WHAT'S THAT STUFF?
varieties "self-destruct" through the loss ofwater, resulting infinecracks 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 GREW UP WITH A MOTHER WHO BEThere are three general groups of gem the Mohs Scale of Hardness, where talc is lieved in superstitions. Besides the to- opals. The first, precious opal, displays rated 1 and diamond ends the scale at 10, be-expected ones relating to ladders, opalescence—spectral color or iridescence opal is rated 5-6. Nevertheless, opal is still black cats, spilled salt, and broken mir- that changes with the angle at which the a premier gemstone. rors, she insisted that anyone who gem is viewed. This is commonly referred Known as the Queen ofGems, opal can stepped over me while I was lying on the to as a"playofcolor" or sometimes as "fire." be made synthetically The Gilson process floor must step back over me in the other Precious opal can be further broken down is considered the chemical process closest direction—something about stunting the into two subgroups: White opal is an to the way nature makes opal. Created by growth of the person who was stepped opaque stone in which the colors appear as Pierre Gilson Sr. in 1974 in a laboratory in over. Such superstitions are often traced flashes or speckles; black opal, which is less France, the process produces opal that back to religious beliefs or belief in magic. common and tends to be costly, contains mimics all the chemical and physical properties of genuine opal and takes 14 to 18 This brings me to opal, which through "fire" with a dark body color. The second group is called fire opal. months to grow. Silicon spheres are genthe ages has been associated with superstitions both good and bad. The early Named for its color, it is transparent or erated and line up in straight-line formation Greeks believed opal bestowed its owner translucent with an orange or red body col- to produce a diffraction grating and the efwith the powers offoresightand proph- or that comes from iron. The third group, fect of spectral colors or refraction. The esy Romans perceived opal as a token of common opal, is amorphous and rather spheres are then surrounded by a material that is strong enough to withstand hope and purity Arabs believed it fell cutting and polishing; only siliconfrom heaven. Medieval peoples, based material can properly be called however, associated opal with the synthetic opaL Evil Eye and even the Black Plague or thought it made a person invisible These opals have all the elements when the gem was wrapped in a bay of natural opal except water, which leaf Queen Victoria boosted opal's makes them equally beautiful but not popularity by making it a court faprone to breaking. Gilson opal can be vorite. More recently, as October's identified by a diagnostic "lizard-skin" birthstone, opal is thought to bring effect that is seen through the magluck—but only to those born in nification of a jeweler's loupe. October. Slocum stones are imitation opals madefromglass; opalite is made from So, what is that stuff that has such plastic. Opal Essence is the name that power over people? has been given to both glass and plasThe word opal comes from upa/a, Sanskrit for "precious stone," and WATER AND FIRE Australian precious opals and tic imitations, which are also difficult evolved to the Greek opallios, mean- Mexican fire opals tantalize but may bring bad luck. to distinguish from precious opal with the naked eye. ing "to see a change of color," and In science and technology, opal has been then to the Roman word opalus. opaque, and some ofits varieties are named found to have potentially useful optical Having no regular arrangement of honey opal, milk opal, and moss opal. properties. Researchers at the University atoms, opal possesses a disordered structure. Gem opal—the mineral Si0 2 * GEOLOGICALLY, opal isformedfrom low- of Toronto describe "opal chips" that use »H20—consists ofa collection of spheres temperature hydrothermal solutions or photons rather than electrons to cany inarranged in parallel planes. These planes when groundwater leaches silica from the formation. They report a way of "fabricatserve as diffraction gratings, resulting in soil, leaving it behind in cracks deep with- ing micrometer-scale patterned singleopal's spectacular iridescence. Or, as Paul in the earth after accumulated groundwa- crystal colloidal crystals in silicon wafers." Pohwat, a geologist with the mineral sci- ter has evaporated. Nearly all ofthe world's This process could be integrated into chip ences department of the Smithsonian In- supply of precious opal comesfromAus- fabrication facilities and would be stitution's Museum of Natural History, tralia, which is famous for its white and amenable to mass production (C&EN,Jan. likes to explain to groups he leads on tours: black precious opal. Hungarian opal comes 22,2001, page 55). "Opal is quartz with water jammed into its from mines in the Czerwenitza area; fire Not many inanimate objects can be structure. That's where iridescence comes opal,fromQueretaro state in central Mex- called tantalizing and two-faced, but opal from." ico. Within the U.S., opal has been found may be one. So believe what you will about Privately, though, Pohwat admits this is in Arizona, Oregon, Idaho, Nevada, and a the powers of opal, but take heed Whoever stepped over me and didn't step an oversimplification. "Opal's structure is few other states. actually comparable to cristobalite, a lessAccording to informationfromthe U.S. back—look out! I never quite made it to 5 er known Si0 2 polymorph, with a lot of Geological Survey, opal is brittle, heat sen- feet, but IVe gpt an opalringand I know nonessential water." sitive, and breaks and scratches easily, some h o w t o use it.—ARLENE GOLDBERG-GIST
OPAL
Made of water and quartz, but filled with fire
I
58
C & E N / J A N U A R Y 2 7 , 2003
HTTP://WWW.CEN-ONLINE.ORG
