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4/9/2015 OpalValue,Price,andJewelryInformationInternationalGemSocietyIGS
http://www.gemsociety.org/article/opaljewelryandgemstoneinformation/ 1/21
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HomeGemstoneEncyclopediaOpalValue,Price,andJewelryInformation
OpalValue,Price,andJewelryInformationDr.JoelArem,DonaldClark,CSMIMG
BarionEmeraldCutOpalfromLambinaMines,AustraliabyDanielStair
OpalValueValuingpreciousopalisahighlyrefinedart.Forfulldetails,seeAppraisingOpalsPart1,NomenclatureandAppraising
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OpalsPart2,ApplyingValue
OpalValueviaGemPriceGuideLastUpdatedOn:October27,2013
BlackOpal,toneN1toN4
CaratWeight
Belowcommercial
Commercial Good Fine Extrafine
1to5 $$/ct
$$/ct
$$,400/ct
$,400$,000/ct
$,000$,000/ct
5to10/ct
$$/ct
$$/ct
$$,400/ct
$,400$,000/ct
$,000$,000/ct
10+ $$/ct
$$/ct
$$,400/ct
$,400$,400/ct
$,400$,000/ct
DarkOpal,toneN5toN6
CaratWeight
Belowcommercial
Commercial Good Fine Extrafine
1to5 $$/ct
$$/ct
$$,200/ct
$,200$,000/ct
$,000$,000/ct
5to10/ct
$$/ct
$$/ct
$$,200/ct
$,200$,000/ct
$,000$,000/ct
10+ $$/ct
$$/ct
$$,200/ct
$,200$,600/ct
$,600$,400/ct
LightOpal,toneN7toN9
BodyTone
Belowcommercial
Commercial Good Fine Extrafine
N7 $$/ct
$$/ct
$$/ct
$$,000/ct
$,000$,400/ct
N8 $$/ct
$$/ct
$$/ct
$$/ct
$$,200/ct
N9 $ $ $ $ $
"3in1Gemstone"SteelyBlueSapphiretreatedornot?ColorCharts?April1eNewsMiniQuiznaturalroughdiamonds
ScholarshipsIGSseekstoencouragetheadvancementofstudentspursuingahighereducation.Learnmoreaboutthe2015ClarkScholarship.
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$/ct $ $/ct $/ct $/ct
CrystalOpal
CaratWeight
Belowcommercial
Commercial Good Fine Extrafine
1to5 $$/ct
$$/ct
$$/ct
$$,000/ct
$,000$,000/ct
5to10/ct
$$/ct
$$/ct
$$/ct
$$,000/ct
$,000$,400/ct
10+ $$/ct
$$/ct
$$/ct
$$,000/ct
$,000$,000/ct
BoulderandMatrixOpal
CaratWeight
Belowcommercial
Commercial Good Fine Extrafine
1to5 $$/ct
$$/ct
$$/ct
$$/ct
$$/ct
5to10/ct
$$/ct
$$/ct
$$/ct
$$/ct
$$/ct
10to15
$$/ct
$$,20/ct
$$/ct
$$/ct
$$/ct
15+ $$/ct
$$/ct
$$/ct
$$/ct
$$/ct
CompositeOpals
Doubletspricesareusually15%to35%ofthevalueofsimilarlookingnatural,(solid,)gem.However,theyoccasionallyrunashighas50%ofanaturalopaliftheyareofexceptionalquality.
Tripletsareonlyabout1%ofthevalueofanequivalentnaturalandpeakat$.
ContraLuz Allsizes
$to$/ct
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FireOpals,faceted
.4to1carat 1to5carats 5caratsplus
$to$/ct
$to$/ct
to$/ct
FireOpals,Cabochons
.4to2carats 2to10carats
Solid $to$/ct $to$/ct
InMatrix $to$/ct
PinkOpal Allsizes
$/ct
GoldMembersOnly(learnmore)
OpalInformationData Value
Name Opal
Varieties CommonOpal,AgateOpal,AndeanOpal,CacholongOpal,Girasol,HoneyOpal,Hyalite,Hydrophane,JasperOpal,MilkOpal,MossOpal,PinkOpal,PraseOpal,Semiopal,WaxOpal,WoodOpal,FireOpal,PreciousOpal,BlackOpal,BoulderOpal,CrystalOpal,HarlequinOpal,MatrixOpal,WaterOpal,WhiteOpal
Crystallography Amorphous.Recentworkshowsthatopaliscomposedofanaggregateoftinysphericalparticles,thatis,asolidifiedgeloftenformsconcretionsbotryoidalreniformstalactitic.
