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Onyx
A chalcedony that ranges in color from a rich black to green, blue, banded (sardonyx) and the natural, or brownish form.
Opal
A hydrated silicon oxide in the quartz family.
Although it has no crystal structure, opal does possess a structure nonetheless. Random chains of silicon and oxygen are packed into extraordinarily tiny spheres. which are irregular in size and inconsistent in concentration. Yet in precious opal, there are many organized pockets of the spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres. This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it. The multicolored flashes of light that opal emits gives it a truly beautiful and valuable look.
Not all opal is precious however. Common opal lacks opalescence, color or luster (e.g., Peruvian opal, African opal).
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