KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Crystal geode. Cut and polished section of a geode. A geode forms when layers of the mineral silica (silicon dioxide) precipitate from liquid onto the inside of hollows in rock. The layers vary from crystalline to non-crystalline depending on the condition. The colours are caused by impurities in the silica.
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