RefractiveIndex
1.441.47
Colors Colorless,white,yellow,orange,andred(variousshades),yellowishbrown,greenish,blue,gray,black,violet.
Luster Vitreous,waxy,pearly.
PolishLuster Vitreoustoresinous
FractureLuster Waxy,subvitreous
Fracture Conchoidaltouneven
Hardness 5.56.5
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Toughness Fairtoverypoor
SpecificGravity 1.992.25
Density 1.992.25orangeredvariety~2.00blackandwhiteopal,2.10greenopal,2.12.
Cleavage None.Fractureconchoidal.Brittle.
Dispersion Verylow.
HeatSensitivity Very
Luminescence GreenfluorescenceinopaloftenduetoincludedUminerals.MuchopalfluorescesstrongwhiteinSW,LW,sometimeswithpersistentphosphorescence.(Seetablebelow.)
Spectral None.
Wearability Poor
SpecialCareInstructions
Veryheatsenstive,cleanwithwarmorroomtemperaturesoapandwater.Avoidwearinggemwhereitwillgetroughtreatment.
AbsorptionSpectrum
Mostnone.Greenlineat660nm,cutoffat470nm.
Phenomena PlayofColor.Asterism,chatoyancy,bothrare.
Birthstone October
Formula SiO2nH2O.Water=121%inopal,usually610%inpreciousopal.
OpticsIsotropicN:1.441.47.Mexicanopalaslowas1.37,usually1.421.43.
FluorescenceinOpal
Locality SW LW
WhiteCliffs,Australia
mediumbluephosphorescent
Park,Wyoming
dullwhitephosphorescent
strongwhitephosphorescent
Queretaro,Mexico
dullwhitephosphorescent
brightbluephosphorescent
VirginValley,Nevada
brightgreen mediumgreen,bluewhitephosphorescent
Quartzite,Arizona
paleyellow brightpaleyellow
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Opalmayalsofluorescebrownish.Blackopalisgenerallyinert.Fireopalluminescesgreenishbrown.Commonopaloftenfluoresces
green.
CommentsbyDonaldClark,CSMIMGOpalsstandinaclassbythemselves.Morethananyothergem,eachopalisdistinctlyanindividual.Nootherstonehasasrichandvariedfolklore.Theyarebothoneoftheluckiestandunluckiestgemsapersoncanown.Theyaresounique,theyhavetheirowndescriptivevocabulary.Opalsarealsothemostdelicategemscommonlyworn.Theyrequirespecialcaretoinsuretheirhealthandlongevity.
ThenameevolvedfromtheGreekOpalluswhichmeanstoseeachangeincolor.Later,theLatinwordopaluscametomeanpreciousstone.
Thereissomedoubtthatancientauthorswerereferringtothesamestonewecallopaltoday.Somescholarsbelievethatmanyreferencesareactuallytoiridescentgems,likeirisagate.
Opalsfirewaslongthoughttobetheresultofiridescence.However,withtheadventofscanningelectronmicroscopes,wenowknowthatitisaresultofdiffraction.
Opalisanamorphousformofsilica,(SiO2.nH2O)chemicallysimilartoquartz,(SiO2)butcontaining3%to21%waterwithinthemineralstructure.Gemgradeopalsareusually6%to10%watercontent.
CommonOpal
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PreciousOpal
Opalisasedimentarystone.Underproperconditions,waterpercolatesthroughtheearth,becomingrichindissolvedsilicates.Whenitentersacavity,thesilicatesaredepositedastinyspheres.Iftheyareuniforminsizeandshape,theywilldiffractlight.Iftheyarerandominshapeandarrangement,wehavecommonopal.
