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About OpalHide

SiO2 · nH2O
Colourless, white, yellow, red, orange, green, brown, black, blue
Vitreous, Waxy, Greasy, Dull
5½ - 6½
Specific Gravity:
1.9 - 2.3
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: contains cristobalite-tridymite
- Opal-C: contains cristobalite
- Opal-AG: amorphous (Amorphous-Gel) (closely packed amorphous silica spheres form a diffraction grating to create precious opal).
Opal-AN: amorphous (Amorphous-Network (found as hyalite).

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

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

Visit for gemological information about Opal.

Classification of OpalHide

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

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

7 : Oxides and Hydroxides
8 : Oxides of Si

Physical Properties of OpalHide

Vitreous, Waxy, Greasy, Dull
Transparent, Translucent
Colourless, white, yellow, red, orange, green, brown, black, blue
5½ - 6½ on Mohs scale
None Observed
Irregular/Uneven, Splintery, Conchoidal
1.9 - 2.3 g/cm3 (Measured)    

Optical Data of OpalHide

RI values:
nα = 1.400 - 1.460
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.
Isotropic minerals have no birefringence
Surface Relief:

Chemical Properties of OpalHide

SiO2 · nH2O
IMA Formula:
SiO2 · nH2O

Geological EnvironmentHide

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

Synonyms of OpalHide

Other Language Names for OpalHide

Simplified Chinese:蛋白石
Traditional Chinese:蛋白石
Vietnamese:Đá opal

Varieties of OpalHide

AlumocalciteA variety of Opal with alumina and lime impurities.
AmatiteOpal formed of thick mounds deposited from hot silica-rich springs.
Amber OpalA yellow-brownish variety of Opal, resembling Amber.
Andean OpalA greenish-blue opal from Peru.
AndenopalGerman name for pink opal variety from Peru (Catamarca?).
Bandfire OpalA precious opal with bands showing a play of colours.
Black OpalThe most precious kind of opal, this variety of precious opal has a dark background colour.
Blue OpalTranslucent 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 OpalOpal replacing fossil bone.
Boulder OpalA variety of Precious Opal found in Queensland, Australia, as cracks or coatings in and around ironstone/sandstone boulders.
Cat's Eye OpalA rare chatoyant form of opal showing a thin line of fire.
Cherry OpalA variety of common or precious opal distinguished by its red-brown color.
ChrysopalGreen 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 OpalTransparent mexican Precious Opal with an intense play of colours (red, green, blue and yellow).
Common OpalA type of opal that doesn't show any fire.
Contra Luz OpalA precious opal where the play of colours is only visible when held up to the light.
Crystal OpalA transparent to translucent variety of Precious Opal where the colour play is visible both on the surface and in the interior.
Fire OpalA red variety of Opal.
ForcheriteA 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.
GirasolA 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 OpalA name for yellow or gold-coloured opal
Harlequin OpalA variety of Precious Opal in which the play of color is arranged in a vivid harlequin, diamond-shaped, or rectangular-shaped pattern.
Honey OpalAn unnecessary name for orange-brown opal.
Hungarian OpalAn old name applied to any opal from Europe. Now even more mistakenly used as a name for white opal.
HyaliteA 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.
HydrophaneA variety of opal that turns translucent or transparent when immersed in water.
IsopyreAn impure dark-red Opal
Jasper OpalA brecciated Jasper cemented by opal.
Jelly OpalA transparent opal with a gelatinous appearance and a bluish sheen.
Lechosos OpalA variety with a milky-white background colour.
Lemon OpalAnother unnecessary name for a colour variety of Opal.
Levin OpalPrecious Opal with long thin lightning-like flashes.
Lluvisnando opalA pale yellow opal
MascareigniteA form of opaline silica, from a vegetable origin. Primarily siliceous remains of grasses and diatoms.
MeniliteGreyish-brown opal.
Originally reported from Ménilmontant, Paris, Ile-de-France, France.
Mexican Fire OpalA transparent red opal from Mexico.
Milk OpalA white Opal.
Moss OpalOpal with dendritic inclusions, usually green silicates.
Mother of OpalA sandstone or ironstone with Opal as a cement.
Mountain OpalUnnecessary name for Opal from an igneous origin.
Onyx OpalA banded Opal resembling Onyx
Oolitic OpalOolitic Opal is a variety of opal made up of interlinked spherical grains, or oolites, of opal.
Opal MatrixA term for a thin layer of opal on a host rock.
Opal-AGA 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-ANOpal-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-AgateA variety of Opal showing agate-like coloured bands.
Opal-CA type of opal consisting of disordered α-Cristobalite.
Opal-CTConsists 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-JasperA variety of yellow jasper-coloured Opal.
OpalinePseudomorphs of Opal after Serpentine. A local term.
Painter BoulderSandstone boulders with a thin coating of opal.
Pineapple OpalPseudomorph of Opal after Ikaite
Pinfire OpalWIth very small pinhead-sized colour flashes.
Pipe OpalOpal filling long cylindrical cavities.
Pitch OpalA blackish opal variety with a pitchy lustre.
Prase OpalA green opal.
Precious Fire OpalFire Opal displaying a play of colour.
Precious OpalA 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, 196...
PyrophaneOpal where colour play appears to wander about at random.
Quinzite OpalRose to pink Opal.
Originally described from Quincy-sur-Cher, Bourges, Cher, Centre, France.
Radiolite OpalA smoky-brown opal coloured by inclusions of radiolaria exoskeletons.
Red Flash OpalPrecious Opal with red colour flashes that appear and disappear as the stone is turned.
Resin OpalA yellow-brown common opal with a resinous luster.
Shell OpalOpal replacing fossil shells.
Slocum StoneA synthetically grown opal
Wash OpalAlluvial pebbles of Opal.
Wax OpalA yellow-to brown opal with a waxy lustre.
White OpalPrecious Opal with a white background.
Wood OpalOpal that has replaced fossil wood.
Yowah NutSmall rounded pebbles of sandstone impregnated with Precious Opal.
Originally reported from Yowah opal field, Queensland, Australia.

