Opal in Volcanic Rock

Honduras, Central America

opal, Honduras [32 kb]

"Rock of the Month # 6, posted December 2001" --- Sample JA-1 (see Descr. 2389).

Opal is a natural silica gel, a cryptocrystalline or amorphous hydrated form of silica, valued as a semi-precious stone. The most economically- important opal mines are in Australia (Coober Pedy, Lightning Ridge and elsewhere), the second most significant producer being Mexico (Queretaro, Guanajuato). Honduras and Nicaragua are both minor opal producers. Opal can also be found locally in western Canada (British Columbia) and in several states of the USA. The gem may display a wide range of colours, hence there are numerous varietal names, and the internal reflections give rise to the term opalescence.

Sample JA-1, courtesy of Joe Arengi, was a "find" at the Bancroft Gemboree 1999. This sample is from Honduras. It is of probable Neogene (late Tertiary) age. It is a black volcanic rock with flashes of fiery opal in the matrix. The hand specimen also displays the alignment of tiny vugs and other features such as feldspar phenocrysts and small rounded clasts. A thin section comprises 82 volume percent flow-banded glass and 10% opal, plus 5% plagioclase feldspar phenocrysts, 3% lithic clasts (partially resorbed, glassy, with feldspar microphenocrysts), and traces of goethite and pyrite. There are small, irregular, frequently elongate patches of opal, generally infilling small voids within the fabric of glass shards, individual patches of opal seldom larger than 600x100 µm. There are intense internal reflections: dominant blue-green tones and lesser, deeper orange-red. There is an internal fibrous texture, sometimes visible in cross-polarized light. The gemstone's fire is best seen when the sample is wet: intense red, green, orange and blue flecks, from the opal infilling small cavities and fractures. Porosity, infilled with opal, is aligned within the directive fabric of the rock, defined by sheets and shards of the predominant yellow-brown glass. The rounded, embayed lithic clasts, up to 1000x600 µm in section, include fine-grained trachyte. 60-second exposure at 160X magnification, long-axis field-of-view 0.7 mm, cross- polarized transmitted light. In plane-polarized light the glassy host of the opal is tinted two-tone, with pale brown strips of glass in a clear glass groundmass.


Leechman,F (1961) The Opal Book. Ure Smith, Sydney, 255pp.

Graham Wilson, for December 2001.

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