Here are some links to online information and photos about geodes:

This Illinois State Geological Survey page has lots of information about geodes found in the state of Illinois:

These two websites have lots of photos and information about the geodes found near the Mississippi River in Illinois and Iowa (which are often called “Keokuk Geodes”):

This Indiana State Museum page has information about geodes found in Indiana:

This web page has information and photos of geodes from all over the world (including Dugway, Utah):