Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
Opal (Hydrated Silica Glass)
by Cake M. Janssen
Physical Geology-GLG 101
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
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).
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 (mineral.galleries.com). 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 (ssdl-delta.tanford.edu/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).
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).
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).
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.
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.