Editing Geode

{{Short description|Hollow formation inside a rock}}
{{other uses}}
{{distinguish|Geoid}}
[[File:Geode inside outside.jpg|thumb|[[Quartz]]-filled geode, shown from inside (top) and outside (bottom)]]

A '''geode''' ({{IPAc-en|ˈ|dʒ|iː|.|oʊ|d}}; {{etymology|grc|{{lang|grc|γεώδης|italic=no}} ({{grc-transl|γεώδης}})|earthlike}}) is a [[geology|geological]] secondary formation within [[sedimentary rock|sedimentary]] and [[volcanic rock]]s. Geodes are hollow, vaguely spherical rocks, in which masses of [[mineral]] matter (which may include crystals) are secluded. The [[crystal]]s are formed by the filling of [[vesicular texture|vesicles]] in volcanic and [[subvolcanic rock]]s by minerals deposited from [[hydrothermal fluid]]s; or by the [[solvation|dissolution]] of syngenetic concretions and partial filling by the same or other minerals [[precipitation (chemistry)|precipitated]] from water, [[groundwater]], or [[hydrothermal]] fluids.

== Formation ==
Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in [[igneous]] and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in [[basalt]]ic [[lava]]; or, as in the American [[Midwest]], in rounded cavities in sedimentary formations. After rock around 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.<ref name=":0">{{Cite web|url=https://geology.com/articles/geodes/|title=Geodes: The rocks with a crystal surprise inside!|website=geology.com|access-date=2017-12-30}}</ref>

== Coloration ==
[[File:Chalcedon Maroko Sidi Rahal.JPG|thumb|left|300px|Reddish [[chalcedony]] geodes]]{{Needs expansion|date=May 2025}}
Geodes and geode slices are sometimes dyed with artificial colors.<ref>{{cite web | url = https://geology.com/articles/geodes/ | title = Geodes | publisher = geology.com }}</ref>

== Occurrence ==
Geodes are found where the geology is suitable with many of the commercially available ones coming from [[Brazil]], [[Uruguay]], [[Namibia]], and [[Mexico]].<ref name=":0" /> Large, [[amethyst]]-lined geodes are a feature of the basalts of the [[Paraná and Etendeka traps]] found in Brazil and Uruguay. Geodes are common in some formations in the United States (mainly in [[Indiana]], [[Iowa]], [[Missouri]], western [[Illinois]], [[Kentucky]], and [[Utah]]). Geodes are also abundant in the [[Mendip Hills]] in [[Somerset]], [[England]], where they are known locally as "potato stones".<ref>{{Cite web|url=http://www.dulcote.com/the-quarries.html|title=The Quarries|website=Dulcote, Somerset|access-date=2017-12-30}}</ref> The term geode generally describes hollow formations. If the rock is completely solid inside, this would be classified as a [[Nodule_(geology)|nodule]] or [[thunderegg]].<ref>{{Cite web |date= |title=Definition of Geode |url=https://www.mindat.org/glossary/geode |archive-url= |access-date=2023-05-22 |website=Mindat}}</ref>

== Crystal caves==
[[File:Amethyst and drusy quartz cathedrals in Crystal Mountain Museum.jpg|thumb|Very large [[amethyst]] and regular quartz geodes]]
'Crystal cave' is both an informal term for any large crystal-lined geode and also used for specific [[geoheritage]] locations such as the [[Crystal Cave (Ohio)]], discovered in 1887 at the Heineman Winery on Put-In-Bay, Ohio, the [[Cave of the Crystals]] (Mexico), and the [[Pulpi Geode]], discovered in 1999 in Spain. In 1999, a mineralogist group discovered a cave filled with giant [[Selenite (mineral)|selenite]] (gypsum) crystals in an abandoned silver mine, Mina Rica, near Pulpi, [[Province of Almeria]], [[Spain]]. The cavity, which measured {{convert|8.0|x|1.8|x|1.7|m}}, was, at the time, the largest crystal cave ever found. Following its discovery, the entrance to the cave was blocked by five tons of rock, with an additional police presence to prevent looters. In the summer of 2019 the cave, a significant [[geotourism]] resource and now named the 'Geoda de Pulpi', [[Pulpi Geode]], was opened as a tourist attraction, allowing small groups (max. 12 people) to visit the caves with a tour guide.<ref>{{Cite web|url=https://www.geodapulpi.es/Servicios/cmsdipro/index.nsf/informacion.xsp?p=Geoda&documentId=6964084B70F2F7E5C1258432003E19F5|title=Geoda de Pulpi|access-date=1 September 2022}}</ref>

==See also==
{{Portal|Minerals}}
*[[Bristol Diamonds]]
*[[Coso artifact]]
*[[Lithophysa]]
*[[Septarian nodule]]
*[[Thunderegg]]

==References==
{{Reflist}}

==Further reading==
*Pough, Fredrick H. ''Rocks and Minerals'', {{ISBN|0-395-91096-X}}
*Middleton, Gerard V. (2003). ''Encyclopedia of Sediments and Sedimentary Rocks''. Springer, {{ISBN|978-1-4020-0872-6}}, p.&nbsp;221 ({{Google books|r_2Ia8wOBlgC|restricted online copy|page=221}})
*Keller, Walter David (1961). ''The Common Rocks and Minerals of Missouri''. University of Missouri Press, {{ISBN|978-0-8262-0585-8}}, S. 67 ({{Google books|haCztne87E4C|restricted online copy|page=67}})
*Witzke, Brian J. [https://web.archive.org/web/20100202195106/http://www.igsb.uiowa.edu/browse/geodes/geodes.htm ''Geodes: A Look at Iowa's State Rock'']. Iowa Geological Survey
*[https://www.uky.edu/KGS/rocksmineral/rm-rocks-geodes.php ''Geodes''] Kentucky Geological Survey (University of Kentucky)

==External links==
{{Commons category|Geodes}}
*[http://www.americangeode.com Indiana geode specimens, facts and stories]
*[http://www.americangeode.com/cracking.php Video of a geode cracking using industrial soil pipe cutter]
*[http://www.amonline.net.au/factsheets/geodes.htm Australian Museum Fact sheet]
*[https://web.archive.org/web/20060430070906/http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm Utah Geode Beds]
*{{cite news|last=Illinois State Geological Survey |title=Geodes – Small Treasure Vaults in Illinois |url=http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070927100407/http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf |archive-date=2007-09-27 }}

[[Category:Rocks]]
[[Category:Mineralogy]]
[[Category:Petrology]]