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Opal

This page kindly sponsored by Opal by Anderson-Beattie.com
Formula:
SiO2 · nH2O
Colour:
Colourless, white, yellow, red, orange, green, brown, black, blue
Lustre:
Vitreous, Waxy, Greasy, Dull
Hardness:
5½ - 6½
Specific Gravity:
1.9 - 2.3
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 a modern commentary by Kostov (2008) questions 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 (found as Hyalite)

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

Studies at low temperature show that water molecules way be organized into ice-like structure, which includes the cubic ice modification (Eckert et al., 2015).


Visit gemdat.org for gemological information about Opal.


Classification of Opal

Approved, 'Grandfathered' (first described prior to 1959)
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
75.2.1.1

75 : TECTOSILICATES Si Tetrahedral Frameworks
2 : Si Tetrahedral Frameworks - SiO2 with H2O and organics
7.8.8

7 : Oxides and Hydroxides
8 : Oxides of Si

Physical Properties of Opal

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

Optical Data of Opal

Type:
Isotropic
RI values:
nα = 1.400 - 1.460
Birefringence:
Opal-AG and Opal-AG are optically isotropic, but may show anomalous birefringence due to strain. The microcrystalline varieties show birefringence: opal-C is length-fast, opal-CT is length-slow, but almost isotropic.
Max Birefringence:
δ = 1.400 - Isotropic minerals have no birefringence
Surface Relief:
Moderate
Pleochroism:
Non-pleochroic

Chemical Properties of Opal

Formula:
SiO2 · nH2O

Synonyms of Opal

Other Language Names for Opal

Basque:Opalo
Bosnian (Latin Script):Opal
Bulgarian:Опал
Czech:Opál
Dutch:Opaal
Esperanto:Opalo
Estonian:Opaal
Finnish:Opaali
French:Opale
Galician:Ópalo
Hungarian:Opál
Italian:Opale
Japanese:オパール
Lithuanian:Opalas
Norwegian (Bokmål):Opal
Polish:Opal
Portuguese:Opala
Romanian:Opal
Russian:Опал
Simplified Chinese:蛋白石
Slovak:Opál
Slovenian:Opal
Swedish:Opal
Traditional Chinese:蛋白石
Turkish:Opal
Ukrainian:Опал
Vietnamese:Đá opal

Varieties of Opal

Alumocalcite

A variety of Opal with alumina and lime impurities.

Amatite

Opal formed of thick mounds deposited from hot silica-rich springs.

Amber Opal

A yellow-brownish variety of Opal, resembling Amber.

Andean Opal

A greenish-blue opal from Peru.

Andenopal

German name for pink opal variety from Peru (Catamarca?).

Bandfire Opal

A precious opal with bands showing a play of colours.

Black Opal

The most precious kind of opal, this variety of precious opal has a dark background colour.

Blue Opal

Translucent blue opal with no play of colours. Cause of colour can be light scattering effects or occasionally microscopic admixture of chrysocolla or other minerals. (See also "Andean Opal")

Bone Opal

Opal replacing fossil bone.

Boulder Opal

A variety of Precious Opal found in Queensland, Australia, as cracks or coatings in and around ironstone/sandstone boulders.

Cat's Eye Opal

A rare chatoyant form of opal showing a thin line of fire.

Cherry Opal

A variety of common or precious opal distinguished by its red-brown color.

Chrysopal

Green nickeloan variety of Opal transitive to Chrysoprase

Chrysopal (of Schumann)

A translucent variety of common opal colored apple green by the presence of nickel.

Claro Opal

Transparent mexican Precious Opal with an intense play of colours (red, green, blue and yellow).

Common Opal

A type of opal that doesn't show any fire.

Contra Luz Opal

A precious opal where the play of colours is only visible when held up to the light.

Crystal Opal

A transparent to translucent variety of Precious Opal where the colour play is visible both on the surface and in the interior.

Fire Opal

A red variety of Opal.

Forcherite

A yellow to orange variety of opal, owing its colour to microscopic inclusions of orpiment and realgar.
Originally reported from Holzbrücken mill, Ingering valley, Knittelfeld, Styria, Austria.

