The Geode was designated the official rock of Iowa in 1967 to promote tourism in the state. Iowa is known worldwide because of the large number of rare and beautiful geodes found in the state. The Latin word "geodes" means earthlike, and geodes are shaped like the earth with a hard outer shell that when cut reveals a core of sparkling mineral crystals (usually quartz and calcite).
Geode bridge in Mount Pleasant, Iowa. Photo by Joe Naylor on Flickr (use permitted with attribution). All Geological Symbols.
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. The geode is also the state rock of Oregon (thunder-egg)
Photo of uncracked geodes (from Keokuk, Iowa) by Ryan Thompson on Flickr (noncommercial use permitted with attribution / share alike). All State Rocks.