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The Learning Kingdom's Cool Fact of the Day for February 25, 1999
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How do opals show rainbow colors?
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Opal is one of the few gemstones that is not a crystal. Although it's
made of silica (silicon dioxide), the same compound as quartz and
agate, the molecules are not arranged in crystalline order.
Opal is made of millions of extremely tiny spheres of silica
molecules, loosely packed together, with water molecules between
them. The water content can be as high as 10% or more.
The colors come from the interaction between light and the silica
spheres. If the spheres are almost the same size, they tend to pack
into semi-regular patterns that diffract light, the same way a
peacock's feather does. Each opal shows its own unique colored
display.
More about opals:
http://www.theimage.com/gemstone/opal/opal.html
http://www.desertusa.com/magfeb98/feb_pap/geo_opal.html
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