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What is opal?

Posted by Miklos Brezanszky on

What is opal?

Opal is a true precious stone which occurs in many varied forms. Opal is amorphous silica with a water content varying from one to twenty percent, depending on the porosity and degree of hydration.
Different black opals from Lightning Ridge, Australia. Also showing some opal rings.
Precious opal usually contains from six to ten percent water.
The chemical formula for opal is SiO2·nH2O
What causes the play of color and different patterns in opal?
In the mid 1960's a group of scientists at the Commonwealth Scientific and Industrial Research Organization (Australia) solved this mystery and put an end to all previous theories.
By using a newly developed electron microscope which magnified 30,000 times they found opal consisted of tiny spheres of silica ranging in size from 0.00005 mm to 0.00004 mm.
By comparing common opal (colorless potch) to precious opal they discovered that precious opal was composed of very constant sized spheres in a tightly packed uniform pattern.
In common potch the spheres are jumbled together in no particular pattern and are of different sizes.

Color is produced by light entering the opal and being broken up into colors by the spheres and spaces between them.
It follows that the space between the spheres must he uniform since the spheres are uniform.
This light phenomenon is called diffraction.
The size or the spheres determines the color one sees. Large spheres (0.00003 mm) (equals one millionth of an inch) produce reds. Blue and greens are produced by medium size spheres. Small spheres produce violet. There is usually not one constant pattern over the whole stone. There are breaks in the size of the spheres or the orientation of the lines of spheres.
These breaks in the alignment of the spheres account for the different patterns of opal.

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