Indiana University
Indiana University Bloomington: Geo Notes: Geodes: One of Nature's Mysteries
Explains what geodes are, how they form, and where they can be found in Indiana.
Indiana University
Indiana University: Reference Documents: Geodes: One of Nature's Mysteries [Pdf]
A fact card describing the properties and characteristics of geodes, where they can be found, and how they may have formed.
Curated OER
National Park Service: Agate Fossil Beds: Geologic Formations
This National Park Service site has information about these fossil beds in Nebraska and where the name Agate came from.