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Two geodes, halved and polished.

Geodes are geological formations common to Indiana, the Keokuk area of Iowa, Kentucky, Brazil and Mexico. Essentially rock bubbles with internal cystal formations, the exterior of a geode is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Geodes work their way up to the surface of the earth crust's after millions of years. Geologists do not yet have a widely accepted theory about how geodes are formed.