Opal

Queensland, Australia
© Antonio Borrelli

Show Opal Photos (397)

Formula:

SiO

·nH

Colour:

Colorless, white, ...

Hardness:

5½ - 6½

Name:

Fron the Sanskrit upala, 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 composed of Cristobalite and/or Tridymite and amorphous silica (closely packed spheres form a diffraction grating to create Precious Opal).

Opal is broken down into four types:

Opal-CT Cristobalite-Tridymite
Opal-C Cristobalite
Opal-AG Amorphous - (Gem)
Opal-AN

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

mindat.org URL:

http://www.mindat.org/min-3004.html
Please feel free to link to this page.

Occurrences of Opal

Geological Setting:

Altering volcanic tuffs, basalts

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

·nH

Essential elements:

H, O, Si

All elements listed in formula:

H, O, Si

Relationship of Opal to other Species

4.DA.05

Quartz

SiO

4.DA.10

Tridymite

SiO

4.DA.15

Cristobalite

SiO

4.DA.20

Mogánite

SiO

4.DA.25

Melanophlogite

46SiO

·6(N

,CO

)·2(CH

)

4.DA.30

Lechatelierite

SiO

4.DA.35

Coesite

SiO

4.DA.40

Stishovite

SiO

4.DA.45

Keatite

4.DA.50

Seifertite

SiO

7.8.1

Quartz

SiO

7.8.2

Coesite

SiO

7.8.3

Tridymite

SiO

7.8.4

Stishovite

SiO

7.8.5

Cristobalite

SiO

7.8.6

Lechatelierite

SiO

7.8.7

Silhydrite

3SiO

·H

7.8.9

Mogánite

SiO

Other Names for Opal

Synonyms:

Common Opal	Gel-Cristobalite	Gel-pristobalite	Half-Opal	Hemiopal
Indivisible Quartz	Iridot	Lechosopal	Lechosos	Neslite
Nevada Opal	Paederos	Potch	Rumanite	Semiopal
Vidrite	Virgin Valley Opal	White 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:

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.

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).