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Roger Weller, geology instructor

Opal (Hydrated Silica Glass)
by Cake M. Janssen
Physical Geology-GLG 101
Spring 2005



Opal (Hydrated Silica Glass) 

Photograph by Roger Weller.  Jewelry courtesy of Opex Opalo.

What is Opal?

What are the differences between precious opal and fire opal and where are they from?

Have opals historically been held to have any magical properties?

      The name of opal probably is derived from the Sanskrit name for precious stone, Upala. It has been mined for
 centuries, at least since Roman times when they extracted the opal from areas now within the Czech Republic. The Aztecs made use of local Mexican sources, as did the Spaniards when they exported the material back to Europe. Today most precious opal comes from Australia with significant sources from Mexico and the Western United States(mineralo/opal/opal.htm).

Opal is not a kind of quartz, but like quartz, it is made of silica, which is silicon dioxide, which can also be a glass. Opal has been a popular gem for many centuries and considered a mineraloid because the structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 –10% and greater. This water can help geologists determine the temperature of the host rock at the time the opal formed (  The Nevada fire opal, shown below has as higher water content than Australian opals.


Photograph by Roger Weller

      Opals are irregular in size and inconsistent in concentration and are found in many organized pockets of the
spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres. This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it, such as the precious opal and fire opal (

Opal from Lightning Ridge, Australia.  Photograph by Roger Weller

      Precious opal refers to an optical phenomenon, used in gems that come from Australia. The delightful flash of
 colors in a fine gem opal is properly called “play-of-color,” and we often react to this phenomenon by saying: “Wow, look at the fire in that one!” Regardless of what we call this rainbow- like effect, what we admire in opals are the colors of the spectrum spread out before us.  When we see these spectral colors in opal, we call the material precious opal. This colorful display is the result of diffraction.  Structurally, precious opal is a three-dimensional array of very tiny spheres of almost equal size aligned in such a way as to separate or diffract white light into some or all of the different colors of the spectrum (Frazier 16).

Australian fire opal courtesy of Opex Opalo.  Photograph by Roger Weller

      Fire opal is an opal with a “fiery” body color, from Queretaro, Mexico. Some have shade of red or yellow, which is due to a very fine dispersion of red or yellow iron oxide or hydroxide particles.  Although other nations, including the United States, produce small amounts of red, yellow, orange, or orange-brown opal without play-of-color, Mexico is so famous for it that it is often referred to as Mexican fire opal.  When fire opals show a play-of-color, they are best referred to as precious Mexican fire opal or precious fire opal (Lapidary Journal 18).

Mexican fire opal.  Photograph by Roger Weller

      Opals historically have been held to have many magical properties.  From the end of the Roman Empire until the dawn of modern mineralogical science in the 1700s, nearly the only interest in gems concerned their reputed magical or medical attributes. There is an enormous literature in what are generically referred to as “mediaeval lapidaries,” which have nothing whatsoever to do with cutting and polishing gems but rather with usually fanciful descriptions of what was called their virtues (Journal 25).

Brazilian opal. Photograph by Roger Weller

      Opal had been more or less shunned since the publication of Sir Walter Scott’s novel, Anne of Geierstein, in which an opal worn in a young woman’s hair is sprinkled with holy water, and soon thereafter she perishes.  From this vague tale, opal developed a reputation for bringing bad luck and the sales of opals dropped 50 percent.  If anything could make women risk life and limb to put opals back in their hair
and around their throats, opal ornaments were it (Lapidary 38).

Australian fire opal.  Photograph by Roger Weller.

      Today, opal is still the favorite gemstone of many people because of the color and the many patterns it has. For me it is like a painting in watercolor.  I noticed when I was at the gem show in Tucson, people moved back to more conventional thinking. Times hasn’t really changed that much. The rich still want their big faceted stones. The big firms are still Cartier and Boucheron.  People, when it comes down to it, want to display their wealth in the old way.







Works Cited

                  Photos of Opals

                  What is Opal?


The difference between precious opal and fire opal

Where are they from?           

Si & Ann Frazier. “Is Opal?”
      Lapidary Journal Vol. 52. No. 5. Frazier. 16-18-38.  August 1998.