opal value, price, and jewelry information - international gem society igs

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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

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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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