Geodes (means "earthlike" because the are often round) are rock formations which occur in sedimentary and certain volcanic rocks. Geodes are rock cavities with internal crystal formations. The outside of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Other geodes are completely filled with crystal, being solid all the way through. These types of geodes are called nodules.

Geodes can form in any cavity, but usually they are more or less rounded formations in volcanic and sedimentary rocks, they can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as in the American Midwest, replacing fossils in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, or jasperbanding or crystals such as calcite, dolomite, celestite, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart. However, geodes from any one locality usually simmilar to each other

Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico. A large geode was discovered in Put-in-Bay, Ohio in the early 20th century. It is known as Crystal Cave, and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a Geode State Park.

 

For more information about Geodes try these links.

http://www.rocksforkids.com/R&M/geodes.html

http://www.igsb.uiowa.edu/browse/geodes/geodes.htm

http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm