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Opal

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Big Photo

Queensland, Australia
© Antonio Borrelli




Formula:
SiO
 
2
· nH
 
2
O
Colour:Colorless, white, ...Hardness:5½ - 6½
Name:The origin of the name is uncertain. It may be from the Sanskrit "upala", meaning "stone" or "precious stone" or from opalus, the ancient Latin name for the gem (Pliny the elder, 75-79). Pliny may have also referred to the gem as paederos, but modern commentary by Kostov (2008) question if that name was actually applied to the opal of modern sense.


Although it is still (2007) regarded as a valid mineral species for historical reasons, Opal is not a true mineral in the accepted sense of the word as it is either composed of Cristobalite and/or Tridymite or composed of amorphous silica.

Opal is classified into four types:

Opal-CT Cristobalite-Tridymite
Opal-C Cristobalite
Opal-AG Amorphous -Gel (closely packed amorphous silica spheres form a diffraction grating to create Precious Opal).
Opal-AN Amorphous-Network (Hyalite)

Transitions between opal-AG, opal-CT and opal-C are common.


Visit gemdat.org for gemological information about Opal. Currently in public beta-test.

Classification of Opal

IMA status:Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
Strunz 8th edition ID:4/D.01-80
Nickel-Strunz 10th (pending) edition ID:4.DA.10

4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
D : Metal: Oxygen = 1:2 and similar
A : With small cations: Silica family
Dana 8th edition ID:75.2.1.1

75 : TECTOSILICATES Si Tetrahedral Frameworks
2 : Si Tetrahedral Frameworks - SiO2 with H2O and organics
Hey's CIM Ref.:7.8.8

7 : Oxides and Hydroxides
8 : Oxides of Si
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Occurrences of Opal

Geological Setting:Altering volcanic tuffs, basalts. Silicous deep-water marine sediments. Opal-C, opal-CT and opal-AG formation is restricted to low pressure and low temperature environments.

Physical Properties of Opal

Lustre:Vitreous, Waxy, Greasy, Dull
Diaphaneity (Transparency):Transparent, Opaque
Colour:Colorless, white, yellow, red, orange, green, brown, black, blue
Streak:White
Hardness (Mohs):5½ - 6½
Tenacity:Brittle
Cleavage:None Observed
Fracture:Irregular/Uneven, Splintery, Conchoidal
Density (measured):1.9 - 2.3 g/cm3

Chemical Properties of Opal

Formula:
SiO
 
2
· nH
 
2
O
Simplified for copy/paste:SiO2·nH2O
Essential elements:H, O, Si
All elements listed in formula:H, O, Si

Relationship of Opal to other Species

Related Minerals - Nickel-Strunz Grouping):

