What is Opal?
Gem quality opal is one of the most spectacular gemstones. A single stone can flash every color of the spectrum with an intensity and quality of color that can surpass the "fire" of diamond. The best opals can command prices per carat that rival the most expensive diamonds, rubies and emeralds. They are very popular gems.
Opal is a wonderful stone for earrings, pendants, brooches and rings. However, it is softer than most other gemstones. Opal has a hardness of about 5.5 to 6.0 on the Mohs hardness scale. Because of that opal works best in earrings, brooches and other pieces that rarely encounter skuffs and impacts. When used in a ring the best designs have a bezel that protects the stone - instead of being placed in a prong setting that allows the edges of the stone to be exposed.
Opal is a hydrous silicon dioxide (SiO2.nH2O). It is amorphous, without a crystalline structure, and without a definite chemical composition. Therefore it is a "mineraloid" rather than a "mineral".
Play-of-Color and Opalescence
Opal is a very common material that is found throughout the world. Most of this opal is "common opal" or "potch" which has a milky or pearly luster known as "opalescence". However, rare specimens of opal produce brilliant color flashes when turned in the light. These color flashes are known as a "play-of-color". Opal specimens that exhibit a play-of-color are known as "precious opal." If the play-of-color is of high quality and large enough to cut, the material can be used to produce valuable gemstones.
A play-of-color in opal can be observed under three situations: 1) when the stone is moved, 2) when the light source is moved, or, 3) when the angle of observation is changed. The video at right illustrates "play-of-color" in an Ethiopian Welo opal.
What Causes "Play-of-Color"?
Areas within an opal that produce a play-of-color are made up of microscopic spheres of silica arranged in an orderly network. As the light passes through the array spheres it is diffracted into the colors of the spectrum. The size of the spheres and their geometric packing determine the color and quality of diffracted light.
Sources of Opal
Although opal is found throughout the world, almost all of that opal is common opal of very little value. Most of the precious opal deposits that have been discovered are in Australia. The mines of Australia produce at least 90% of the world's precious opal.
Famous mining areas in Australia include: Coober Pedy, Mintabie, Andamooka, Lightning Ridge, Yowah, Koroit, Jundah and Quilpie. Other countries that produce precious and fancy varieties of common opal include: United States, Mexico, Hungary, Indonesia, Brazil, Peru, Honduras, Guatemala, Nicaragua, Slovakia, Czech Republic and Ethiopia.
http://geology.com/gemstones/opal/#harlequin
