Article
Article
Opal
Article By:
Darragh, Peter J. Commonwealth Scientific and Industrial Research Organisation, Wembley, Australia.
Klein, Cornelis Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico.
Last reviewed:December 2019
DOI:https://doi.org/10.1036/1097-8542.469700
A natural hydrated form of silica. Opal is a relatively common mineral in its nongem form, which is known as common opal and lacks the play of color for which gem, or precious, opal is known (see illustration). All opal is of relatively simple chemical composition, SiO2 ยท nH2O, in which SiO2 represents silicon dioxide and n represents a variable amount of water (H2O) that is housed in the internal structural arrangement of the mineral. The water content generally ranges from about 4 to 10 wt %, but it may be as high as 20 wt %. The hardness of opal on the Mohs hardness scale ranges from 5 to 6, the specific gravity from 2.25 to 1.99 and the refractive index from 1.455 to 1.435. Both specific gravity and refractive index decrease as a function of increasing water content. See also: Hardness scales
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