- +
4.DA.05Quartz
SiO
 
2
4.DA.10Tridymite
SiO
 
2
4.DA.15Cristobalite
SiO
 
2
4.DA.20Mogánite
SiO
 
2
4.DA.25Melanophlogite
46SiO
 
2
· 6(N
 
2
,CO
 
2
) · 2(CH
 
4
,N
 
2
)
4.DA.30Lechatelierite
SiO
 
2
4.DA.35Coesite
SiO
 
2
4.DA.40Stishovite
SiO
 
2
4.DA.45Keatite
SiO
 
2
4.DA.50Seifertite
SiO
 
2
Related Minerals - Hey's Index Grouping:

- +
7.8.1Quartz
SiO
 
2
7.8.2Coesite
SiO
 
2
7.8.3Tridymite
SiO
 
2
7.8.4Stishovite
SiO
 
2
7.8.5Cristobalite
SiO
 
2
7.8.6Lechatelierite
SiO
 
2
7.8.7Silhydrite
3SiO
 
2
· H
 
2
O
7.8.9Mogánite
SiO
 
2

Other Names for Opal

Synonyms:
Gel-CristobaliteGel-pristobaliteHalf-OpalHemiopalIndivisible Quartz
IridotLechosopalLechososNesliteNevada Opal
PaederosPotchRumaniteSemiopalVidrite
Virgin Valley OpalWhite Cliffs Opal
Other Languages:
Arabic:العقيق
Basque:Opalo
Bosnian (Latin Script):Opal
Bulgarian:Опал
Czech:Opál
Dutch:Opaal
Esperanto:Opalo
Estonian:Opaal
Finnish:Opaali
French:Opale
Galician:Ópalo
German:Opal
Livit
Ljardit
Neslit
Rumanit
Vidrit
Hebrew:אופאל
אופל
Hungarian:Opál
Italian:Opale
Japanese:オパール
Latin:Opalus
Achates unguium colore oculus mundi
Lithuanian:Opalas
Norwegian (Bokmål):Opal
Polish:Opal
Portuguese:Opala
Romanian:Opal
Russian:Опал
Simplified Chinese:蛋白石
Slovak:Opál
Slovenian:Opal
Spanish:Ópalo
Neslita
Rumanita
Vidrita
Swedish:Opal
Traditional Chinese:蛋白石
Turkish:Opal
Ukrainian:Опал
Vietnamese:Đá opal
Varieties:
AlumocalciteAmatiteAmber OpalAndean OpalAndenopal
Bandfire OpalBlack OpalBlue OpalBone OpalBoulder Opal
Cachalong OpalCherry OpalChrysopalChrysopal (of Schumann)Claro Opal
Common OpalContra Luz OpalCrystal OpalDiatomiteFiorite
Fire OpalForcheriteGeyseriteGirasolGold Opal
Harlequin OpalHoney OpalHungarian OpalHydrophaneIsopyre
Jasper OpalJelly OpalLechosos OpalLemon OpalLevin Opal
Lluvisnando opalMascareigniteMeniliteMexican Fire OpalMilk Opal
Moss OpalMother of OpalMother of Pearl OpalMountain OpalOnyx Opal
Oolitic OpalOpal MatrixOpal-AGOpal-ANOpal-Agate
Opal-COpal-CTOpal-JasperOpalinePainter Boulder
Pineapple OpalPinfire OpalPipe OpalPitch OpalPrase Opal
Precious Fire OpalPrecious OpalPyrophaneQuinzite OpalRadiolite Opal
Red Flash OpalResin OpalShell OpalSlocum StoneWash Opal
Wax OpalWhite OpalWood OpalYowah Nut

Other Information

Fluorescence in UV light:Yellow to green (uranyl)
Health Warning:No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Industrial Uses:Gemstones

References for Opal

Reference List:

- +
Jones, J.B. and E.R. Segnit (1971) The nature of opal. I Nomenclature and constituent phases. Journal of the Geological Society of Australia: 18: 57-68.

Anal. Chem. Acta (1994): 286: 107.

Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. In: Reviews in Mineralogy, Volume 29, Silica - Physical behavior, geochemistry and materials applications. Mineralogical Society of America, Washington, D.C.

Li, D., Bancroft, G.M., Kasrai, M., Fleet, M.E., Secco, R.A., Feng,X.H., Tan, K.H., and Yang, B.X. (1994) X-ray absorption spectroscopy of silicon dioxide (SiO2) polymorphs: the structural characterization of opal. American Mineralogist: 79: 622-632.

Lapis Extra No. 10, Opal (1996).

Adamo, I., Ghisoli, C. & Caucia, F. (2010) A contribution to the study of FTIR spectra of opals. Neues Jahrbuch für Mineralogie - Abhandlungen 187, 63-68.

Caucia, Franca; Ghisoli, Christian; Marinoni, Luigi; Bordoni, Valentina (2013): Opal, a beautiful gem between myth and reality. Neues Jahrbuch für Mineralogie - Abhandlungen, 190, 1-9.
Kostov, Rusian I. (2008) Orphic Lithica As A Source Of Late Antiquity Mineralogical Knowledge, Annual Of The University Of Mining And Geology “ST. Ivan Rilski”, Vol. 51, Part I, Geology And Geophysics, p. 109-115.

Mindat.org articles about Opal

Article entries:
Welo Ethiopia Opal Jon Young

Internet Links for Opal

Search Engines:
  • Look for Opal on Google
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  • External Links:
  • Look for Opal on Webmineral
  • Look for Opal on Athena Mineralogy
  • Look for Opal on Wikipedia
  • Look for Opal on Mineralien Atlas
  • Raman and XRD data at RRUFF project
  • American Mineralogist Crystal Structure Database
  • Mineral Dealers:
  • Buy RARE Minerals from Rocks of Africa
  • Lapis Mineral Magazin
  • Search for Opal on Well-Arranged Molecules
  • Fabre Minerals - search for Opal specimens
  • SpiriferMinerals.com - high quality low prices
  • Buy fine minerals and gemstones from Pala International
  • Opal specimens for sale - minfind.com
  • rare and unusual minerals mainly crystallized
  • Rare and Unusual minerals at Mineralogical Research Company
  • Buy from David K Joyce minerals
  • DAKOTA MATRIX offers Cabinet and Rare Species from Worldwide Localities.
  • Top quality minerals from Kristalle of California
  • Specimens:The following Opal specimens are currently listed for sale on minfind.com.

    Localities for Opal

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