What Is a Geode?

Geodes form in two main geological settings: volcanic (in gas bubbles trapped in lava flows) and sedimentary (in cavities left by dissolved fossils or buried organic matter). When silica-rich groundwater percolates into these cavities, dissolved minerals slowly precipitate on the walls over thousands to millions of years, growing crystal layers inward. The most common crystals are varieties of quartz: clear quartz, smoky quartz, amethyst (purple, coloured by iron impurities and radiation), and chalcedony (microcrystalline quartz that often forms the outer banding). Some geodes contain calcite, celestite, or even rare minerals. The exterior of a geode is typically nondescript — a rough, rounded rock indistinguishable from ordinary stones — making the discovery of their crystal-filled interiors a thrilling surprise. The world largest known geode is the Pulpi Geode in Spain: a 10.7-metre-long cave lined with metre-long transparent selenite (gypsum) crystals.