Scroll to the bottom of the page to learn the complete history of the Keokuk Geode.

What Is A Geode?

The term geode is derived from the Greek word Geoides which means "earthlike." A geode is a sphere or oval shaped rock which contains a hollow cavity lined with crystals. Most geodes have a hollow are partially hollow center with quartz crystals and various mineral formations inside. Some geodes are completely filled with small compact inward growing crystal formations such as quartz, agate, jasper or chalcedony and are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.
Geodes begin as bubbles in volcanic rock, animal burrows, balls of mud or tree roots in sedimentary rock. Over time, the outer shell hardens and water containing silica precipitation forms on the inside walls of the hollow cavity within the geode. The silica precipitation can contain many varieties of dissolved minerals, the most common being quartz, but calcite, amethyst and many others are also found.

How did Keokuk Geodes form?

During the Paleozoic Era, much of the Midwest portion of North America was covered by a vast sea. The later half of the Paleozoic Era called Mississippian Period (360 to 320 million years ago) was the last time that Iowa was covered by carbonate producing seas.
In this Mississippian Period, the Keokuk, Iowa area lay almost directly under the equator. The climate is thought to have been tropical, hot and arid. Different types of rock sediment in the area suggest fluctuations in the sea level over long periods of geologic time. Sediments containing small fossils and quartz sand represent periods of shallow water, while sediment absent of fossils and quartz represent periods of relatively deep water. It was in the sediment of these deep sea bottoms that the concretions (mineral masses which are caused by replacing rock sediment) formed which were much later to become the Keokuk Geodes.

Large invertebrate shell life inhabited these seas and probably some of these creatures burrowed into the sea floor to escape predators. Some of these invertebrates died in their safe burrows. As with all decaying organic matter, the decomposition of these creatures by bacteria released chemicals which caused the water to become very alkaline. Calcite, a mineral found in limestone and marble, is attracted to these areas of alkalinity. Balls of calcite began forming around the decaying matter and concretions began to grow. Later silica replaced the calcite and began to form the chalcedony (a crystalline variety of silicon dioxide) shell which is found on all Keokuk Geodes. Minerals were deposited into the hollow core by flowing water which passed through tiny fractures in the chalcedony shell. Quartz is the most common and most attractive of minerals found in Keokuk Geodes but many others also formed during these early formations such as, kaolinite, calcite, pyrite, sphalerite. During the latter stages of the Paleozoic Era, the Pennsylvanian Period, fluctuations in the sea level and pressure allowed other minerals, barite, dolomite, aragonite, marcasite, geothite and chalcopyrite to begin forming crystals. Over a period of millions of years, layers of silica cool, forming crystals of different minerals within the cavity. Different types of silica cool at varying temperatures, thus creating layers of different types of mineral crystals.

 

These fabulous specimens containing true gem quality crystals
can only be found in the tiny 35 mile radius of Keokuk Iowa.