Precious opal (Andamooka Opal Fields, South Australia)
<p>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. Opal is made up of extremely tiny spheres (colloids - www.uwgb.edu/dutchs/acstalks/acscolor/OPALSPHR.jpg) that can be seen with a scanning electron microscope (SEM).
</p><p>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 &amp; blue &amp; green colors are produced. Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).
</p><p>Not all opals have the famous play of colors, however. Common opal has a wax-like luster &amp; 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.
</p><p>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.
</p>
Locality: unrecorded locality in the Andamooka Opal Fields, ~eastern edge of the Stuart Range Plateau, west of northern Lake Torrens, southeast-central South Australia (environs of 30° 26' 50" South latitude, 137° 09' 55" East longitude)
James St. John
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/34547177805">flickr</a>