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Volume 23 No. 6. April 1993 TheJournal of Gemmology THE GEMMOLOGICAL ASSOCIATION AND GEM TESTING LABORATORY OF GREAT BRITAIN OFFICERS AND COUNCIL Past Presidents: Sir Henry Miers, MA, D.Sc., FRS Sir William Bragg, OM, KBE, FRS Dr. G.F. Herbert Smith, CBE, MA, D.Sc. Sir Lawrence Bragg, CH, aBE, MC, B.Sc., FRS Sir Frank Claringbull, Ph.D., F.lnst.P., FGS Vice-Presidents: R K. Mitchell, FGA A.E. Farn, FGA D.G. Kent, FGA Council ofManagement D.}. Callaghan, FGA C.R Cavey, FGA T.}. Davidson, FGA E.A. jobbins, B.Sc., C.Eng., FIMM, FGA I. Thomson, FGA V.P. Watson, FGA, DGA RR Harding, B.Sc., D.Phil., FGA, C. Geol. Members' Council A.}. Allnutt, M.Sc., J. Kessler I. Roberts, FGA Ph.D., FGA G. Monnickendam E.A. Thomson, P. J. E. Daly, B.Sc., FGA L. Music Hon. FGA R Fuller, FGA, DGA }.B. Nelson, Ph.D., FGS, R Velden B. Jackson, FGA F. lnst. P., C.Phys., FGA D. Warren G.H. Jones, B.Sc., Ph.D., P. G. Read, C.Eng., C.H. Winter, FGA, DGA FGA MIEE, MIERE, FGA, H. Levy, M.Sc., BA, FGA DGA Branch Chairmen: Midlands Branch:D.M. Larcher, FBHI, FGA, DGA North-West Branch: I. Knight, FGA, DGA Examiners: A. J. Allnutt, M.Sc., Ph.D., FGA G. H. Jones, B.Sc., Ph.D., FGA L. Bartlett, B.Sc., M.Phil., FGA, DGA D. G. Kent, FGA E. M. Bruton, FGA, DGA R D. Ross, B.Sc., FGA C. R Cavey, FGA P. Sadler, B.Sc., FGS, FGA, DGA S. Coelho, B.Sc., FGA, DGA E. Stern, FGA, DGA A.T. Collins, B.Sc., Ph.D Prof. I.Sunagawa, D.Sc. B. Jackson, FGA C. Woodward, B.Sc., FGA, DGA E. A. Iobbins, B.Sc., C. Eng., FIMM, FGA Editor: E. A. Jobbins, B.Sc., C.Eng., FIMM, FGA Editorial Assistant: Mary A. Burland Curator: C. R Cavey, FGA The GemmologicalAssociation and GemTestingLaboratory ofGreatBritain 27 Greville Street, London EClN 8SU Telephone: 071-404 3334 Fax: 071-404 8843 ^Journal of Gemmology VOLUME 23 NUMBER SIX APRIL 1993 Cover Picture The Hematita mine in Minas Gérais in 1987 - probably the richest alexandrite concentration ever known. Note the whitish kaolinitic gravel (which carries the gem material) in the foreground, the soil profile in the centre and the valley confluence in the background. (See 'The location, geology, mineralogy and gem deposits of alexandrite, cat's-eye and chrysoberyl in Brazil' pp. 333-54) ISSN: 0022-1252 F.G.AF.G.A.. youyourr kekeyy ttoo ththee fascinatingfascinating worlworldd ooff GemmologGemmologyy The Gemmological The Gemmological The Gemmological The Gemmological Association and Gem Association's course Association and Gem Associations course Testing Laboratory of has been specially Testing Laboratory of has been specially Great Britain has been compiled into a series Great Britain has been compiled into a series actively involved in of teaching modules actively involved in of teaching modules the very beginning of that are both readable the very beginning of that are both readable Gemmology as an and lavishly Gemmology as an and lavishly independent science. illustrated. independenToday, thet science . illustratedTogether. they AssociationToday, the takes provideTogethe a r they Associatiopride in itsn takes providcomprehensivee a step­ pridinternationale in its comprehensivby-step guide toe stepthe - internationareputation las a learned by-stefascinatingp guid worlde to thofe reputatiosociety ndedicated as a learne to d i fascinatingemmologyg worl andd anof societthe promotiony dedicate dof t o gemmologopportunityy toan earnd an thgemmologicale promotion o f opportunitthe covetedy initialsto earn gemmologicaeducation andl the thFGAe covete - Fellowd initial of thes educatiospread nof an d the FGA-FelloGemmologicalw o f the spreagemmologd of ical The GemmologicaAssociation. l gemmologicaknowledge. l AssociationTo find out. more knowledgeThe Association's. Gemmological Association completeTo find outhet morcoupone covetedThe Association Fellowships is andGemmologica Gem Testingl Associatio Laboratoryn completbelow ande th wee coupowill n covetea markd Fellowshiof excellencep is and Gem Testing Laboratory beloimmediatelyw and w eforward will a onlymar kbestowed of excellenc upone Of Great Britain immediatelthe Associaty ioforwarn's d onlmembers,y bestowe whod upo n Of Great Britain thprospectuse Association givings memberssuccessfu, whlly oundertake the Association's full details of this uniqprospectuue course.s giving successfullwritten andy undertakpractical eDiploma the Association examinations.s full details of this unique course. writte1 -n an--d practica- --l Diplom- a---- examinations-. .