Opal: All The Colors in One Gem
Mysterious opals contain all the colors of the rainbow at once - the wonders of the skies - sparking rainbows, fireworks, and lightning - shifting and moving in their depths. Opal has been treasured throughout history by many cultures around the world. Roman historian Pliny described the beauty of opal as the combination of the beauty of all other gems: "There is in them a softer fire than the ruby, there is the brilliant purple of the amethyst, and the sea green of the emerald - all shining together in incredible union." Opal was much loved and valued highly by the Romans, who called it opalus.
At the same time, opal was also sought in what would become the
Americas. The Aztecs mined opal in South and Central America. So Opal has been found in at
least small amounts in many locations. However, by far the vast majority of the opal on
the market today comes from country of Australia.
The Australian Gem of Water Born in the Desert
The story of opal in Australia begins more than 100 million years ago when the deserts of central Australia were a great inland sea, with silica-laden sediment deposited around its shoreline. After the sea receded and disappeared to become the great Artesian basin, weathering 30 million years ago released a lot of the silica into a solution which filled cracks in the rocks, layers in clay, and even some fossils. Some of this silica became precious opal. Opal is one of the few gemstones that is sedimentary in origin. Opal still contains 6 to 10 percent water, a remnant of that ancient sea. Gold panners in Australia found the first few pieces of precious opal in 1863. Mines at White Cliffs began producing in 1890.
Only opal with a perfectly aligned grid of silica spheres will show play of color, which is created through diffraction of light across those spheres. The size of the spheres determine the wavelengths and therefore the colors seen. The brilliance of the colors are determined by the regularity of the grid. The strength of the colors seen in opal also depend on the background body color and the transparency of the stone. The body color determines the variety of opal and has a large impact on the value.
Black opal, a form of opal with a black to dark gray contrasting body color, has the most brilliant colors and is the most valuable. Crystal opal, the next most costly type of opal, is transparent with flashes and is highly valued due to the brilliance of its colors and the fact that many layers of color within the stone can also be seen. White and milky opals tend to have more diffused colors due to the light background color. This is the most affordable type of opal. Another more unusual type of opal is boulder opal, which has opal with an ironstone host rock matrix which creates a natural dark background to view its fire. These sometimes occur in "splits" a matched pair of opals created when a piece of boulder opal is split along the opal vein. These are particularly favored for earrings, since they are mirror images of each other.
In Australia, black opal is found only at Lightning Ridge, the most famous opal deposit in the world since it was discovered in 1903, and in Mintabie, which also produces large quantities of light opal. Another large opal producing area in Australia is Coober Pedy, which produces light opal. The name Coober Pedy is an Aboriginal name meaning "white man in a hole." If you visit Coober Pedy, you will understand how it got its name: many houses - and even a church! - are burrows dug into the ground called dugouts. This type of dwelling is quite practical and cool as temperatures soar in the daytime. Andamooka is known for producing crystal opal and light opal. Boulder opal is produced in several areas in western Queensland.
OPAL FROM AROUND THE WORLD
In addition to Australia, a small quantity of precious opal is
produced in Brazil. Mexico and the state of Oregon in the United States produce a volcanic
opal called fire opal. Mexican opal has a generally orange body color. Fire opal is
transparent opal ranging in color from colorless to yellow, orange, and red. Sometimes
the Mexican opal
also shows play of color in addition to its bright orange body color. Large
amounts of Fine Mexican opal is cut and polished within Mexico and sold in
Mexican jewelry stores, especially in popular tourist areas. A low quality opal was
recently discovered in Ethiopia. A green translucent opal that resembles chrysoprase or
jade, which is called prase opal, is found in Tanzania. Tanzania also produces an unusual
variety of opal that shows a strong cats eye effect. A beautiful blue-green opal is found
in Peru in the Andes Mountains. These Peruvian types of opal do not display play of color.
CHOOSING AN OPAL
Opal is a gem valued for its play of color, the shifting light showing through from its depths. The intensity of that play of color is the chief factor in evaluating a gem for purchase. After this consideration, the colors seen and the pattern of the colors will also influence value. Generally, opal with red fire is the most valued because opal that shows red will also show other colors when rolled back and forth: it contains the whole spectrum. The pattern of the play of color also influences value. Generally large flashes and broad patterns are more rare and valuable than small pinfire patterns. When purchasing a fine opal, the play of color shown when in moderate lighting should be evaluated. Most opals are shown in very bright light where they are at their best. One should also consider the play of color in more normal lighting situations. Most opal varieties are usually cut in smooth-domed cabochon shapes so nothing distracts from the play of color. Fire opal is usually faceted, to add sparkle to the juicy color.
Fire opal breaks the standard rules for opal. Body color is normally only a backdrop for the main attraction - the opal's play of color. But the body color of fire opal is hard to ignore: hot yellows, oranges, and reds so bright they look as though they might glow in the dark. Fire opal sometimes does have play of color but it does not need this to take a starring role in jewelry. Fire opal has become much more popular in the last few years as more jewelry designers have grown to appreciate its bold presence and bright color. Because it is light as well as bright, fire opal is especially good for earrings, where even small sizes have a big punch of color.
Though Nevada opal may have the highest water content, all opal contains a significant water content, and can crack if abused. As a result, opal should be protected from heat and prolonged exposure to strong light, which could dry it out. All opal is relatively soft and should be in a protective mounting if set in a ring. Opal rings should not be worn in situations involving heavy household or outdoor work where the opal may be hit or scratched. Be especially careful with the points of marquise and pear shapes. The hardness of opal ranges from 5.5 to 6.5 on the Mohs scale. It should be protected from heat and strong light, which can dry it out, causing cracks. Ultrasonic cleaners, metal polish, acids, and any strong solvents should be avoided. Opal is best set in a protected mounting.
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