OPAL

 

Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid, but most geologists don't mind calling it a mineral.  Opal is made up of extremely tiny spheres (colloids) that can be seen with a scanning electron microscope (SEM).

 

Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).

 

Not all opals have the famous play of colors, however.  Ordinary opal has a wax-like luster &  is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.

 

Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails/scouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.

 


 

 


 

 

Common opal (4.2 cm across) from Nevada.  The milky white color is usually attributed to an abundance of microscopic inclusions.

 


 

Cacholong opal from Tampa Bay, Florida, USA.  Cacholong opal is a non-lustrous, microporous variety of common opal.  Cacholong opal will stick to your tongue, a consequence of its microporous and hygroscopic nature (public display, Field Museum of Natural History, Chicago, Illinois, USA).

 


 

Jelly opal in volcanic breccia from Mexico.

 


 

Opal in vesicular basalt from Humboldt County, Nevada, USA. (CMNH 11028, Cleveland Museum of Natural History, Cleveland, Ohio, USA)

 


 

Bacon opal (a.k.a. candy stripe opal) from the Thomas Range of Utah, USA.

 


 

Precious opal in matrix (2.2 cm across) from Queretaro State, Mexico.

 


 

Black opal (polished) from Lightning Ridge, northern New South Wales, eastern Australia.  Lightning Ridge produces most of the world's precious black opal from fluvial sediments of the Finch Clay (upper Lower Cretaceous).

 


 

 


 

 


 

Precious opal in matrix (Field Museum of Natural History public display, Chicago, Illinois, USA).

 


 

Precious opal from Western Australia.  (CSM public display, Colorado School of Mines Geology Museum, Golden, Colorado, USA)

 


 

Precious opal in matrix from Australia.  Specimen owned by Stan & Pris Woollams.

 


 

Precious opal vein intruding sandstone fragment (6.6 cm across) (above & below).

Locality: Andamooka Opal Fields, erosive edge of the Stuart Range Plateau, west of northern Lake Torrens, South Australia

Stratigraphy of host rock: Bulldog Shale, Marree Subgroup, Aptian Stage, upper Lower Cretaceous

 


 

Precious opal from Andamooka, South Australia (CIS 212-125, Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA).

 


 

Precious opal from Andamooka, South Australia (CIS 212-133, Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA).

 


 

Precious opal from Queensland, Australia (CIS 212-148, Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA).

 


 

Precious opal from Queensland, Australia (CIS 212-152, Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA).

 


 

Precious opal from Virgin Valley, Humboldt County, Nevada, USA (CIS 212-10, Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA).

 


 

Precious opal (3.3 cm across) from the Yita Ridge-Guy Meda Wihe River-Gift Ridge area of Menz Gishe District, northeastern Shewa (Shoa) Province, northeast-central Ethiopia.  The opal there occurs in nodules in an Early to Middle Miocene-aged rhyolitic welded tuff (Alaji Rhyolite Formation, a.k.a. Amba Alaji Rhyolites, a.k.a. Aliyu Amba Ignimbrites).

Ethiopian opal has been identified as opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.

The Australian opals (see above) are composed of opal-A, which consists of amorphous, hydrous silica spheres (colloids).

 


 

Opal (polished) from the Coober Pedy Opal Field of South Australia (CSM # A83.301, Colorado School of Mines Geology Museum, Golden, Colorado, USA).

 


 

Opal (polished) (Colorado School of Mines Geology Museum, Golden, Colorado, USA).

 


 

Black opal (polished, 87 carats) (public display, Field Museum of Natural History, Chicago, Illinois, USA)

 


 

Black opal (polished) from Australia.  (public display, Field Museum of Natural History, Chicago, Illinois, USA)

 


 

Photo gallery of opal

 


 

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