The Keokuk geode beds

Where Iowa, Illinois and Missouri meet, the Warsaw Formation weathers out geodes packed with quartz, calcite, pyrite and sometimes kaolinite, and Keokuk geodes are known to collectors worldwide. They erode out of the creek banks whole, look like plain grey cannonballs, and split open to crystal. Geode State Park protects a stretch of it; collecting there is not allowed, but pay-to-dig farms nearby will let you fill a bucket.