File:Hydrophane opal (precious opal).jpg

English: Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia)

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).

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).

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.

The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres. This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids). The photo shows the sample dried out. Three days earlier, it was immersed in water for several hours, during which time it turned transparent. After drying out, it returned to its "normal" translucent state. Opal that changes its opacity in water is called hydrophane opal.