6.5" Keokuk Calcite Geode with Iridescent Chalcopyrite - Missouri

Unlike most geodes that form in volcanic rock, Keokuk geodes are found in sedimentary rock. They started out as concretions of mud that formed around organic material about 340 million years ago. The outer shells of these concretions were subsequently replaced by chalcedony and the interiors of the concretions were dissolved, leaving a hollow space into which quartz crystals could grow. Most geodes are 2 to 5 inches wide, though specimens as large as two feet across have been found.

Keokuk geodes contain a variety of minerals, but quartz is dominant in most. Many geodes are filled with clear to white quartz crystals. Micro-crystalline quartz, or chalcedony, whose component crystals are too small to be seen with the naked eye, forms the outer shell. 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 also a common mineral in many geodes, though seventeen other minerals have been identified in Keokuk Geodes, including pyrite and sphalerite.

The area around Keokuk, Iowa is sometimes referred to as “the geode capital of the world". In 1967, these geodes were even named the official state rock of Iowa. Geodes have been collected from the Lower Warsaw Formation within about 100 miles of the city for over 150 years.