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.
Why Are Keokuk Geodes Special?
Keokuk Geodes are famous for their sparkling interiors containing some of the most
beautiful crystals and minerals in the entire world. Quartz is the dominant mineral
found in Keokuk Geodes and shimmering white to transparent quartz crystals fill the
walls of many of these geode cavities but 20 additional minerals have also been identified.
Keokuk Geodes can be found in museums and are sought after by collectors worldwide.
Nowhere else on the planet can geodes like these be found. Iowa named the geode their
state rock in 1967. The prime hunting location for geodes is in a 35-
These fabulous specimens containing true gem quality crystals
can only be found in the tiny 35 mile radius of Keokuk Iowa.
