Opal
This iridescent stone, known in modern times as the birthstone for October, is comprised of water and quartz.
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The word opal comes from upala, Sanskrit for “precious stone,” and evolved to the Greek opallios, meaning “to see a change of color,” and then to the Roman word opalus.
Having no regular arrangement of atoms, opal possesses a disordered structure. Gem opal–the mineral SiO2*nH2O–consists of a collection of spheres arranged in parallel planes. These planes serve as diffraction gratings, resulting in opal’s spectacular iridescence. Or, as Paul Pohwat, a geologist with the mineral sciences department of the Smithsonian Institution’s Museum of Natural History, likes to explain to groups he leads on tours: “Opal is quartz with water jammed into its structure. That’s where iridescence comes from.”
There are three general groups of gem opals. The first, precious opal, displays opalescence–spectral color or iridescence that changes with the angle at which the gem is viewed. This is commonly referred to as a “play of color” or sometimes as “fire.” Precious opal can be further broken down into two subgroups: White opal is an opaque stone in which the colors appear as flashes or speckles; black opal, which is less common and tends to be costly, contains “fire” with a dark body color.
The second group is called fire opal. Named for its color, it is transparent or translucent with an orange or red body color that comes from iron. The third group, common opal, is amorphous and rather opaque, and some of its varieties are named honey opal, milk opal, and moss opal.
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Excerpted with permission, Chemical & Engineering News
Copyright © 2003 American Chemical Society