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Opal

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Queensland, Australia
© Antonio Borrelli

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Formula:
SiO
 
2
 · nH
 
2
O
Colour:Colorless, white, ...Hardness:5½ - 6½
Name:From the Sanskrit "upala", meaning "stone" or "precious stone".


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 broken down 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
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
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
Slovak:Opál
Slovenian:Opal
Spanish:Ópalo
Neslita
Rumanita
Vidrita
Swedish:Opal
Turkish:Opal
Ukrainian:Опал
Vietnamese:Đá opal
Varieties:
AlumocalciteAmatiteAmber OpalAndean OpalAndenopal
Bandfire OpalBlack OpalBone OpalBoulder OpalCachalong Opal
Cherry OpalChrysopalChrysopal (of Schumann)Claro OpalContra Luz Opal
Crystal OpalDiatomiteFioriteFire OpalForcherite
GeyseriteGirasolGold OpalHarlequin OpalHoney Opal
Hungarian OpalHydrophaneIsopyreJasper OpalJelly Opal
Lechosos OpalLemon OpalLevin OpalLluvisnando opalMascareignite
MeniliteMexican Fire OpalMilk OpalMoss OpalMother of Opal
Mother of Pearl OpalMountain OpalOnyx OpalOolitic OpalOpal Matrix
Opal-AGOpal-ANOpal-AgateOpal-COpal-CT
Opal-JasperOpalinePainter BoulderPineapple OpalPinfire Opal
Pipe OpalPitch OpalPrase OpalPrecious Fire OpalPrecious Opal
PyrophaneQuinzite OpalRadiolite OpalRed Flash OpalResin Opal
Shell OpalSiliceous sinterSlocum StoneVianditeWash 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.

Mindat.org articles about Opal

Article entries:
Welo Ethiopia Opal Jon Young

Internet Links for Opal

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  • Raman and XRD data at RRUFF project[Link Broken?]
  • American Mineralogist Crystal Structure Database[Link Broken?]
    • Mineral Dealers:
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    • Specimens:The following Opal specimens are currently listed for sale on minfind.com.
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