Often found in North America, Central America, South America and Namibia, and a common sight in museum gift shops, geodes are rock formations that involve a variety of minerals. At its most basic, geodes are rocks with an interior cavity lined with another mineral.
The name geode comes from the Greek word “geode” meaning “earthlike.” This name is fitting as many geodes are round, like little planets--worlds of light and stone unto themselves capturing fascination wherever they are discovered.
The term geodes generally conjures up images of lumpy rocks cut in half with an interior lining of sparkly crystals or shiny opaque layers, but there are other types of geodes. Other types include logs, vugs and nodules. Log geodes are oblong and can attain great lengths.
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Nodules occur when the geodes' cavity is completely filled with a mineral. These are also known as thunder eggs. Vugs are geodes that are in the veins of rocks rather than contained inside of separate spherical rocks.
Scientists are not completely sure of how geodes form, but they do have some theories. The idea so far is that they form in igneous rock gas bubbles. The rock around the bubble hardens, and minerals (carbonates or silicates or both) gradually deposit on all available surfaces. These dissolved minerals are contained in hydrothermal or groundwater. They can also develop in the spherical hollow places located in sedimentary strata. Common minerals found within geodes include minerals, such as celestite, agate, jasper, amethyst and chalcedony.
Geodes may not always be dry inside. When broken open, water from the time the geodes developed may be found. Due to the variation of mineral solutions, geodes vary in color and content. Quartz crystals that are clear are the most common. Purple amethyst crystals decorate the interior of others. All geodes are unique when it comes to the color and orientation of the inside.
Not all geodes that are sold are naturally colored. Geodes are sometimes cut into slices and artificially dyed.
Iowa, a state known for its geode deposits, has the geode as its state rock (officially designated in 1967), and has a state park called Geode State Park. This celebration of geodes does not stop with Iowa. In Ohio, a large geode called Crystal Cave is open for tours.