File:Precious opal-knapped arrowhead (Coober Pedy Opal Field, South Australia) 5 (30524168151).jpg
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| Description |
Precious opal-knapped arrowhead. (~23.5 millimeters long) Knapper: Chris Miller Opal is hydrous silica (SiO2·nH2O) and is made up of extremely tiny spheres (colloids - <a href="https://www.uwgb.edu/dutchs/acstalks/acscolor/OPALSPHR.jpg" rel="nofollow">www.uwgb.edu/dutchs/acstalks/acscolor/OPALSPHR.jpg</a>) 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. Common 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. Host rock: Bulldog Shale, lower Marree Subgroup, Aptian Stage, upper Lower Cretaceous Locality: unrecorded site in the Coober Pedy Opal Field, north-central South Australia State, southern Australia Photo gallery of opal: <a href="http://www.mindat.org/gallery.php?min=3004" rel="nofollow">www.mindat.org/gallery.php?min=3004</a> Reference cited: Carr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program. Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51. 68 pp. |
| Date | |
| Source | Precious opal-knapped arrowhead (Coober Pedy Opal Field, South Australia) 5 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/30524168151 (archive). It was reviewed on 5 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 05:41, 5 December 2019 | | 720 × 1,430 (675 KB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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