Volcanicashgivesblackopalitscolor,butinclusionshavenothingtodowiththeplayofcolor.Thatisdueentirelytothetinyspheres.Theymustbesmallerthan1500angstromsforblueandvioletcolors,butnolargerthan3500angstromstoproduceorangesandreds.Toputthatinperspective,20,000spheresareaboutthesizeoftheperiodattheendofthissentence.
Opalgrowsbyfillingincavities,regardlessoftheirshape.Hence,wehavemanypseudomorphs,materialswithshapesthatareunrelatedtothechemicalcontent.Themostcommonareopalizedwoodandseashells.
OpalizedWood
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OpalizedShell
OpalCareOpalsaredelicate,butwellworththecare.Theirmostsignificantweaknesshastodowiththewatercontent.Ifanopalisallowedtodry,itwillcrackandcraze.Inmostcases,theydonotneedanyspecialcarewhilestored.However,ifyouliveinaverydryclimate,orkeeptheminadehumidifiedroom,someprecautionsarenecessary.Keepingtheminatightplasticbag,withadamppieceofcottonorfabricwillpreventdehydration.
Storinganopalinoilorglycerinisnotrecommended.Itisunlikelytodamagetheopal,butitisunnecessaryandrequirestediouscleaning.
Becauseoftheirwatercontent,opalsarealsohighlysensitivetosuddenchangesintemperature.IknowofawomaninPennsylvaniawhoworeabroochontheoutsideofhercoat.Asshepassedfromthewarmthofherhousetothewintercold,therewasanaudiblecrackasheropalselfdestructed.
Opalsdonotmindbeinghotorcold,itistherateofchangethatdamagesthem.Youneedtoavoidsituationliketheoneabove,goingfromawarmhousetothewinterscold.Simplywearinganopalunderclothingwillprotectthem.Also,donotstoreopalsnearaheatsource,anopenwindow,etc.,wheretheycanbeexposedtosuddentemperaturechanges.
(ClickImagesbelowtoseesomemoreexamples)
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OPAL:BlackOpal,Australia,lightingposition#2(stonein
ring~10cts)
OPAL:Contraluz,Mexico,illuminatedfromrear(~4)
OPAL:Contraluz,Mexico,illuminatedfromfront(~4)
Opal:Triplet,Australianrough,ceramicbase/quartz
top.(30x23mm)
OPAL:Blackopal,Australia(freeformcabochons~30
caratseach)
OPAL:BlackOpal,Australia,lightingposition#1(stonein
ring~10cts)
OPAL:Mossopal,Idaho
OPAL:BlackOpal.Australia(stoneinbracelet~20
carats)
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(5.70)
OPAL:Fireopal,Idaho(11.74),Mexico(5.15,
hyalite)//Mexico(0.76,8.04)
OPAL:Semiblackopal.Australia(~10carats)
Beingsomewhatsoft,theyscratcheasily.Realizethatalargecomponentofdustisquartzat7inhardness.At5.5to6inhardness,simplywipingthedustoffanopalwillgraduallyreduceitspolish.Thesolutionistocleanyouropalsusingasoftclothorbrush,amilddetergent,androomtemperaturewater.Thenrinsethejewelstoremoveanyresidue.Cleandoubletsandtripletswiththesamemethod,butdonotsoakthem.Soakingcandissolvetheglueholdingthelayerstogether.
ProtectiveOpalSettings
Preventionisthebestsolutiontoscratchingandchipping.Opalsarebestsuitedforearrings,brooches,andpendants.Thesejewelsreceivelittlecontactwithharderobjects,
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comparedtowhatafingerringexperiences.Ifyoudogetanopalring,chooseasettingthatprotectsitfromcomingincontactwithotherobjects.
Understandthat,ifyouwearanopalringonaregularbasis,itwillrequireoccasionalrepolishing.Reservingyourringforspecialoccasionswillgreatlyreducetheriskofdamage.
Makesureyouremoveyourringbeforephysicalactivitieslikegardeningandsports.Also,donotimmersethegeminliquidchemicalsolutions,likedishwater.Opalsareporousandabsorbliquids.