- - - --- --- - - I To GAGTL Education Departmenl, 27 Greville Streel, London ECl N 8SU I Please send details and application form for the Diploma in Gemmology Course. rI To GAGTL Education Department, 27 Greville Street, London ECl N 8SU I Please send details and application form for the Diploma in Gemmology Course. I Name: I I Name: I I Address: I I Add ress: I I I I Telephone: GEM; I L ____ ___ ______ _ _ _ __ _ __ _ _ ~ Telephone: . GEM; L: _l 323323 JJ.. Gemm.,Gemm., 1993,1993, 2323,, 6 A methodmethod forfor obtainingobtaining opticoptic figuresfigures fromfrom inclusioninclusionss John l.I. KoivulaKoivula ChiefChief ResearchResearch Gemologist,Gemologist, GemologicaGemologicall InstitutInstitutee ofof AmericAmericaa GeGemm TradTradee LaboratoryLaboratory,, Inc.,Inc., SantaSanta Monica,Monica, California,California, USUSAA AbstractAbstract A methodmethod forfor obtainingobtaining opticoptic figures fromfrom doublydoubly refractive inclusionsinclusions inin transparenttransparent gemsgems usingusing a a stan­standard dard gemmologicalgemmological microscopmicroscopee iiss introducedintroduced.. SomeSome usefuluseful equipmenequipmentt andand ththee limitationslimitations ooff thithiss techniquetechnique are discussed. IntroductionIntroduction InIn gemmologygemmology,, non-destructivnon-destructivee testintestingg iiss thethe normnorm.. ThThee vasvastt majoritmajorityy ooff ththee gemsgems encounteredencountered cannot be be examined examined in ina manner a manner that thatis destructive is destruc­ tiveor evenor even potentially potentially destructive destructive to their to their Fig.Fig. 1.1. TheThe stoneholdcr-conoscopcstoneholder-conoscope mademade byby HaroldHarold OateOatess useusedd toto resolveresolve optiopticc figures from indusionsindusions.. NoteNote ththee sisixx diff'crent­different- sizedsized interchangeableinterchangeable sphericalspherical lenseslenses.. Photograph by RobertRoben FigFig.. 2.2. OneOne axisaxis ofof a biaxiabiaxiall opticoptic figurefigure obtainedobtained bbyy thithiss methodmethod Weldon from aann inclusioninclusion iinn a pinpinkk spinelspinel octahedronoctahedron from StiSri Lanka.Lanka. Photomicrograph by by John John I. I. KoivuJa Koivula appearanceappearance.. ThThee microscopimicroscopicc studystudy ooff inclusionsinclusions playsplays a majomajorr rolrolee iinn non-destructivenon-destructive gemgem identifi­identifi­ cationcation.. InclusionInclusionss exposedexposed aatt ththee surfacesurface durinduringg thethe initialinitial cuttincuttingg processprocess araree ideallyideally suitedsuited forfor detaileddetailed examinationexamination.. HoweverHowever,, itit iiss rarelyrarely possiblepossible toto actu­actu­ allallyy grindgrind dowdownn a gegemm toto exposeexpose aann inclusioinclusionn atat thethe surfacesurface foforr X-rayX-ray diffractiodiffractionn oorr chemicalchemical anal­anal­ ysisysis.. YeYett inclusioninclusion recognitionrecognition anandd identificationidentification isis ononee ofof thethe mormoree importantimportant scientificscientific disciplinesdisciplines iinn modermodernn gemmology.gemmology. IInn thithiss arenaarena mosmostt gem­gem- mologistsmologists musmustt workwork 'with'with theitheirr handshands tied'.tied'. AlthougAlthoughh itit iiss veryvery importanimportantt ttoo bbee ableable toto deter­deter­ minemine iiff a gegemm isis naturanaturall oorr synthetic,synthetic, andand treatedtreated oorr untreated,untreated, thithiss mustmust bbee accomplishedaccomplished iinn waysways whichwhich areare nonott harmfuharmfull toto thethe gemgem itself.itself. AAtt ththee retailretail anandd wholesalwholesalee levelevell wherwheree a greatgreat manymany gegemm identificationsidentifications araree donedone,, gemmologistsgemmologists andand appraiserappraiserss dodo notnot havhavee immediateimmediate accessaccess toto testingtesting instrumentsinstruments suchsuch aass X-raX-rayy difI'ractometers,diffractometers, electronelectron microprobemicroprobess andand RamaRamann laselaserr probes.probes. InclusionInclusion identificatioidentificationn iiss generallygenerally accomplishedaccomplished bbyy meansmeans ofof ththee gemmologicagemmologicall microscope.microscope. CertaiCertainn patternpatternss andand typetypess ooff inclusioninclusionss thathatt havehave beebeenn seenseen beforbeforee areare recognizedrecognized agaiagainn whenwhen theythey araree encounteredencountered.. IInn manymany cases,cases, althoughalthough thethe overalloverall inclusioninclusion scenescene presentpresentss testimonytestimony thathatt a © CopyrightCopyright thethe GemmologicalGemmological AssociationAssociation ISSN:ISSN: 0022-12520022-1252 J. Gemm., 1993,23,6 324 gem is natural, individual mineral inclusions apparently colourless, doubly refractive crystal within the scene remain unidentified. If such that measured approximately 1.5 x 1.0 millimetres inclusions are well formed, then their external was used, rather than the calcite-in-ruby described morphology gives evidence of their identity. above. This was done because spinel is isotropic Colour and degree of transparency are also useful and there is no

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