Vol. 20 No. 3 July 1986 TheJournal of Gemmology GEMMOLOGICAL ASSOCIATION OF GREAT BRITAIN OFFICERS AND COUNCIL President: Sir Frank Claringbull Ph.D., F.lnst.P., F.G.S. Vice-Presidents: J. R. H. Chisholm, M.A., F.G.A. R. K. Mitchell, F.G.A. Chairman: D. ]. Callaghan, F.G.A. Vice-Chairman: N. W. Deeks, F.G.A. Honorary Treasurer: N. B. Israel, F. G. A. Fell&Ws elected to Council: C. R. Cavey B. Jackson M. J. O'Donoghue, L. F. Cole C. B. Jones M. A., F.G.S. R. W. Croydon D. M. Larcher P. G. Read, C.Eug., P. J. E. Daly, B.Sc. D. Morgan M.I.E.E., M.I .E. R.E. A. J. French G. Neary A. W. R. Round S. E. Hiscox J. B. Nelson, Ph.D., F.R.M.S., E. Stern J. A. W. Hodgkinson F. InsLP., F.G.S. C. H. Winter D. Inkersole W. Nowak, C.Hug., F.R.Ae.S. Branch Chairmen: Midlands Branch: P. West, F.G.A. Norlh-West Branch: S. Hill, F.G.A. South Yorkshire & Dislrict Branch: J. I. Reynolds, F.G.A. Examiners: E. A. Jobbins, B.Sc., F.I.M.M., F.G.A. A. J. Allnutt, M.Sc., Ph.D., F.G.A. R. R. Harding, B .Sc., D . Phil. , F.G .A. G. H. Jones,B.Sc., Ph.D., F.G.A. D. G. Kent, F.G.A. E.M.Bruton,F.G.A. P. Sadler,B.Sc., F.G.S., F.G.A. C. Woodwara., B.Sc., F.G.A. A.E. Farn, F.G.A. M. Font-Altaba, D.Sc. ]. M. Bosch-Figueroa, D.Se. M. Virkkunen, M.Phil., F.G.A. 1 nstruc tors: V. G. Hinton, F.G.A. S. B. NikonCooper, B.D., F.G.A. P. A. Waters, F.G.A. J. Edwards, F.G.A. D. Pratt, F.G.A. R. J. Peace, B.Sc., F.G.A. Editor: E. A. Jobbins, B.Sc., F.I.M.M., F.G.A. Consultant Editor: J. R. H. Chisholm, M.A., F.G.A. Editorial Assistant: Mary A. Burland Curator: C. R. Cavey, F.G.A. Secretary: Con Leuan, F.G.A. Deputy Secretary: Jonathan Brown, Barrister-at-Law, F.G.A. Saint Dunstan's House, Carey Lane, London EC2V 8AB (By Goldsmith's Hall) Telephone:01-7264374 J. Gemm., 1986,20,3 137 TheJournalof^Journal of GemmologyGemmology VOLUMEVOLUME 2020 NUMBERNUMBER THREETHREE JULYJULY 19819866 Cover PicturePicture ProtogeneticProtogenetic crystalscrystals ofof alteredaltered hornblendehornblende werewere first coatedcoated withwith anan initiainitiall layelayerr ofof opalopal andand thethenn laterlater werewere completelycompletely engulfedengulfed byby thisthis MexicanMexican opal.opal. TransmittedTransmitted andand obliqueoblique illumination.illumination. 4Sx.45x. SeeSee'Inclusion 'Inclusionss inin opal',opal', pppp.. 139-44.139-44. Photograph from'from 'PhotoatlasPhotoatlas ofofinclusions inclusions in in gemstones'. gemstones'. ISSN:ISSN: 0022-12520022-1252 138 J. Gemm., 1986, 20, 3 NEW GEMMOLOGY COURSE The Gemmological Association of Great Britain is proud to announce that from September 1986 it is introducing a new home study course in gemmology. This will prepare students for the examinations leading to the award of the Association's Fellowship Diploma. The new course is radically different from other gemmological courses, and presents a new, friendly, step-by-step approach to learning that should be welcomed by students all over the world. For further details, contact the Education Department, Gemmological Association of Great Britain, Saint Dunstan's House, Carey Lane, London, EC2V 8AB. Tel: 01-726 4374. Cables: GEMINST. JJ.. Gemm.Gemm.,, 1986,1986, 2020,, 3 139139 The long-awaitedlong-awaited new new bookbook about about inclusions inclusions in ingemstones gemstones by DrbyDrE. E. Giibelin Giibelin and and M r Mr J.I. J. KoivulaI. Koivula will will appear appear on on the marketmarket this this Autumn Autumn underunder the the title title of'Photoatlas of'Photoatlas of ofinclusions inclusions in gemstones'fin gemstones''f and and we arewe aredelighted delighted to learn to learn that that this bookbook is is dedicated dedicated to to Basil Basil W. W. Anderson. Anderson. Dr E. Giibelin has beenbeen successful successful in in securing securing the the co-operation co-operation ofProfessor of Professor H. 0 H.O.A. .A. Meyer Meyer and and Dr EDr . RoedderE. Roedder in in the UnitedUnited States, States, andand ofofProfessor Dr Dr H.A. H.A. StalderStalder inin Switzerland. Switzerland. The volume resultingresulting from from this this collaboration collaboration reveals reveals a spectacular a spectacular triumph triumph ofcolour of colour photomicrography, photomicrography, and the and the work isis obviouslyobviously destined destined to to become become a classic a classic in the in thefield. field. We Weare aretherefore therefore pleased pleased to publish to publish a foretaste a foretaste of this of work this work in thethe form form ofofthe the chapter chapter on on inclusions inclusions in inopal. opal. It isIt especiallyis especially appropriate appropriate to include to include it here, it here,since thesince subject the subject has not has not been presentedpresented in in the the Journal Journa lbefore. before. We congratulate Dr Dr Giibelin Giibelin and and his his team team on on their their great great achievement. achievement. The Editor InclusionInclusionss inin opalopal Dr Edward Gubelin,Gübelin, e.G.,C.G., F.G.A.