OpalTermsSiliceoussintergeyserite:massive,glassyopalthatformsaroundhotspringsandgeysersnogemsignificance.Diatomaceousearthtripoli:finegrained,powderymassesofopalorthesiliceousremainsofmicroscopicmarineanimalscalleddiatoms.Oftenusedaspolishingagents,fillers.Pseudomorphs:opalmay,inpercolatingthroughtheground,replace(onamicroscopicorevencellularbasis)wood,bone,andshells.Hyalite:transparent,colorless,orwhitetograyopal,glassy,occasionallyfacetedbutgenerallynogemsignificance.Commonopal:opaqueorglassyopal,inawiderangeofcolors,sometimeswithawaxylusteroftenfluorescentseldomcutintogems.Wateropal:transparent,colorlessopalthatmayhavefireinit.
Note:Thetermfirereferstothemagnificentplayofcolordisplayedbyopal,whichisduetolightdiffractionfromneatlystackedlayersofthemicroscopicspheresofwhichopaliscomposed.Commonopalisajumbleofspheresofrandomsizes,butinpreciousopalthespheresarethesamesize,andtheyarelayeredinneatrows.Theparticularcolorseendependsonthesizeofthespheresandtheangleofviewing.
Fireopal:transparenttotranslucentredororangeopal,whichmayormaynothavefireinit!Thetermfireopalreferstoabodycolor,nottoplayofcolor.Preciousopal:opalofanycolorwithfireorplayof
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colorsdisplayed.Whiteopal:whitebodycoloropal,usuallywithplayofcolor.Grayopal:lighttodarkbodycolor,withplayofcolorsuperimposed.Blackopal:blackbodycolorwithfire,oftenspectacularagainstdarkbackground.Bodycoloralsoverydarkbluish,greenish,orbrownish.Semiblackopal:anotherwayofdescribinggrayopal.Milkopal:milkwhite,translucent,alsoyellowishorgreenishincolor.Crystalopal:wateropalormilkopal,generallyrichinfiretransparenttotranslucentintransmittedlightcolorsseenbyreflectedlight.Contraluzopal:veryraretype,usuallyfromMexico,withcolorplayinbothtransmittedandreflectedlight.Hydrophane:lightcolored,opaque,becomesiridescentandtransparentwhensoakedinwater.Jasperopal:reddishbrownopal,opaque,resemblesjasper.Cachalong:porcelaniferous,oftenbluishwhite,veryporousadherestothetongue.Praseopal:translucentoropaquegreenopalacommonopalresemblingprase.Mossopal:whitetobrownishopaqueopalthatcontainsdendriticinclusions.Menilite:opaquegraytobrownopalwithaconcretionarystructure.Tabasheer:opalinesilicaoccurringinthejointsofbamboo.Girasol:opalthatisalmosttransparentandhasabillowylighteffectwithinit,resemblingmoonstone.Chrysocollainopal:bluematerial,withfinelydisseminatedchrysocollathatgivesthecolor.Liveropal:termsometimesusedforbrowncommonopal.Resinopal:yellowishbrowncommonopalwithawaxyluster.Playofcolor:thephenomenaofflashingormovingcolorsduetodiffractionandnotrelatedtothebodycolor.Nobleopal,orPreciousopal:Opalwithplayofcolor.Crazing:cracksthatdevelopasanopaldehydrates.
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VarietyNamesPrimaryTerms
DoubletsandTripletsmakeuseofopalthatistoothintouseasasolidgemstone.Adoubletisathinlayerofpreciousopalgluedtoablackbase.Atripletaddsatransparent,quartzcap.
Puristspreferthebasematerialtobecommonopal.However
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manyblackmaterialsareused,includingoldphonographrecords.
