Lightning Ridge, Finch Co., New South Wales, Australia
© Mineral Classics
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.
|Photos of natural/un-cut material from mindat.org|
|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: 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 Hardness||5 to 6.5Blue Chart Gem Identification (2010) , More from other references|
|Specific Gravity||1.98 to 2.25Blue Chart Gem Identification (2010) , Highly porous opal may show much lower SGMore from other references|
|Tenacity||BrittleGemstones of the world (2001) ,|
|Cleavage Quality||NoneGemstones of the world (2001) ,|
|Fracture||Conchoidal,SplinteryGemstones of the world (2001) , More from other references|
|Heat Sensitivity||When heated opal is easily fractured.The constituent water is expelled below a red-hit, when the specimen, if not so, becomes cloudy and opaque.Precious Stones (1968) Dover Publications ,|
|Optical Properties of Opal|
|Refractive Index||1.37 to 1.52Gemstones of the world (2001) , More from other references|
|Optical Character||IsotropicGemstones (2009) ,|
|Birefringence||NoneGemstones of the world (2001) ,|
|Pleochroism||AbsentGemstones of the world (2001) ,|
|Dispersion||NoneGemstones of the world (2001) ,|
|Colour (General)||Black, white, colourless, red, yellow, bluishGemmological Tables (2004) , More from other references|
|Causes of Colour||Multicolors (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 inclusionsPragmatic Spectroscopy For Gemologists (2011) ,|
|Transparency||Transparent,Translucent,OpaqueGemmological Tables (2004) , More from other references|
|Lustre||VitreousGemstones (2009) ,|
|Fluorescence & other light emissions|
|Fluorescence (Short Wave UV)||Commonly green or yellow, occasionally green-yellow, rarely yellow or redGemmological Tables (2004) ,|
|Fluorescence (Long-Wave UV)||Many WHITE and light opals glow bluish-white and phosphorescence yellowish-greenBlue Chart Gem Identification (2010) , More from other references|
|Crystallography of Opal|
|Crystal System||AmorphousBlue Chart Gem Identification (2010) , More from other references|
|Habit||Kidney- or grape-shaped aggregatesGemstones of the world (2001) , More from other references|
|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).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|
|Mineral information:||Opal information at mindat.org|
|Significant Gem Localities|