Geodes are geological rock formations which occur in sedimentary and certain volcanic rocks. They are essentially rock cavities or vugs with internal Crystal formations or concentric banding. The exterior is generally Limestone while the interior contains Quartz Crystals and/or Chalcedony deposits. If the Geode is completely filled with Crystal, being solid all the way through, it is then called a nodule.
Keokuk Geodes are found in the outcropping of the Mississippian lower Keokuk bed within an 70 mile radius of the intersection of the Mississippi and De Moines Rivers where the greatest abundance and variety are found. 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.