Dugway Geodes

Approximately 100 miles west of where we live in Riverton, Utah, out in the Utah desert lie the Dugway geode beds. Six to eight million years ago volcanic activity occurred in western Utah and deposited an extrusive igneous rock called rhyolite. Trapped gasses formed cavities within the rhyolite, and millions of years of ground-water circulation allowed minerals to precipitate into the cavities. The resulting geodes have roughly spherical shapes and (hopefully) crystal-lined cavities. For a period of as long as 20,000 years ago starting 30-35 thousand years ago, an inland body of water known as Lake Bonneville covered most of western Utah. The lake's wave activity eroded the geode-bearing rhyolite and redeposited the geodes several miles away in the Dugway geode bed area as lake sediments.

The geodes I’ve found are in very interesting white clay which is veined with purple strips (look closely at the second picture). Many geodes are hollow while others are completely filled with massive, banded quartz or, worse yet, turn out to be mostly rheolite. The most common mineral deposited inside the geodes is quartz in various colors: clear (rock crystal), purple (amethyst), and pink (rose). Dugway is particularly known for blue colored quartz.

It's kind of hard to figure out what you've got when you dig them up because they're all covered with clay. And often you break them either when you are digging or when you are trying to get them out as they often have seams in them that split, or weak sides, etc. But every once in a while you get a good one like the one shown in the third picture that is relatively large. The most typical ones are more in the range of two to six inches. Once you find them, it's like Forest Gump's box of chocolates: you never know what you are going to get until you cut them open. Sometimes its a pleasant surprise and other times it's "oh well".