Term for a glass-like effusive rock (volcanic glass, obsidian) that is formed by chemical and physical processes. Devitrification occurs when water enters along the cracks and microscopic crystals of quartz, cristobalite and feldspar form from the amorphous glass. In the advanced stage, a loose rock of bluish green, brownish or grey colour is formed. In winemaking, it is used as a filter aid during filtration. It is also used in nurseries for the cultivation of vines. See also under soil type.
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