Opal

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Black Opal
Lightning Ridge, Finch Co., New South Wales, Australia
1.34 carats
© Mineral Classics

The name is derived from an Indian (Sanskrit) word for "stone". Opal consists of silica and water. The amount of water varies in different specimens.

Opal is divided into three subgroups: Precious Opal, yellow-red Fire Opal, and Common Opal. Their physical properties vary considerably. Precious Opal is conspicuous amongst all others for the magnificent play of colours produced by the refraction and reflection of light in its colourless substance.

The lustre is usually of the common vitreous type, though in some opals it may be greasy, resinous, or waxy. The lustre of natural specimens is only moderately strong, it is increased by cutting and polishing, but even then is in no way remarkable. Both common and precious varieties of opal are, as a rule, cloudy, being at the best only translucent or semi-transparent. In a pure condition the mineral is perfectly colourless, the tint of the coloured varieties is due to the presence of impurities. In colour these varieties are usually brown, yellow, or red, of various shades. Green opal is rare, black opal is known, and the cloudy varieties of milk opal are white.

Opal Gemstones by Colour

This table shows the variety of hues this gemstone can be found in. Click on a photo for more information.
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Opal Gemstones by Size

This table shows distribution of Opal gemstone sizes that are listed on this site. This can give a good indication as to the general availability of this gemstone in different sizes.
Contributed photos
Lightest:0.10 cts
Heaviest:206.2 cts
Average:9.79 cts
Total photos:233
Do you have a larger Opal? Why not upload a photo?
Significant stones
Galaxy Opal3,749 cts
Flame Queen Opal263 cts
0.10ct to 20.71ct20.71ct to 41.32ct41.32ct to 61.93ct61.93ct to 82.54ct82.54ct to 103.15ct103.15ct to 123.76ct123.76ct to 144.37ct144.37ct to 164.98ct164.98ct to 185.59ct185.59ct to 206.20ct
General Information
Varieties/Types:
Common Opal - Opaque variety of Opal, shows no play of colour.
Agate Opal - Opal showing concentric banding as in agate.
Andean Opal - A greenish-blue variety of opal found in Peru.
Girasol - Bluish-white translucent Opal with reddish reflections.
Honey Opal - An orange-brown Common opal.
Hyalite - Colourless, water-clear opal with a strong sheen.
Hydrophane - A milky variety of Opal that turns translucent or transparent when immersed in water.
Jasper Opal - A brecciated Jasper cemented by opal.
Milk Opal - A white Common opal.
Moss Opal - Milky opal containing dendritic inclusions
Prase Opal - A green variety of Common opal.
Semiopal - Historic name for an opal without opalescence
Wax Opal - A yellow-to brown Opal with a waxy lustre.
Wood Opal - Petrified wood replaced with Opal.
Fire Opal - A red to yellow variety of Opal.
Precious Opal - A variety of Opal that displays play-of-colour.
Black Opal - The variety of Precious Opal with a dark background colour.
Boulder Opal - A variety of Precious Opal found in Queensland, Australia.
Crystal Opal - Transparent to translucent Precious Opal where colour play is visible both on the surface and in the interior.
Harlequin Opal - Transparent to translucent Precious Opal with effective mosaic-like colour patterns.
Matrix Opal - A rock consisting of a (usually) silicified matrix containing veins or blebs of Precious opal.
Water Opal - A transparent Precious opal with a gelatinous appearance and a bluish sheen.
White Opal - Precious Opal with a white background.
Chemical Formula
SiO
 
