My Blog

Thundereggs are not geodes, but they are so similar that they deserve at least one locality mention in this article. The state of Oregon is the most famous thunderegg locality in the world. Thundereggs are found in rhyolite and tuff deposits in many parts of the state. In 1965 the Oregon Legislative Assembly issued a…

Some unusual geodes and nodules found at the Inspiration Mine in Gila County, Arizona are lined with gem silica, a rare, beautiful and valuable form of blue chalcedony. Some have been found with gem silica stalactites!

Woodbury geodes occur in the area around Woodbury, Tennessee. They originated in the limestones and dolostones of the Warsaw Formation and can be seen where these rock units are outcropping. Liberated geodes are found in residual soils above the rock units in which they formed, and in the sediments of the valleys that drain these…

In south-central Indiana geodes can often be seen in exposures of the Harrodsburg Limestone and Ramp Creek Formations. The Indiana Geological Survey reports that geodes are abundant along streams and scattered on the ground for several miles on either side of their outcrop areas.

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…