Blackopal,preciousopalwithblackbodycolor.Alsousedforblackpotchcoveredwiththinlayerofcrystalopalthatletstheblackunderlayershowthrough.Semiblack,orgrayopal,preciousopalwithdarkbodycolor.Whiteopal,preciousopalwithwhitebodycolor.Crystalopal,transparenttosemitransparent,colorlessbodywithplayofcolor.Blackcrystalopal,transparenttosemitransparent,withdarkbodycolorandplayofcolor.Fireopal,Translucenttotransparent,withyellow,orange,orredbodycolor.Mayormaynothaveplayofcolor.AlsocalledMexicanopalorSunopal.Boulderopal,thinseamofopalnaturallyoccurringonironstonematrix.Cachalongopal,translucenttoopaque,bluishwhiteopalthatresemblesporcelain.Veryporous,willsticktotongue.AlsocalledKalmuckagateopal.Chrysocollaopal,blueChrysocolla,finelydistributedincommonopal.Contraluzopal,transparentopalshowingplayofcolorinbothreflectedandtransmittedlight.Girasolopal,semitransparenttotranslucentopalwithmoving,billowylightresemblingmoonstone.Goldopal,fireopalwithgoldbodycolororpreciousopalwithonlygoldfire.Hyalite,transparenttotranslucent,whitecommonopal.AlsocalledWaterstoneorWateropal.Hydrophane,commonopalthatshowsplayofcolorwhensoakedinwateroroil.AlsocalledMagicstone.Ironopal,redtoyellow,commonopal.Jasperopal,reddishbrown,commonopal.JellyorWateropal,colorless,transparenttosemitransparentwithlittleornoplayofcolor.Lechosesopal,hasonlygreenplayofcolor.
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Liveropal,brown,commonopal.Louisianaopal,composedofquartz,opalandpyrite.FoundinLouisiana,USA.Matrixopal,thinseamsorspotsofpreciousopalinmatrix,(usuallyironstone.)AlsocalledMassopal.Menilite,opaquegraytobrownopalwithconcretionstructure.Milkopal,whitecommonopal.Mossopal,commonopalwithinclusionsresemblingmoss.OnyxopalorAgateopalbandedlayersofpreciousandcommonopal.Ooliticopal,astonewithsmallblackorbrownsphericalinclusionsresemblingfishroe,overplayofcolor.Opalizedwood,bone,orshell,pseudomorphswhereorganicmaterialisreplacedbyopal.Opalizedwoodalsocalledxylopal.Praseopal,greentoyellowishgreencommonopalresemblingchrysoprase.AlsocalledChrysopal.Resinopal,yellowishbrownopalwithresinousluster.RollingFlashFireplayofcolortravelsacrosssurfaceasstoneismoved.Veryrare.Siliceoussinter,glassyopalthatformsaroundhotspringsandgeysers.AlsocalledGeyserite.Tabasheer,opallikesilicafoundinthejointsofsomebamboo.TripoliorDiatomaceousearth,finegrainedopalandsiliceousremainsofdiatoms.Useasapolishorfiller.Waxopal,yellowishopalwithwaxyluster.Zeasite,opalizedwood.
OpalPatternsNote:thedescriptionsofopalareasbroadasthehumanimagination.Patternsincludestainedglasswindows,butterflies,birds,andothernaturalandmanmadeobjects.Belowarethemostcommonterms.
Chaffthin,linearbandsofcolor.ChinesewritingresemblesAsianscript.Usuallyingoldandgreen,somewhatrare.Fernleaflacypatternsthatresembleafernleaf.Mostlyinlargergreengems,rarelyred.
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Flagstonelarge,broadpatchesofcolor,setclosetogether.Flameopal,bandsorstreaksofredfirethatmoveacrossthesurface.Flashopal,playofcolorthatquicklyflashesoffandonasthestoneismoved.Floralavarietyofrepeateddesignandcolor,reminiscentoffloraldressmaterial.Harlequin,orMosaic,closesetpatchesofbroad,angularplayofcolor.Peacockopal,predominantlygreenandblueplayofcolor,withsomeresemblancetoapeacocktail.PinfireorPinpoint,verysmallareasoffiresetclosetogether.Ribbonslightlycurved,parallelbandsofcolor.Usuallyparallelandonblackordarkbase.Rare.Strawresemblesflattenedstraws,crisscrossingeachother.Windmillapatternoffirethatradiatesaroundacentralpoint.Veryrare.
