Geode
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robot Adding: [[eu:Geoda]]
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[[Image:geode_angle_300x267.jpg|thumb|Geode, halved and polished]]
[[Image:Geode_rings_300x182.jpg|thumb|Geode, halved and polished]]
[[Image:Geodes in unusual rock formation.jpg|thumb|Geodes in unusual rock formation]]
[[Image:Large_Geode_Carefree_AZ.jpg|thumb|Large hollow geode at the Carefree Resort in [[Carefree,_Arizona]]. Booklet shown for scale is 8.5 by 11 inches.]]
'''Geodes''' ([[Greek language|Greek]] ''geoides'', "earthlike") are [[Geology|geological]] [[Rock (geology)|rock]] formations which occur in [[Sedimentary rocks|sedimentary]] and certain [[volcanic]] rocks. Geodes are essentially rock cavities or [[vug]]s with internal [[crystal]] formations or concentric banding. The exterior 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 [[nodule (geology)|nodules]].
Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks, while the more general term ''"vug"'' is applied to cavities in fissures and veins. They can form in gas bubbles in [[igneous rock]]s, such as vesicles in basaltic [[lavas]], or as in the American [[Midwest]], rounded cavities in sedimentary formations. After rock surrounding the cavity hardens, dissolved [[silicate]]s and/or [[carbonate]]s 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.
The size of the crystals, including their form and shade of color, vary—making each geode unique. Some contain clear, pure quartz crystals, and others have rich purple [[amethyst]] crystals. Still others can have [[agate]], chalcedony, or [[jasper]] or minerals 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 have a more restricted variety of interior mineralization.
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 roughly the size of a minivan was discovered in [[Put-in-Bay, Ohio]] in the early 20th century. It is known as [[Crystal Cave (Ohio)|Crystal Cave]], and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a [[List of Iowa state parks|Geode State Park]].
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.
==See also==
*[[concretion|Septarian nodule]]
*[[Lithophysa]]
*[[Thunderegg]]
==Sources==
* ''Rocks and Minerals'', written by Fredrick H. Pough, ISBN 0-395-91096-X
==External links==
{{Commons|Geode}}
*[http://www.rocksforkids.com/R&M/geodes.html Geode images, locations and discussion]
*[http://www.amonline.net.au/factsheets/geodes.htm Australian Museum Fact sheet]
*[http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm Utah Geode Beds]
*{{cite news | first= | last=Illinois State Geological Survey | coauthors= | title=Geodes—Small Treasure Vaults in Illinois | date= | publisher= | url =http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf | work = | pages = | accessdate = | language = }}
[[Category:Rocks]]
[[Category:Mineralogy]]
[[Category:Petrology]]
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