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X GAGTL EDU GemmolosX"^ The Journal of "I y Volume 27 No. 8 October 2001 tj J Cultured Cat's-eyes and Nephrite Synthetic pearls star stones jade moissanite The Gemmological Association and Gem Testing Laboratory of Great Britain Gemmological Association and Gem Testing Laboratory of Great Britain 27 Greville Street, London EC1N 8TN Tel: 020 7404 3334 Fax: 020 7404 8843 e-mail: [email protected] Website: www.gagtl.ac.uk President: Professor A.T. Collins Vice-Presidents: A.E. Farn, R.A. Howie, D.G. Kent, R.K. Mitchell Honorary Fellows: Chen Zhonghui, R.A. Howie, R.T. Liddicoat Jnr, K. Nassau Honorary Life Members: H. Bank, D.J. Callaghan, E.A. Jobbins, H. Tillander Council of Management: T.J. Davidson, R.R. Harding, I. Mercer, J. Monnickendam, M.J. O'Donoghue, E. Stern, I. Thomson, J-P. van Doren, V.P. Watson Members' Council: A.J. Allnutt, S. Burgoyne, P. Dwyer-Hickey, S.A. Everitt, J. Greatwood, B. Jackson, L. Music, J.B. Nelson, P.G. Read, P.J. Wates, C.H. Winter Branch Chairmen: Midlands - G.M. Green, North West - I. Knight, Scottish - B. Jackson, South West - R.M. Slater Examiners: A.J. Allnutt, M.Sc, Ph.D., FGA, L. Bartlett, B.Sc, M.Phil., FGA, DGA, S. Coelho, B.Sc, FGA, DGA, Prof. A.T. Collins, B.Sc, Ph.D, A.G. Good, FGA, DGA, J. Greatwood, FGA, G.M. Howe, FGA, DGA, S. Hue Williams MA, FGA, DGA, B. Jackson, FGA, DGA, G.H. Jones, B.Sc, Ph.D., FGA, Li Li Ping, FGA, DGA, M. Newton, B.Sc, D.Phil., C.J.E. Oldershaw, B.Sc. (Hons), FGA, DGA, H.L. Plumb, B.Sc, FGA, DGA, R.D. Ross, B.Sc, FGA, DGA, P.A. Sadler, B.Sc, FGA, DGA, E. Stern, FGA, DGA, S.M. Stocklmayer, B.Sc. (Hons), FGA, Prof. I. Sunagawa, D.Sc, M. Tilley, GG, FGA, CM. Woodward, B.Sc, FGA, DGA The Journal of Gemmology Editor: Dr R.R. Harding Assistant Editors: M.J. O'Donoghue, P.G. Read Associate Editors: Dr C.E.S. Arps (Leiden), G. Bosshart (Zurich), Prof. A.T. Collins (London), Dr J.W. Harris (Glasgow), Prof. R.A. Howie (Derbyshire), Dr J.M. Ogden (Hildesheim), Prof. A.H. Rankin (Kingston upon Thames), Dr J.E. Shigley (Carlsbad), Prof. D.C. Smith (Paris), E. Stern (London), Prof. I. Sunagawa (Tokyo), Dr M. Superchi (Milan), CM. Woodward (London) Production Editor: M.A. Burland Vol 27, No. 8, October 2001 ISSN: 1355-4565 Cultured pearls and colour-changed cultured pearls: Raman spectra Li Liping and Chen Zhonghui Gemmological Institute, China University of Geosciences, Wuhan 430074, P.R. China ABSTRACT: The numbers of coloured pearls on the market have grown over the past few years and include dyed pearls, irradiated pearls and so-called 'lasered' pearls. In this study, different pearl samples have been experimentally dyed or irradiated. Their Raman spectra have been obtained and found to be useful in identifying the colour origin of pearls. Keywords: colour origin, dyed pearl, irradiated pearl, natural coloured pearl, Raman spectrum Introduction coloured freshwater cultured pearls. Our research work has shown that the so-called ver the years, the range of coloured Tasered' freshwater cultured pearls are actu­ freshwater cultured pearls generally ally dyed(1). Our studies have confirmed that Oavailable has increased significantly colour features, colour distribution, internal 451 and includes black, peacock green, dark characteristics, ultra-violet fluorescence, and purple and bronze varieties. Some are dyed, chemical tests are useful in identifying some some are irradiated freshwater cultured dyed or irradiated pearls (op. cit.). However, pearls, and some are even labelled as many treated cultured pearls do not show Tasered'. These are in addition to naturally any distinctive features. Furthermore, when #•% i Figure 1: Some of the pearl samples studied: dyed cultured pearls (left) and irradiated cultured pearls (right). © Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355-4565 Table I: Chemical data for five pearls. Sample Freshwater pearls Tahiti pearl white orange purple di/erf rfarfc Element purple at.% F-l F-2 F-3 FA F-6 Ca 37.98 37.78 37.91 38.17 38.12 Na 0.26 0.26 0.28 0.27 0.75 ppm Mg 39 25 48 40 79 Si 840 790 1800 1300 1600 P 210 220 190 170 170 Al 9 8 13 24 14 K 56 52 69 238 238 Sr 394 332 402 373 900 Ba 410 550 390 330 190 Fe 5 36 11 21 49 Mn 286 323 643 241 1 451 Cu 1 1 1 1 2 Zn 4 3 4 4 5 Ni 1 1 2 <1 <1 Co <1 <1 <1 <1 <1 Cr <1 <1 <1 <1 <1 N.B.: Results obtained from Hitchi 180-70 atomic absorption spectrometer. a pearl is mounted, identification of its green line (514.5 nm) of an Ar-ion laser colour origin is made more difficult. as excitation. In order to obtain comparable Raman spectra, the power and exposure For this study, some of the samples were times were kept constant for all the samples. obtained directly from the market and others were treated by dyeing or