Opal is yet one more form of silica, but in opal the silica molecules have to be bound to some water molecules. The water content can vary from as little as 6 percent to as much as 21 percent.
Opal is neither a crystal nor a glass but a third form of solid matter called a gel. It forms on or just below the surface of the earth in relatively cool and damp conditions. One variety can even form inside the joints of bamboo stems (Tabasheer). Quite frequently, high water content opal will crack as it dries out, and it can also lose its color. Soaking in water can for a time restore the color to such unstable opal.
Opal is graded principally by its stability and the play of color that it shows. The cheapest material shows no color and is opaque white or barely translucent. This is called common opal. Common opal is sold under a variety of trade names depending on the interesting intrusions of other material into the opal. There is also a green common opal (rare), and common opal from Peru is known to exist naturally in a range of colors.
Where the opal shows a single orange-red color, it is offered as Fire Opal. This comes mainly from Mexico and is significantly higher priced than common opal. The price rises as the transparency of the material improves. Fire opal should be treated with care since it cracks easily.
The best grades of opal show strong play of color patches against a milky, black, or colorless background. Typically, the play of color will show a mix of some or all of the colors in the spectrum. This type of opal is sometimes called precious opal. For the play of color to form, nanometer-scale silica spheres have to form and be held in the gel matrix as regimented as are atoms in a crustal matrix. But the silica spheres are large enough to interfere with white light passing through, splitting the white light up into the different colors of the spectrum. This process is called diffraction and is unique to opal.
A range of treatments are used to improve the physical stability and body color in opal. Synthetic precious opal is also available in the market as are a range of imitations, mainly based on glass or plastic.
The main source of precious opal is Australia, but there is now also significant production from Ethiopia.
Image courtesy of irocks.com.
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