A rounded, hollow rock formation with crystal-lined interior cavity, often with quartz or other minerals coating the inside walls.
From Greek geōides ('earth-like'), from gē ('earth') + eidos ('form' or 'appearance'). The term entered European languages via Latin in the 18th century as geological science developed.
Geodes are geological time machines—the crystals inside grew slowly over millions of years, layer by layer, so the size of a geode tells you roughly how long mineral-rich water seeped through it.
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