Keokuk Geodes have long been objects of curiosity, their sparkling interiors containing some of the most beautiful crystals to be found anywhere in the Midwest. Although Geodes are known from many localities around the world, one of the most productive and famous collecting regions is encompassed within a 35-mile radius of Keokuk, Iowa. Rock collectors commonly refer to geodes from this region as "Keokuk Geodes." In keeping with the world-renowned status of the Iowa Geodes, the Iowa General Assembly declared the Geodes as the official "State Rock" in 1967.
Geodes from the Keokuk area contain a variety of minerals, but Quartz is dominant in most. Quartz is silicon dioxide, the primary mineral in ordinary sand. Beautiful transparent to white Quartz crystals cover the walls of many Geode cavities. These crystals become larger and fewer in number towards the center of the Geode, and terminate in characteristic pointed hexagonal pyramid shapes. Micro-crystalline Quartz, or Chalcedony, whose component crystals are too small to be seen with the naked eye, forms the outer shell in all "Keokuk Geodes." Chalcedony layers also encrust the interior walls of many Geode cavities, covering the surfaces of the earlier-generation Quartz crystals in a variety of colors, including white, gray, blue, yellow and orange. Calcite is a common and attractive calcium carbonate mineral in many Geodes, which occurs in a variety of crystal habits and colors. An additional 17 minerals have been identified in "Keokuk Geodes.
Iowa's renowned "Keokuk Geodes" can be found in specific stream drainages and excavations in parts of southeastern Iowa, especially in Lee, Henry, and Van Buren Counties, including the area near Geode State Park. Most Geodes are derived from strata of the lower Warsaw Formation, a widespread rock unit of Mississippian age. Muds deposited in a shallow sea about 340 million years ago were primarily calcium carbonate and clay, and were subsequently lithified to form the shales, shaley dolomites, and limestones that we see today. Collecting Geodes can be both fun and educational. Once you've located exposures of lower Warsaw strata or a Geode-bearing stream course, all that's required is a little patience and a good bricklayer's or rock hammer. A sharp blow with a hammer is usually sufficient to crack open individual Geodes, exposing their crystalline interiors to daylight for the first time. However as a Warning, remember that most Geode-collecting localities are now on private land, and written permission must be secured before entering and collecting the Geodes.