irradiation The chemical compositions of natural- (Figure 1). Some of the cultured pearls were coloured and dyed pearls were obtained by dyed black by immersion over a twelve day dissolving small samples of pearl and period in either 0.025N silver nitrate, or in analysing the solutions with a Hitachi 180-70 silver nitrate diluted with ammonia atomic absorption spectrometer; the results (Figure 2). Some were treated by electron are shown in Table I. accelerator. Eighteen samples were tested In this study, all the freshwater cultured using the Renishaw MK1-1000 laser Raman pearls are non-nucleated and all the spectrometer at room temperature with the seawater cultured pearls are nucleated. /. Gemm., 2001, 27, 8, 449-455 Figure 2: Cultured pearls dyed with AgN03 only (left) and pearls dyed using AgN03 diluted by ammonia (right). Results and discussion The black Tahiti cultured pearl samples show very strong fluorescence under the Naturally-coloured cultured pearls green laser, and in the Raman spectrum Naturally-coloured freshwater cultured (Figure 4) there is a broad low peak at 1605 pearls are usually white, orange or purple as cm4 beside the typical sharp peaks at 1083 shown in Figure 3. It is easy to distinguish and 702 cm1 of aragonite. As can be seen naturally-coloured freshwater cultured from the chemical analysis (Table I, column pearls from black Tahiti cultured pearls F-6) the Tahiti pearl contains very small (whether nucleated or non-nucleated) on quantities of minor elements, which proba­ colour alone. However, the lighter grey bly would have little influence on the colour. Tahiti cultured pearls may prove to be more The peak at 1605 cm4 is relatively low and difficult to differentiate. comparatively wide, indicating that it may 451451 451 Figure 3: Naturally-coloured Chinese freshwater cultured pearls. Cultured pearls and colour-changed cultured pearls: Raman spectra '083 25000 1605 20000 ~ 15000 0 II 10000 702 5000 '600 '400 ' 200 '000 800 600 400 200 Ramanshift (cm-t) fi gure 4: Raman spectrum of Tahiti cultured pearl. 1084 '5000 10000 1524 452 5000 3046 2650 ~ 1132 A 226 1 151 JU ' 90 :r 27' 3500 3000 2500 2000 ' 500 1000 500 Raman shift (em-I) f igure 5: Raman spectrum of naturally-colouredorangefres hwatercultured pearl. 6000 1083 6000 1510 15 19 1127 00 C 4000 263 1 5 3730 3024 II 2000 '295 1017 702 3500 3000 2500 2000 1500 1000 500 Raman shift (cm-r) Figure 6: Raman spectrum of naturally-coloured purplefreshwater cultured pearl. J. Gemm., 2001, 27, 8, 449-455 12000 1579 10000 8000 ~ 6000 6 0 4000 2000 3500 3000 2500 2000 1500 1000 500 Raman shift (cm-t) Figure 7: Ramanspectrum offreshwater culturedpearl dyed black by silvernitrate. be caused by a substance of low crystalline Dyed pearls structure, which would include organic Many freshwater and saltwater cultured materials. It has been suggested by other pearls are dyed various colours. The dyed researchers (e.g. see(2)) that pigments such as black cultured pearls often show very good porphyrin seem to be the cause of colour in iridescence. The iridescent peacock green and Tahiti cultured pearls. purple colours associated with the highly For naturally-coloured orange freshwater valued natural-coloured black Tahiti cul­ 453 cultured pearls, the Raman spectrum tured pearls can also be seen in the so-called (Figure 5) is very different to that seen in 'Iasered' freshwater cultured pearls. The Tahiti cultured pearls. In addition to the colour distribution in both types is quite sharp and strong peaks of aragonite at 1084 even. The 'lasered' freshwater cultured and 702 cm', there are sharp peaks at 1524 pearls are actually dyed - the process and 1132 cm' and also some weak peaks at involves immersion for about one year in a 2650 and 2261 cm'. The sharp peaks indicate new organic dye. that the impurities are in the crystal structure. All the dyed cultured pearl samples show The chemical analyses for the orange fresh­ very different Raman spectra to the naturally­ water cultured pearls reveal iron (Table I, F-2) coloured cultured pearl samples. The three being present in a comparatively high black cultured pearls dyed by silver nitrate enough concentration to be a possible methods produced differing colour satura­ contributory cause of the orange colour. tions, but similar Raman spectra. The Further research is needed to establish if this absorption peaks of aragonite are commonly is the cause of colour. present but, compared with Raman spectra The naturally-coloured purple freshwa­ of undyed cultured pearls, what was also ter cultured pearls show a complicated found were peaks relating to carbon as seen Raman spectrum, with sharp peaks at 1127, in the likes of jet'3) (Figure 7).
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