Other rocks considered for official status were limestone and fossil coral. In Latin, the word geode means earthlike. Geodes are shaped like the earth and average about four inches in diameter. Geodes are found in limestone formations and have a hard outer shell. When carefully broken open, a sparkling lining of mineral crystals, most often quartz and calcite, is revealed.
Geologists attribute the crystal growth to the percolation of groundwater in the geologic past.
Southeastern Iowa is one of the state's best Geode collecting areas. Geode State Park in Henry County is named for the occurrence of the geode.