* andJohnI.and John I. Koivula, C.G.,F.G.A.**C.G.,F.G.A.** *Ratna*Ratna Mahal,Mahal, BenzeholzstrassBenzeholzstrassee 11II,, CH-604CH-60455 MeggeMeggenn LU,LU, SwitzerlandSwitzerland **Gemological**Gemological InstituteInstitute ofof America,America, Santa Monica,Monica, CACA 90406,90406, USAUSA PreciousPrecious opal,opal, prizeprizedd forfor itsits spectralspectral plaplayy ooff andand thethe satisfyingsatisfying harmonyharmony ofof thethe livelylively colour­colour- colour,colour, iiss commonlycommonly describedescribedd asas anan amorphousamorphous play,play, isis thethe contemplationcontemplation ofof ththee manifoldmanifold uniqueunique silicasilica gel.gel. ButBut thethe amorphousamorphous statestate ofof opal'sopal's inclusionsinclusions - mainlymainly inin opalopal fromfrom Mexico.Mexico. ThisThis iiss structurestructure isis questionable.questionable. ItIt isis nownow knownknown thatthat opalopal especiallyespecially soso sincesince ththee greatgreat majoritymajority ofof opalsopals foundfound isis structurallystructurally composedcomposed ofof a more-or-Iessmore-or-less orderly,orderly, allall overover thethe worldworld eithereither havehave nono inclusionsinclusions oror thesethese close-packedclose-packed arrangementarrangement ofof submicroscopicsubmicroscopic cannotcannot bbee seenseen becausebecause ththee hosthost gemgem isis tootoo opaque.opaque. spheresspheres ofof twotwo crystallinecrystalline mineralmineralss - cristobalitecristobalite TheThe brownisbrownishh opalopal termedtermed hyalite,hyalite, founfoundd inin andand tridymite.tridymite. InIn MexicanMexican opal,opal, whichwhich formedformed atat Mexico,Mexico, containscontains whitewhite spherulitesspherulites asas largelarge asas oneone higherhigher temperaturestemperatures thanthan itsits AustralianAustralian cousin,cousin, thethe millimetremillimetre inin diameter.diameter. ThisThis hyalitehyalite isis calledcalled irisiris spheresspheres consistconsist ratherrather ofof cristobalite.cristobalite. InIn thethe opalopal whenwhen itit displaydisplayss anan iridescenceiridescence resultingresulting AustralianAustralian opalopal theythey areare mormoree commonlycommonly tridymite.tridymite. fromfrom a finelyfinely layeredlayered internalinternal botryoidalbotryoidal structure.structure. TheThe sizesize ofof thesethese spheresspheres andand thethe interstitialinterstitial spacesspaces BrazilianBrazilian whitewhite opalsopals maymay containcontain tinytiny darkdark greygrey betweenbetween themthem dictate,dictate, throughthrough thethe diffractiondiffraction ooff octahedraoctahedra ofof thethe nickelnickel ironiron sulphide,sulphide, bravoite.bravoite. whitewhite light,light, thethe playplay ofof colourscolours anan opalopal willwill display.display. FineFine particlesparticles ofof sand-likesand-like matrixmatrix andand anan unevenuneven FormedFormed iinn sedimentarysedimentary oror volcanicvolcanic rocks,rocks, throughthrough distributiondistribution ofof body-colourbody-colour areare notednoted iinn somesome thethe invasioninvasion ofof low-temperaturelow-temperature silica-bearingsilica-bearing AustralianAustralian stones.stones. AsAs a resultresult ofof theirtheir translucenttranslucent toto solutions,solutions, opalopal isis essentiallyessentially composedcomposed ofof silica,silica, clearclear transparenttransparent opal-medium,opal-medium, oneone cancan seesee thatthat thethe withwith variablevariable waterwater ofof hydration.hydration. OpalsOpals may,may, inin firfiree opalsopals foundfound iinn MexicoMexico areare alivealive withwith inclusions.inclusions. time,time, looseloose theirtheir waterwater andand dehydratedehydrate toto chalcedony.chalcedony. NeedleNeedless ofof hornblende,hornblende, whichwhich onceonce protrudedprotruded fromfrom A smallsmall percentagepercentage ofof additionaladditional elementselements suchsuch asas thethe rhyoliterhyolite matrix,matrix, areare oftenoften trapped.trapped. Quartz,Quartz, aluminium,aluminium, iron,iron, calcium,calcium, magnesiummagnesium andand sodiumsodium goethitegoethite andand eveneven two-two- andand three-phasethree-phase inclusionsinclusions inin thethe formform ofof submicroscopicsubmicroscopic mineralmineral grains,grains, maymay areare bubutt a fewfew ofof thethe inclusionsinclusions identified.identified. TheThe alsoalso bbee present.present. followingfollowing richlrichlyy varyingvarying photomicrographsphotomicrographs ofof opalopal TheThe bodybody colourcolour ofof precioupreciouss opalsopals rangesranges fromfrom inclusionsinclusions offeroffer anan impressiveimpressive showshow ofof theirtheir unsuspec­unsuspec­