MARTIN LAND / SCIENCE PHOTO LIBRARY MARTIN LAND / SCIENCE PHOTO LIBRARY
A polished cross-section of a geode showing bands of agate and purple amethyst crystals growing into its cavity. Agate is a form of chalcedony (an opaque microcrystalline variety of the mineral quartz) that displays variegated bands or zones. 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 rock cavity. The purple colour of the amethyst crystals, which consist of another type of quartz, is caused by iron impurities.
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