels, i.e. they contain too much aluminium. Airbubbles in a Verneuil synthetic spinel. As a consequence, there crystal structure is slightly deformed, resulting in strain double refraction under the polariscope and in broadening of the peaks in the Raman spectrum. © Swiss Gemmological Institute SSEF 9 Flux-melt synthetic Spinel Flux-melt synthetic spinels are known since the 90ies, but quite rare. As a consequence of the increased interest and value of red and intense blue spinels, the SSEF has tested recently a number of such synthetic spinels from the market. Flux residue in synthetic spinel Flux-melt synthetic spinel form as nice octahedral crystals, mostly in saturated red and blue colours. These stones are not easy to separate from natural spinel. © Swiss Gemmological Institute SSEF Raman spectra of flux-melt synthetic spinel © Swiss Gemmological Institute SSEF 10 Luminescence spectra of flux-melt synthetic spinel © Swiss Gemmological Institute SSEF Sources of spinel As for ruby and sapphire, spinel of fine quality is often certified. The clients often require an origin determination of the spinel, especially for stones from Burma, Tajikistan (Pamir), Vietnam and Tanzania. At SSEF, origin determination is based on microscopy (inclusions), trace element distributions and UV-Vis and Raman spectroscopy. The origin of spinels can not be determined in all cases due to overlapping properties of spinels from different sources. LA-ICP-MS, for analysis of trace elements and isotopes. © Swiss Gemmological Institute SSEF 11 Major sources of spinel Tajikistan Afghanistan Vietnam Burma (Myanmar) Siam (Thailand) Ceylon (Sri Lanka) Tanzania Madagascar © www.mygeo.info © Swiss Gemmological Institute SSEF Tajikistan (Pamir) Tajikistan is a classical source for large spinels since ancient times. they usually show a pinkish red to pink colour. © Swiss Gemmological Institute SSEF 12 Tajikistan (Pamir) Pierre Lefèvre, gemmologist at SSEF with a superb spinel of 110 cts from Tajikistan. © Swiss Gemmological Institute SSEF Burma (Myanmar) Spinel from Burma is highly appreciated in the market especially for its outstanding saturated red colour. © Swiss Gemmological Institute SSEF 13 Burma (Myanmar) But Burma is also the source of spinels in many pastel colours. © Swiss Gemmological Institute SSEF Madagascar Colour changing spinel (84 ct) from Madagascar © Swiss Gemmological Institute SSEF 14 Vietnam Apart from red and pink spinels, Vietnam has recently produced fine spinels in light purple, light blue and saturated cobalt blue. © Swiss Gemmological Institute SSEF Vietnam Characteristic crossed inclusions in spinel from Vietnam. © Swiss Gemmological Institute SSEF 15 Tanzania Main sources for spinel in Tanzania are Matombo, Mahenge and Tunduru in the south. Matombo Mahenge Tunduru © Swiss Gemmological Institute SSEF Tanzania, Mahenge © Swiss Gemmological Institute SSEF 16 Tanzania, Mahenge © H.A. Hänni, SSEF Oriented planes of fine „dust“ precipitates in spinel from Tanzania © Swiss Gemmological Institute SSEF Tanzania, Mahenge Matombo Mahenge © Swiss Gemmological Institute SSEF 17 Tanzania, Mahenge © Swiss Gemmological Institute SSEF Tanzania, Mahenge Spinel from Mahenge (3.4 ct), showing a characteristic attractive bright red colour. Geological map of the Mahenge fold nappe. Indicated in blue are the marble rocks. The spinels (and rubies) are found at different spots in these marbles or near-by eluvial and aluvial gravels. © Swiss Gemmological Institute SSEF 18 Tanzania, Mahenge In 2007, a 52 kg (260‘000 cts) spinel was discovered in Mahenge. Although only partly of gem quality, this resulted in a sudden supply of large (up to 50 cts) and many small vibrant pinkish red spinels (totally several © W. Spaltenstein and D. Weinberg, Multicolour, Thailand thousand carats) in the market. © Swiss Gemmological Institute SSEF Trip to Mahenge, October 2009 © Swiss Gemmological Institute SSEF 19 Tanzania, Mahenge © Swiss Gemmological Institute SSEF Tanzania, Mahenge Mahenge Morogoro Province © Swiss Gemmological Institute SSEF 20 Going to the mine: Epangko, south of Mahenge © Swiss Gemmological Institute SSEF Epangko, Spinel mines Gemmologist Walter Balmer at the Epangko spinel mines. © Swiss Gemmological Institute SSEF 21 Epangko, Spinel mines © Swiss Gemmological Institute SSEF Epangko, Spinel mines © Swiss Gemmological Institute SSEF 22 Epangko, Spinel mines © Swiss Gemmological Institute SSEF 23 © Swiss Gemmological Institute SSEF Epangko, Spinel mines © Swiss Gemmological Institute SSEF 24 © Swiss Gemmological Institute SSEF Epangko, Spinel mines © Swiss Gemmological Institute SSEF 25 26 Thank you for your attention... High Noon in Mahenge... 27.
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