Optical Properties of Opal

by Karen Dawes

Lithosphere (May 1995); Fallbrook Gem and Mineral Society, Inc.; Fallbrook, CA

[Ed. Note: This article won first place in the adult article competition in the California Federation of Mineralogical Societies in 1995.]

I love opals. Even as a child, I was fascinated by the ever-changing play of colors as I moved my grandmother's opal ring in the light.

There are many kinds of opal, but only precious opal flashes color. Opal is composed of silicon, oxygen, and water. The chemical formula is SiO2 · nH2O. Opal contains from 1 percent to 27 percent water. The mineral itself is amorphous, never crystalline. Opal is most often found in cracks, cavities, and in veins within rock.

Why does precious opal flash color? The opal structure consists of numerous, tiny spheres of silica, hydrous silica, and water packed together in a pattern to form layers. The index of refraction of opal is 1.44. The index of refraction of water is 1.33. The refraction of light, back and forth, between the layers causes the light to break up into spectral colors. Unequal refraction of different wavelengths of visible light result in bright flashes of different prismatic colors.

The size of the tiny spheres determines which colors are visible. If the particles are large enough to allow the longest wavelengths of visible light through, then all the colors of the spectrum will be seen. If the particles are very small and only the shorter wavelengths of visible light can pass through, then only the colors produced by the shorter wavelengths will be seen. I have a piece of opalized petrified wood that only flashes blue and violet. This tells me that this piece of opal consists of very tiny spheres of material.

Precious opal is never faceted like a gemstone. It is polished "in cabochon" in order to capture as much of the flash of color as possible.


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Rocks and Minerals; Ridge Press, Inc.; New York; 1973.

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Kourimsky, Jiri;
The Illustrated Encyclopedia of Minerals and Rocks; Octopus Books Limited; London; 1977.

Leechman, Frank;
The Opal Book; Lansdown Press; Australia; 1961.

World Book Encyclopedia; 1991.

Copyright © 1995 by Fallbrook Gem and Mineral Society, Inc.

The preceding article was originally published in the May 1995 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).

Permission to reproduce and distribute this material, in whole or in part, for non-commercial purposes, is hereby granted provided the sense or meaning of the material is not changed and the author's notice of copyright is retained.

Last updated: 18 September 2002