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Individual shapes and minerals vary but your item will resemble the pictured example.
About 6 to 8 million years ago, during the Miocene epoch, volcanic activity in western Utah deposited an igneous rock called rhyolite. Trapped gasses formed cavities within the rock, which, over millions of years, was filled with minerals by circulating ground water, resulting in spherical, crystal lined geodes like these. Until about 15,000 years ago, a large body of water known as Lake Bonneville covered most of western Utah. The movement of lakewater eroded geode bearing rhyolite and redeposited the geodes several miles away, in the Dugway geode bed where they can be found today. Most geodes are hollow, though some are completely filled with quartz (the most common mineral in the geodes).
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