Geodes are geological formations common to the United States (Indiana, the Keokuk area of Iowa and 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.
