Geodes are rock formations found in sedimentary or volcanic rock all over the world. A geode looks like a nondescript spherical rock from the outside -- a bit lumpy and ugly -- but inside it contains mineral deposits or crystals. Geologist call the outer layer of rock, usually limestone, the rind. Hollow geodes may have quartz crystals inside them. Sometimes the mineral deposits completely fill the inside; this kind of formation is a nodule.

Some people call geodes thunder eggs and they are found in many areas of the United States. In the Midwest, they are commonly found in stream beds, and in the West they can be found in dry valleys and deserts that are volcanic ash beds.

The state rock of Iowa is the geode. In southeastern Iowa, near the town of Keokuk, is Geode State Park. The area within a 70 mile radius of the junction of the Des Moines River and the Mississippi River contains some of the most varied geodes anywhere. The crystal geodes from Iowa are sought after by collectors, and they are in museums around the world. Keokuk has an annual Geode Fest, which is a great opportunity for new or experienced collectors to meet and rock hunt. You'll find plenty of geodes, from pea-size to more than a foot in diameter.

The limestone areas of south-central Indiana south of Bloomington are rich with geodes. Pick up geodes around the Monroe Reservoir, or go stream hunting along Bear Creek near the town of Trevlac. Some streams run through private property, and permission to hunt might be necessary. The 200,000-plus acre Hoosier National Forest is in this area, and public rivers and streams that run there are also good places to find geodes.

Kentucky is home to great geode sites. The Fort Payne and Warsaw-Salem Formations in east-central Kentucky provide good hunting along the creek beds. The Green River area in south-central Kentucky also has good findings, and it is known for large geodes measuring two feet in diameter.

Outstanding geodes have been found at Dugway Geode Beds in Juab County, Utah. These are volcanic geodes, which have igneous rhyolite rinds rather than the sedimentary limestone or dolomite of the Midwest. Drive to the site, select a spot and dig. You will reach a layer of clay, and the geodes are buried in the clay, usually one to four feet deep. A few mining claims are in this area; check so you won't trespass.

California has several geode sites. The area near Blythe is known for the Hauser Geode Beds. These geodes are found in the desert, and take along plenty of food and water and drive a 4-wheel vehicle if you want to drive closer to the dig. Another geode spot close to Blythe is known as Potato Patch Thunder Eggs because the geodes here are about the size of potatoes. They are often found scattered about on the surface, and you only need to pick them up.