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 rhyolite and deposited them several miles away in lake sediments of what is now known as the Dugway Geode Beds.

Today, lots of people search for Dugway geodes because they are thrilled to find their agate and crystal-lined centers. But, some of the Dugway Geodes contain another hidden surprise – trace amounts of uranium incorporated into their chalcedony lining cause the geode’s interior lining to exhibit a spectacular lime-green fluorescence under ultraviolet light. See accompanying image. They probably get lots of fluorescent mineral collectors interested in geode collecting.