File:Precious opal in basalt (Honduras) 2.jpg

English: Precious opal ("black matrix opal") in Tertiary-aged vesicular basalt from Honduras.

The tiny spots of rainbow colors in this basalt sample (click once or twice on the picture to zoom in) are composed of precious 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. Opal is made up of extremely tiny spheres called collloids that can be only be seen with a scanning electron microscope.

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, and 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 a play of colors - common opal usually has a wax-like luster and 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.

Locality: unrecorded site in Honduras (possibly - probably? - from the Tablon Mine, near Erandique, southeastern Lempira Department, western Honduras)

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Photo gallery of opal: www.mindat.org/gallery.php?min=3004

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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.