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WHAT IS AN OPAL?
Opal is a mineral composed of Silica with a water content SiO2.nH2O. The water content of opal can range from 3% to 20 %.
Opal is believed to form as a result of precipitation from low temperature silica rich water solutions. The opal forming silica precipitates in an amorphous mass of closely packed "micro-spheres" which are generally random in size and randomly oriented.
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COMMON OPAL Incident light passing through this material emanates from the amorphous mass as random oriented white light. This is common opal which has little value except when it occurs in a clear (transparent) form which is often used for cutting "facetted" gemstones (particularly Mexican opal). |
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PRECIOUS OPAL Occasionally, opal formation conditions are such that the "micro-spheres" of silica precipitate in uniform sizes and orderly orientation. When this happens the white incident light which enters the opal is diffracted by the orderly spheres into the colors of the rainbow as it emanates from the amorphous opal mass.This is precious opal. |
The play of color is therefore an optical rather than a chemical property. This play of color is the sole distinguishing feature between common opal and precious opal.
What is an Opal? | Opal Value | The Six Cuts | Okanagan Opal