Girasol

A bluish-white translucent opal with reddish reflections. Name dates back to at least 1837. Compare with Girasol Quartz which is named after this material.

Gold Opal

A name for yellow or gold-coloured opal

Harlequin Opal

A variety of Precious Opal in which the play of color is arranged in a vivid harlequin, diamond-shaped, or rectangular-shaped pattern.

Honey Opal

An unnecessary name for orange-brown opal.

Hungarian Opal

An old name applied to any opal from Europe. Now even more mistakenly used as a name for white opal.

Hyalite

A colourless variety of Opal. Hyalite is opal-AN, an amorphous silica-glass containing about 3-8% water.
Because it has a different structure, it does not show the opalescence (play of color) that is found in precious opal.

Hyalite occurs as globular a...

Hydrophane

A variety of opal that turns translucent or transparent when immersed in water.

Isopyre

An impure dark-red Opal

Jasper Opal

A brecciated Jasper cemented by opal.

Jelly Opal

A transparent opal with a gelatinous appearance and a bluish sheen.

Lechosos Opal

A variety with a milky-white background colour.

Lemon Opal

Another unnecessary name for a colour variety of Opal.

Levin Opal

Precious Opal with long thin lightning-like flashes.

Lluvisnando opal

A pale yellow opal

Mascareignite

A form of opaline silica, from a vegetable origin. Primarily siliceous remains of grasses and diatoms.

Originally reported from Réunion Island.

Menilite

Greyish-brown opal.
Originally reported from Ménilmontant, Paris, Ile-de-France, France.

Mexican Fire Opal

A transparent red opal from Mexico.

Milk Opal

A white Opal.

Moss Opal

Opal with dendritic inclusions, usually green silicates.

Mother of Opal

A sandstone or ironstone with Opal as a cement.

Mountain Opal

Unnecessary name for Opal from an igneous origin.

Onyx Opal

A banded Opal resembling Onyx

Oolitic Opal

Oolitic Opal is a variety of opal made up of interlinked spherical grains, or oolites, of opal.

Opal Matrix

A term for a thin layer of opal on a host rock.

Opal-AG

A type of Opal consisting of aggregated spheres of amorphous silica, with water filling the gaps in between. Precious Opal and Potch Opal consist of this kind of structure - the difference being in the regularity of the sizes of the spheres and packing.

Opal-AN

Opal-AN is a term for amorphous opal with a glass-like structure (Graetsch, 1994). The "A" in the name stands for amorphous; the subscript "N" is to imply its structure is network-like similar to silica glass; however, it still contains about 3-8% water, ...

Opal-Agate

A variety of Opal showing agate-like coloured bands.

Opal-C

A type of opal consisting of disordered α-Cristobalite.

Opal-CT

Consists of packed microscopic (150-300 nm) spheres made up of tiny microcrystalline blades of Cristobalite and/or Tridymite, with water content as high as 10 wt%.

Opal-Jasper

A variety of yellow jasper-coloured Opal.

Opaline

Pseudomorphs of Opal after Serpentine. A local term.

Originally described from Quicksilver region, Napa Co., California, USA.

Painter Boulder

Sandstone boulders with a thin coating of opal.

Pineapple Opal

Pseudomorph of Opal after Ikaite

Pinfire Opal

WIth very small pinhead-sized colour flashes.

Pipe Opal

Opal filling long cylindrical cavities.

Pitch Opal

An opal variety with a pitchy lustre

Prase Opal

A green opal.

Precious Fire Opal

Fire Opal displaying a play of colour.

Precious Opal

A valuable gem-variety of opal.
Most precious opal is opal-AG. It shows a play of colours ("opalescence") that is due to diffraction of light from the regular packing of submicroscopic silica spheres of roughly equal size (Jones et al, 1964; Sanders, 19...

Pyrophane

Opal where colour play appears to wander about at random.

Quinzite Opal

Rose to pink Opal.
Originally described from Quincy-sur-Cher, Bourges, Cher, Centre, France.

Radiolite Opal

A smoky-brown opal coloured by inclusions of radiolaria exoskeletons.

