Geodes have been a mystery though out the ages. The word "geode" is derived from a Latin word meaning "earthlike”, a round spherical shape. Geodes are formed with an inward growth of crystals on the walls and are not wholly filled. Geodes that are completely filled are referred to as “nodules” and are no longer considered a geode. Hollow geodes are lighter than ones that are completely filled. The most sought after geodes are hollow ones and sometimes they will even rattle with loose crystals inside. Come and discover the hidden treasures of the Keokuk Geodes!

     325-360 million years ago, the Mississippian time period, mud was deposited into shallow seas in Iowa, Illinois, and Missouri areas. The mud was calcium carbonate and clay that turned into shale and limestone. This is host rock that geodes are found in. It is weaker than the outer shell of the geode. Geodes are more resistant to weathering than the host rock that surrounds them. This means geodes will eventually “fall out” of the host rock by natural weathering.

Recent research has established three general points on how geodes are formed:

1) Geode precursors were concretions (nodules formed by outward growth around some nucleus) which grew within soft, unlithified sediment.

2) The outer shells of these concretions were replaced subsequently by chalcedony.

3) The interiors of the concretions were dissolved, leaving a hollow space into which quartz crystals could grow. The composition of the original concretions is unclear, though geologists propose they were either limestone or anhydrite, a fairly soluble calcium sulfate mineral related to gypsum. (Iowa Geology 1987 No. 12; Iowa Department of Natural Resources)

     Many people wonder how the crystals got inside the geodes. Some of the minerals were trapped inside when forming. Also over time mineral rich waters seeped into the geodes through the outer shell. As the solutions precipitate it forms the crystals inside from the minerals that were left. The most common mineral in geodes is quartz. Secondary minerals are also common to find inside. There are up to 22 different minerals found and probably more.

     The geode is the Official State Rock of Iowa. Outcrops are located in a 35 mile radius of Keokuk, Iowa. The Lower Warsaw Formation is where geodes are commonly found. The Upper Warsaw and Upper Keokuk Formation also contain some geodes. Fossils from the sea that once stood in the area are also found. Some times fossils are found on the outer shell of a geode or a fossil itself can be a geode!

     A collector can mine geodes out of the host rock with a chisel and rock hammer. A geo-paleo pick works great to break up the shale layer to find the hidden geodes inside. A person can also walk along outcrop areas and pick up the geodes that have “weathered or eroded out”. Many outcrops are located on private land so permission must be obtained from the land owners first. Although there are a few pay-fee mines located around the area.

     Geodes can be broke open with a rock hammer or cut with a rock saw. The only problem with cutting a geode is that it can destroy any secondary minerals inside. Cut and polished geodes make very beautiful specimens for displaying. A soil pipe cutter is also a very nice tool that can be used to break hollow geodes open. It makes a loud “pop” when they open and you get two matching halves more often. The cutter makes great flat breaks with out damaging much of the secondary minerals inside.

Geodes must be opened to see what’s inside, that’s the mystery!