A polished cross-section of a geode. It is composed of agate, a form of chalcedony that has variegated bands or zones. Chalcedony is an opaque microcrystalline variety of the mineral quartz (silicon oxide). Any colours it might have arise from the presence of various impurities. The multi-coloured concentric bands, visible here, represent successive layers of precipitation onto the interior surface of a rounded cavity within rock.
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