Common AssociatesHide

Associated Minerals Based on Photo Data:
30 photos of Opal associated with ChalcedonySiO2
28 photos of Opal associated with QuartzSiO2
21 photos of Opal associated with CalciteCaCO3
15 photos of Opal associated with TopazAl2(SiO4)(F,OH)2
13 photos of Opal associated with Goethiteα-Fe3+O(OH)
12 photos of Opal associated with CinnabarHgS
11 photos of Opal associated with Phillipsite subgroup
11 photos of Opal associated with MagnesiteMgCO3
10 photos of Opal associated with FluoriteCaF2
10 photos of Opal associated with IkaiteCaCO3 · 6H2O

Related Minerals - Nickel-Strunz GroupingHide

4.DA.Carbon Dioxide IceCO2
4.DA.05QuartzSiO2Trig. 3 2 : P31 2 1
4.DA.10TridymiteSiO2Tric. 1
4.DA.15CristobaliteSiO2Tet. 4 2 2 : P41 21 2
4.DA.25Melanophlogite46SiO2 · 6(N2,CO2) · 2(CH4,N2)Tet.
4.DA.35CoesiteSiO2Mon. 2/m : B2/b
4.DA.40StishoviteSiO2Tet. 4/mmm (4/m 2/m 2/m) : P42/mnm
4.DA.50SeifertiteSiO2Orth. mmm (2/m 2/m 2/m) : Pbcn

Related Minerals - Hey's Chemical Index of Minerals GroupingHide

7.8.1QuartzSiO2Trig. 3 2 : P31 2 1
7.8.2CoesiteSiO2Mon. 2/m : B2/b
7.8.3TridymiteSiO2Tric. 1
7.8.4StishoviteSiO2Tet. 4/mmm (4/m 2/m 2/m) : P42/mnm
7.8.5CristobaliteSiO2Tet. 4 2 2 : P41 21 2
7.8.7Silhydrite3SiO2 · H2OOrth.

Fluorescence of OpalHide

Yellow to green (uranyl)

Other InformationHide

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:

Opal in petrologyHide

References for OpalHide

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.
Kalokerinos, A. (1981) Opal. Edelstein der tausend Farben. Ein Buch für Sammler und Liebhaber. Stuttgart.
Komotauer, S.K. (1983) Mineralarten im Bild: Opal. Mineralien-Magazin, Hefte 1-3. Stuttgart
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.
Bustillo, M.A. & Martínez-Frías, J. (2003): Green opals in hydrothermalized basalts (Tenerife Island, Spain): alteration and aging of silica pseudoglass. Journal of Non-Crystalline Solids 323, 27-33.
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.
Curtis, N.J., Gascooke, J.R., Johnston, M.R., Pring, A. (2019) A review of the classification of opal with reference to recent new localities. Minerals: 9(5): 299. []

Internet Links for OpalHide

Localities for OpalHide

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