TermsforColorandColorDistributioninOpal
Onyxopalandagateopal:alternatinglayersofpreciousandcommonopal.Incatseyeopalthecolorplayisconcentratedintheformofaneyeorband.Matrixopalconsistsofspecksofpreciousopalinarockmatrix,usuallysandstonethistypeofopalisoftendyedblacktoenhancethecolorplay.ironstoneopalisinabrown,hard,compacttypeofsandstone.Matrixopalmayalsobelayersorstringersofopalinarockmatrix.Flameopal:sweepingreddishstreaksandbandsmoveacrossthegem,resemblingflickeringflames.Flashopal:asthegemismovedbackandforth,flashesofcolorappearanddisappearatvariousspots.Harlequinopal:thecolordisplayisintheformofangularorquiltlikepatches,allincontactwitheachother,likeamosaic.Pinfireopal:thecolorisintheformoftinydotsorspeckles,setclosetogether.Peacockopal:manycolorsappearinthesamegem,resemblingthedisplayofthetailofthemalepeacock.Alsogoldopal(goldfire),blueopal(bluish
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fire),lechososopal(greencolors).
EnhancementsImpregnatedwithoil,wax,orplastic.Improvesplayofcolor,disguisescrazing.Common.Stabilitypoorforoilandwax.
Detectoilandwaxwithhotpoint.Plasticusuallyrequiresmajorlabequipment.
Impregnatedwithblackplastic.Giveappearanceofblackopal,disguisescrazing.Stabilityexcellent.Somewhatcommon.
Detection,SGtoolow,concentrationofcolorincracks.Mayrequiremajorlabequipment.
Impregnatedwithsmoke.Darkensbodycolor.Common.Stability,fairtopoor.
Detection,magnification,lowSG,(mayfloatinwateruntilitabsorbsenoughtosink,)loosesplayofcolorwhenwet,returnswhendry.UnusuallylowRI,1.38to1.39.
Treatmentwithanilinedye,silvernitrate,orsugar.Darkensbodycolor.Common.Stabilitygood.
Detection,magnificationshowsblackconcentrations.
OtherDisclosures
Hydrophaneopals,(commontoEthopia,)willabsorbwaterandchemicalswhichhaveanegativeaffectontheirappearance.Theymustbekeptdryandawayfromallsourcesofcontamination.
IdentifyingCharacteristicsNatural
Playofcolor.Mayhavephosphorescence.
Synthetic
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Strongplayofcolor,usuallyinmosaicpattern.Insidemosaicthereisacellular,scalelike,snakeskin,orchickenwirestructure.(Highmagnification,toporbacklighting.)Mayshowdendriticstructure,(highmagnification,transmittedlight.)
Doesnotphosphoresce.Stickstotongue.Whiteopalshowscolumnarstructurefromside.
PlasticImitations
Soft,canprobewithsharpneedle.Doesnotphosphoresce.
GlassImitations
Typicalglassbubblesandswirlmarks.Doesnotphosphoresce.SGandRIusuallyhigher.
OccurrenceInsedimentaryrocksorwherelowtemperaturesolutionsbearingsilicacanpercolatethroughrocks.
