January 27, 2003
Volume 81, Number 04
CENEAR 81 04 pp. 59
ISSN 0009-2347


Made of water and quartz, but filled with fire

I grew up with a mother who believed in superstitions. Besides the to-be-expected ones relating to ladders, black cats, spilled salt, and broken mirrors, she insisted that anyone who stepped over me while I was lying on the floor must step back over me in the other direction--something about stunting the growth of the person who was stepped over. Such superstitions are often traced back to religious beliefs or belief in magic.

This brings me to opal, which through the ages has been associated with superstitions both good and bad. The early Greeks believed opal bestowed its owner with the powers of foresight and prophesy. Romans perceived opal as a token of hope and purity. Arabs believed it fell from heaven. Medieval peoples, however, associated opal with the Evil Eye and even the Black Plague or thought it made a person invisible when the gem was wrapped in a bay leaf. Queen Victoria boosted opal's popularity by making it a court favorite. More recently, as October's birthstone, opal is thought to bring luck--but only to those born in October.

WATER AND FIRE Australian precious opals and Mexican fire opals tantalize but may bring bad luck.
So, what is that stuff that has such power over people?

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."

Privately, though, Pohwat admits this is an oversimplification. "Opal's structure is actually comparable to cristobalite, a lesser known SiO2 polymorph, with a lot of nonessential water."

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.

GEOLOGICALLY, opal is formed from low-temperature hydrothermal solutions or when groundwater leaches silica from the soil, leaving it behind in cracks deep within the earth after accumulated groundwater has evaporated. Nearly all of the world's supply of precious opal comes from Australia, which is famous for its white and black precious opal. Hungarian opal comes from mines in the Czerwenitza area; fire opal, from Querétaro state in central Mexico. Within the U.S., opal has been found in Arizona, Oregon, Idaho, Nevada, and a few other states.

According to information from the U.S. Geological Survey, opal is brittle, heat sensitive, and breaks and scratches easily; some varieties "self-destruct" through the loss of water, resulting in fine cracks that extend over the surface until they intersect and cause the gem to crumble. This process, called crazing, is one of the reasons why opal has been called a bad-luck stone. On the Mohs Scale of Hardness, where talc is rated 1 and diamond ends the scale at 10, opal is rated 5–6. Nevertheless, opal is still a premier gemstone.

Known as the Queen of Gems, opal can be made synthetically. The Gilson process is considered the chemical process closest to the way nature makes opal. Created by Pierre Gilson Sr. in 1974 in a laboratory in France, the process produces opal that mimics all the chemical and physical properties of genuine opal and takes 14 to 18 months to grow. Silicon spheres are generated and line up in straight-line formation to produce a diffraction grating and the effect of spectral colors or refraction. The spheres are then surrounded by a material that is strong enough to withstand cutting and polishing; only silicon-based material can properly be called synthetic opal.

These opals have all the elements of natural opal except water, which makes them equally beautiful but not prone to breaking. Gilson opal can be identified by a diagnostic "lizard-skin" effect that is seen through the magnification of a jeweler's loupe.

Slocum stones are imitation opals made from glass; opalite is made from plastic. Opal Essence is the name that has been given to both glass and plastic imitations, which are also difficult to distinguish from precious opal with the naked eye.

In science and technology, opal has been found to have potentially useful optical properties. Researchers at the University of Toronto describe "opal chips" that use photons rather than electrons to carry information. They report a way of "fabricating micrometer-scale patterned single-crystal colloidal crystals in silicon wafers." This process could be integrated into chip fabrication facilities and would be amenable to mass production (C&EN, Jan. 22, 2001, page 55).

Not many inanimate objects can be called tantalizing and two-faced, but opal may be one. So believe what you will about the powers of opal, but take heed: Whoever stepped over me and didn't step back--look out! I never quite made it to 5 feet, but I've got an opal ring and I know how to use it.


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