Geodes are crystal-lined cavities in rocks. The crystals are formed when minerals precipitate (settle) out of groundwater in underground rock cavities.
The minerals deposited may vary, but in Kansas most geodes consist of quartz, chalcedony (a type of quartz), and calcite. They commonly form in limestone and can be found in many places in the Flint Hills region. In particular, many good quartz geodes have been reported from Chase County. They also have been found in Wallace, Logan, and Trego counties in western Kansas.
Kansas Rocks and Minerals, Kansas Geological Survey Educational Series 2.