Red Flash Opal

Precious Opal with red colour flashes that appear and disappear as the stone is turned.

Resin Opal

A yellow-brown common opal with a resinous luster.

Shell Opal

Opal replacing fossil shells.

Slocum Stone

A synthetically grown opal

Wash Opal

Alluvial pebbles of Opal.

Wax Opal

A yellow-to brown opal with a waxy lustre.

White Opal

Precious Opal with a white background.

Wood Opal

Opal that has replaced fossil wood.

Yowah Nut

Small rounded pebbles of sandstone impregnated with Precious Opal.
Originally reported from Yowah opal field, Queensland, Australia.

Common Associates

Associated Minerals Based on Photo Data:
Chalcedony30 photos of Opal associated with Chalcedony on mindat.org.
Quartz28 photos of Opal associated with Quartz on mindat.org.
Calcite21 photos of Opal associated with Calcite on mindat.org.
Topaz15 photos of Opal associated with Topaz on mindat.org.
Goethite13 photos of Opal associated with Goethite on mindat.org.
Cinnabar12 photos of Opal associated with Cinnabar on mindat.org.
Phillipsite11 photos of Opal associated with Phillipsite on mindat.org.
Magnesite11 photos of Opal associated with Magnesite on mindat.org.
Fluorite10 photos of Opal associated with Fluorite on mindat.org.
Ikaite10 photos of Opal associated with Ikaite on mindat.org.

Related Minerals - Nickel-Strunz Grouping

4.DA.Carbon Dioxide IceCO2
4.DA.05QuartzSiO2
4.DA.10TridymiteSiO2
4.DA.15CristobaliteSiO2
4.DA.20MogániteSiO2
4.DA.25Melanophlogite46SiO2 · 6(N2,CO2) · 2(CH4,N2)
4.DA.30LechatelieriteSiO2
4.DA.35CoesiteSiO2
4.DA.40StishoviteSiO2
4.DA.45KeatiteSiO2
4.DA.50SeifertiteSiO2

Related Minerals - Hey's Chemical Index of Minerals Grouping

7.8.1QuartzSiO2
7.8.2CoesiteSiO2
7.8.3TridymiteSiO2
7.8.4StishoviteSiO2
7.8.5CristobaliteSiO2
7.8.6LechatelieriteSiO2
7.8.7Silhydrite3SiO2 · H2O
7.8.9MogániteSiO2

Fluorescence of Opal

Yellow to green (uranyl)

Other Information

Health Risks:
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

Opal in petrology

An essential component of (items highlighted in red)
Common component of (items highlighted in red)

References for Opal

Reference List:
Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Jones, J.B., Segnit, E.R. (1971) The nature of opal. I Nomenclature and constituent phases. Journal of the Geological Society of Australia: 18: 57-68.
Elzea, J.M., Odom, I.E., Miles, W.J. (1994) Distinguishing well ordered opal-CT and opal-C from high temperature cristobalite by X-ray diffraction. Analytica Chimica Acta 286: 107-116.
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., Yang, B.X. (1994) X-ray absorption spectroscopy of silicon dioxide (SiO2) polymorphs: the structural characterization of opal. American Mineralogist: 79: 622-632.
Weise, C., publisher (1996) Opal - extraLapis Nr.10. Christian Weise Verlag, München.
Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., Ostroumov, M. (2008) Common gem opal: An investigation of micro- to nano-structure. American Mineralogist: 93: 1865-1873.
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.
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, F., Ghisoli, C., Marinoni, L., Bordoni, V. (2013) Opal, a beautiful gem between myth and reality. Neues Jahrbuch für Mineralogie - Abhandlungen: 190: 1-9.
Eckert, J., Gourdon, O., Jacob, D.E., Meral, C., Monteiro, P.J.M., Vogel, S.C., Wirth, R., Wenk, H.-R. (2015) Ordering of water in opals with different microstructures. European Journal of Mineralogy: 27: 203-213.

Internet Links for Opal

mindat.org URL:
https://www.mindat.org/min-3004.html
Please feel free to link to this page.
Specimens:
The following Opal specimens are currently listed for sale on minfind.com.

Localities for Opal

Mineral and/or Locality  
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