Geode is a Latin word which loosely translates to “earthlike”. The geode specimens are near perfect spheres which normally range from 2 inches to 6 inches in diameter and in some cases up to 2 feet in diameter. Inside the geodes are crystal linings that tend to grow inward. There are two types of geodes, sedimentary geodes which form within sedimentary rock formations and the Volcanic Geodes that form within Igneous or volcanic rocks.
The geodes formation requires presence of cavity in the rocks. In the case of the volcanic geode it starts out as a gas bubble within the volcanic/igneous rock. With the case of sedimentary rocks especially where limestone is found a rounded cavity would suffice and the process that follows is equal for both the sedimentary and volcanic geodes.
The next step is the hardening of the rock cavity walls, followed by flow of mineral (mainly carbonates and silicates) content by way of mineral saturated groundwater. The deposition of these minerals leads to crystal formation inside the cavity around the cavity hardens as minerals namely carbonates and silicates form a layer on top of earlier layers going inward. The necessary conditions for crystal growth are temperature, pressure changes and evaporation. When these conditions are combined in a perfect nexus, they allow for mineral matter to precipitate and leave solid crystal formations inside these cavities. The geode’s outer shell is a crystal quartz lining called chalcedony. This shell is much harder than the average rock and it is the reason why the geode remains intact even after the surrounding bedrock weathers away.
The colors of the geodes may vary cream, tan, purple, blue to colorless hues. The colors are mostly dependent on the impurities that were soaked and deposited into the cavity using groundwater medium during the formation process. There is no way of determining what color of crystals are inside the globes unless the geodes are split open. In some cases the crystals can be dyed in order to bring out the desired effect. The process of a geode formation is a long drawn up process and usually takes anywhere from 200 to 250 million years!
Around the US, geodes are prevalent in the Midwest and lower Midwestern states. Internationally they have been discovered in Brazil, Namibia, Australia, Mexico just but to name a few areas.
Interesting tidbit: Geode is the official designated state rock of Iowa in 1967. There are wide geode deposits found in Keokuk Iowa. In fact the geode found there is referred to as the Keokuk Geode.
Sources:
http://www.usgs.gov
http://australianmuseum.net.au/Concretions-Thunder-Eggs-and-Geodes/
http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf
http://www.igsb.uiowa.edu/browse/geodes/geodes.htm