Honduras:depositsknownsincebefore1843,perhapsmucholderthanthat.Occursasveinsindarkreddishtoblacktrachyterock.Whiteopalcontrastsstronglywiththedarkcoloredmatrix.Piecesnotlarge,seldomveryspectacular.Czechoslovakia:sourceofopalknowninRomantimes,nearthevillageofCzerwenitza(formerlyinHungary).Opaloccursasseamsingrayishbrownandesiterock.Theopalisamosaicofstrongcolorsandisveryattractive,againstamilkywhitebackgroundcolor.Muchofthisisharlequinopal.Indonesia:verylittleknownmaterial,asthinseamsindarkrock.MuchofitiswateropalandresemblesmaterialfromMexico.ThewhiteopalresemblespoorgradeAustralian.Someblackopalisproducedthatisveryunusualandconsistsofreddishflecksofcolorswimminginatranslucentbutverydarkbrownbody.Mostgemsareverysmall(lessthan10carats)fromthislocality,andproductionisverysmall.Mexico:Mexicanopaloccursinsiliceousvolcaniclavas,incavities,andinmanylocalities.Yellowandredfireopalcomesfromatrachyteporphyryat
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ZimapaninHidalgo.Hyaliteandpreciousopalthatiscompletelytransparent,colorless,andrichinfireoccursatSanLuis,Potosi,Chihuahua.Queretaroisawellknownopalproducinglocality.FineMexicanopalisveryrareinlargesizes(over50carats)butisamongthemostbeautiful.VirginValley,Nevada:opaloccursinHumboldtCountyascracksandseamsinopalizedWood.Thiswasdiscoveredabout1900.Theopalismagnificent,butisveryhydrousandhasastrongtendencytocrackduetolossofwaterwhenexposedtotheair.Thisbehaviorisknownascrazing,or,whenonthesurface,checking.Wholeskeletonsofextinctanimalshavebeenreplacedbyfinepreciousopalatthislocality.SimilaropalisfoundinIdaho.Brazil:opaloccursinsandstoneinPiauiSlate,northernBrazil,andalsonearManaus,northernBrazil.ThematerialiswhiteandfieryandsometimesresemblesgoodqualityAustralianwhiteopal.Itisperhapsthemostdurableopal,lowinwaterandnotheatsensitiveIhaveseenacutgemheldforahalfaminuteoveracandleflamewithnoadverseeffects.ThematerialseemstobeabundantandmuchofitisshippedtoHongKongwhereitiscutandsold,oftenasAustralianopal.Poland:greenpraseopal,coloredbynickel.Tanzania:nickeliferousopalresemblingchrysopraseoccursinTanzania,associatedwithbrownlimonite.TheR.I.(1.452)islowerthanthatofchrysoprase(1.535),asisthegravity(2.125versus2.620).Stonesizestendtobesmall.Australia:thefirstdiscoverieswereprobablyabout1850.butmajorfindsweremadeinQueenslandabout1872.Australianopalisinvarioustypes:Boulderopal:shellsofcoarse,hardened,sandyclaywithlayersofopalinbetween.Yowahnuts:walnutsizedconcretions,inaregularlayer,likeaconglomerate.Theopalisthecentralkernelandneverreachestheouteredge.Seamopal:thintothickseamsofwhiteorblackopalinsandstonematrix.Alsoknownassandstoneopal.Largestonesareveryrareinthismaterial.MajorfindsofAustralianopalarebestknownfromspecificlocalities:LightningRidge:blackopalinnodules,worldsfinestofthismaterialfirstminedcommerciallyabout1905.CooberPedy,South
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4/9/2015 OpalValue,Price,andJewelryInformationInternationalGemSocietyIGS
http://www.gemsociety.org/article/opaljewelryandgemstoneinformation/ 20/21
Australia:discoveredabout1915onlywhiteopalsfoundhere,insandstoneandclaystonematrix,butsomeveryfine.Andamooka,SouthAustralia:opalsfoundhereabout1930verydistinctiveopal,whiteandalsobrownishincolormaybeartificiallyblackenedtoenhancetheappearanceofthefireinthematrix.WhiteCliffsarea:startedabout1889,buttheopalisusuallysmall,withveinletsofpreciousopalwithincommonopal.Gabanintha(MurchisonGoldfield):brightgreenopal,coloredbycopper,isfoundinquartz.Mintabie:minedsince1931,about350kmnorthwestofCooberPedy.Thisareahasnowbeenextensivelyprospected.
Australiaisthebestknownopalproducingareaintheworld,butthedepositshavebeenworkedsointenselythattheyarebecomingdepleted.Manyfewerminersarenowworkingtheopalfieldsthan10yearsago,andnewdiscoveriesarerare.Thisfactor,plusworldwidedemand,isputtingtremendouspressureonopalprices.
Source/Attribution:Dr.JoelArem,DonaldClark,CSMIMG
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4/9/2015 OpalValue,Price,andJewelryInformationInternationalGemSocietyIGS
http://www.gemsociety.org/article/opaljewelryandgemstoneinformation/ 21/21
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