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When volcanic activity occurred in what is now western Utah some 8 million years ago, deposits of the igneous rock rhyolite formed. As the rock cooled, pockets of volcanic gases became trapped and formed cavities. The circulation of groundwater allowed silica and other minerals to precipitate into these cavities and form layers of agate and crystal quartz over the course of millions of years. Today the result is a beautiful and unique variety of geodes known as Dugway Geodes, composed of banded agate and crystal quartz captured in rhyolite. They are found in western Juab Co., UT.
