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Opal is a variety of the mineral quartz. It is widely used as a gemstone. The chemical formula is SiO2·nH2O, hydrated quartz. Opal has different colors which produces a beautiful brilliance of color under the influence of light. The colors are caused by the structure of the opal. Opal can be formed by diagenesis in sediments. For instance fossil wood can be converted into opal. Opal also occurs in volcanic rocks.

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