One of the most interesting geode deposits in the United States is in Juab County, Utah. Between 32,000 and 14,000 years ago, Lake Bonneville covered much of what is now western Utah. Wave action along the shores of the lake eroded geode-bearing rhyolite flows. The wave action, along with weathering, liberated the geodes from the…

The Geological and Natural History Survey of Wisconsin reports numerous occurrences of geodes, Lake Superior agate nodules, and thundereggs within the state. These have been in Ashland, Chippewa, Clark, Crawford, Douglas, Dunn, Grant, La Crosse, Milwaukee, Pepin, Pierce, St. Croix, Sheboygan, Trempealeau, and Washburn Counties.

Parts of the Fort Payne and Warsaw-Salem Formations of Kentucky contain large numbers of geodes. These have weathered out of their host rock units and are now found in stream valleys. Other areas where numerous geodes are found in Kentucky stream valleys include the Green River in the south-central part of the state and along…

The Lake Superior agate is a fortification agate that fills cavities in basalt flows that formed over a billion years ago in the Lake Superior region. Over time, silica-rich groundwaters filled these cavities with agate and crystalline quartz. Most of them have been completely infilled and are more properly called a “nodule.” However, some still…

One of the best-known occurrences of geodes in the world is an area surrounding the community of Keokuk, Iowa. It is located near the three-state intersection of Illinois, Iowa and Missouri, with geodes of this locality being found in all three states. The geodes formed in the limestones and dolomites of the Mississippian-age Warsaw Formation.…

A few areas in the United States are well-known for their geodes and geode-like objects. Geodes are so popular in a few states that they have achieved the status of “official state rock” or “official state gem.” The Iowa State General Assembly designated the “quartz geode” as the official “state rock” in 1967. The Oregon…

Geodes, nodules, vugs, concretions and thundereggs are all sites in the earth where substances dissolved in subsurface waters precipitate to form crystals or rounded objects. These objects share many common features and form by similar processes. They all also produce objects that attract attention and stimulate debate. These objects are often confused with one another,…

As with most popular or valuable objects, fake “geodes” have been manufactured by people and offered for sale as naturally-formed objects. If you are a collector paying serious money for a spectacular geode, you need to know enough about geodes and the mineral materials that occur in them to spot a fake. Expert gemologists, mineral…

Everyone expects to find bright, shiny, geometric crystals when they open a geode. But, sometimes there are other interesting landscapes inside. One of the most common finds when geometric crystals are not present is a geode lined with chalcedony, a microcrystalline variety of quartz. Chalcedony crystals as so small that they cannot be seen with…

Small, thin-walled geodes are often sold in kits by department, education, science, tourist and novelty stores. The vendor product descriptions encourage teachers, parents and students to purchase the kits and break the geodes open with a hammer. These geode kits are extremely popular. If you do a Google search for “geode kit” you will find…