2
+ H
 
2
O
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Significant stones
ImageNameWeightCountry of OriginCurrent Owner
Galaxy Opal3,749.00 ctsBrazil
Flame Queen Opal263.18 ctsAustraliaJack Plane Collection
Andamooka Opal203.00 ctsAustraliaQueen Elizabeth II
Photos of natural/un-cut material from mindat.org
Opal Simulants
Several good imitations made from glass or plastic are known. In 1970, a synthesis of White opal and Black opal succeded. Fakes are prepared by colouring Black opal or Matrix opal in order to liven up the play-of-colour. - Gemstones of the world, Walter Schumann, 2001, p 152
Synthetic Opal
Synthetic opal: RI 1.440 - 1.450; opt. isotropic; SG 1.74 - 2.07; Colours: colourless, white, black, red; Play of colour: red, orange, yellow, green, blue, violet; Transparent to opaque; Hardness 5.5 - 6.5. Clearly separated colour patches, "lizard-skin effect", columnar structure perpendicular to the "lizard-skin effect" - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 1
Physical Properties of Opal
Mohs Hardness5 to 6.5
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Specific Gravity1.98 to 2.25
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Highly porous opal may show much lower SGMore from other references
TenacityBrittle
Walter Schumann, Gemstones of the world (2001)
Cleavage QualityNone
Walter Schumann, Gemstones of the world (2001)
FractureConchoidal,Splintery
Walter Schumann, Gemstones of the world (2001)
More from other references
Heat SensitivityWhen heated opal is easily fractured.The constituent water is expelled below a red-hit, when the specimen, if not so, becomes cloudy and opaque.
Max Bauer, Precious Stones (1968) Dover Publications
Optical Properties of Opal
Refractive Index1.37 to 1.52
Walter Schumann, Gemstones of the world (2001)
More from other references
Optical CharacterIsotropic
Arthur Thomas, Gemstones (2009)
BirefringenceNone
Walter Schumann, Gemstones of the world (2001)
PleochroismAbsent
Walter Schumann, Gemstones of the world (2001)
DispersionNone
Walter Schumann, Gemstones of the world (2001)
Colour
Colour (General)Black, white, colourless, red, yellow, bluish
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Causes of ColourMulticolors (play of color opal), diffraction by the regular stacking of silica spheres. Orange to red (fire opal), microscopic to sub-microscopic inclusions of iron hydrous oxides. Green (prase opal), microscopic to submicroscopic nickeliferous clay-like inclusions
W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011)
TransparencyTransparent,Translucent,Opaque
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
LustreVitreous
Arthur Thomas, Gemstones (2009)
Fluorescence & other light emissions
Fluorescence (Short Wave UV)Commonly green or yellow, occasionally green-yellow, rarely yellow or red
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
Fluorescence (Long-Wave UV)Many WHITE and light opals glow bluish-white and phosphorescence yellowish-green
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Crystallography of Opal
Crystal SystemAmorphous
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
HabitKidney- or grape-shaped aggregates
Walter Schumann, Gemstones of the world (2001)
More from other references
Geological Environment
Where found:Much of the finest opal occurs in thin seams and has to be recovered together with its underlying rock (matrix). Alternatively some fine opal is found as nodules.
(‘nobbies’ in Australia).
Michael O’Donoghue, Gems, Sixth Edition (2006)
Inclusions in Opal
Swirl-like exsolutions, goethite, chalcedony etc., liquid- and multiphase inclusions - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 1
Further Information
Mineral information:Opal information at mindat.org
Significant Gem Localities
Australia
 
  • New South Wales
    • Finch Co.
Thomas et al. (2006)
    • Yungnulgra Co.
Thomas et al. (2006)
  • Queensland
    • Paroo Shire
Cooper and Neville (2007)
    • Quilpie Shire
      • Kyabra Co.
        • Keeroongooloo
Opal, Lithographie, 2007
[Precious Opal var: Boulder Opal] Opal, Lithographie, 2007
[Precious Opal] Cooper and Neville (2007)
    • Winton Shire
Cooper and Neville (2007)
Wise (1993)
  • South Australia
    • Central North
Townsend (2006, 2009)
      • Stuart Creek
Townsend (2006, 2009)
    • North West Province
      • Mintabie district
Townsend (2006, 2009)
    • Stuart Shelf
      • Andamooka Ranges
Townsend (2006, 2009)
  • Victoria
    • Sunbury
Opal, Lithographie, 2007
Brazil
 
  • Pará
    • São Geraldo do Araguaia
Farrar (2007)
  • Piauí
    • Pedro II
Knigge and Milisenda (1997)
[var: Fire Opal] H. Henriques
Canada
 
  • British Columbia
    • Vernon Mining Division
      • Vernon
Downing (2003)
Ethiopia
 
  • Afar Province
Mazzero et al. (2009)
      • Delanta
        • Wegeltena (Wegel Tena)
Mazzero et al. (2009)
  • Amhara Province
    • Mezezo
Mazzero (2003)
Honduras
[var: Fire Opal] Banerjee and Wenzel (1999)
  • Lempira Department
Viti and Gemmi (2009)
Indonesia
 
Laurs (2001)
    • Banten Province
[Precious Opal] Laurs (2001)
  • Sumatera Island (Sumatra Island)
    • Bengkulu Province (Benkoelen Province; Benkulen Province)
      • Rejang Lebong District (Rediang Lebong District)
Lambert and Brown (1994)
Mexico
 
  • Durango
    • Mun. de Mapimí
[Common Opal var: Pink Opal]
  • Hidalgo
    • Mun. de Zimapán
[var: Fire Opal] Coenraads and Zenil (2006)
  • Jalisco
    • Mun. de Magdalena
[var: Fire Opal] Michelou (2006)
[Precious Opal] mindat.org
[Precious Opal] mindat.org
Peru
 
  • Arequipa Department
    • Caraveli Province
Hyrsl (2001, 2007)
  • Ayacucho Department
  • Ica Department
    • Ica Province
      • Ica
[Precious Opal var: White Opal]
    • Pisco Province
[Common Opal var: Honey Opal]
Tanzania
 
  • Arusha Region
    • Dumbawanga District
http://www.ganoksin.com/borisat/nenam/tanzania-mines.htm
  • Dodoma Region
Keller (1992)
USA
 
  • Idaho
    • Clark Co.
      • Spencer
Sinkankas (1997)
  • Louisiana
    • Vernon Parish
[Precious Opal] Gaber (2007)
  • Mississippi
Gaber (2007)
  • Nevada
    • Humboldt Co.
Sinkankas (1997)
  • Oregon
    • Lake Co.
[var: Fire Opal] Laurs and Quinn (2003)
    • Morrow Co.
Holzhey (1997)
Copyright © Jolyon & Katya Ralph 1993-2014. Site Map. Locality, gem & photograph data are the copyright of the individuals who submitted them. For more information please contact the . Gemdat.org is an online information resource dedicated to providing free gemmological information to all.