{{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 geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter (which may include crystals) are secluded.
The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syngenetic concretions and partial filling by the same or other minerals 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 basaltic lava; or, as in the American Midwest, in rounded cavities 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.<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]]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> The colours found inside geodes result from the minerals and trace elements present when the crystals formed. As mineral-rich water slowly deposits crystals within a cavity, different elements produce different hues: iron oxide and cobalt can yield reds, yellows or purples; titanium can create blues; chromium and nickel may produce greens; and manganese can give pink tones.<ref>{{Cite web |title=Guide to Geodes |url=https://online.maryville.edu/education-degrees/guide-to-geodes/ |access-date=2026-01-25 |website=Maryville University Online |language=en-US}}</ref><ref name=":1">{{Cite web |last=Schott |first=Sachiko |date=2018-03-13 |title=What Colors Are Geodes Naturally? |url=https://www.sciencing.com/colors-geodes-naturally-7456633/ |access-date=2026-01-25 |website=Sciencing |language=en-US}}</ref>
Many geodes contain banded agate or chalcedony, where changes in the chemistry of the solution over time produce the distinctive layers of colour. The type of mineral also affects colour: for example, quartz may appear colourless, white, or purple (as amethyst), while calcite, celestite or other minerals can produce earthy, pastel, or blue tones.<ref name=":2">{{Cite web |last=Andrei |first=Mihai |date=2024-03-29 |title=Geodes: Nature's Hidden Treasures |url=https://www.zmescience.com/feature-post/natural-sciences/geology-and-paleontology/rocks-and-minerals/geodes/ |access-date=2026-01-25 |website=ZME Science |language=en-US}}</ref><ref name=":1" />
Some geodes on the market are artificially dyed, often in bright, unnatural colours such as electric blues or pinks, which would not form naturally. Generally, the intensity and pattern of colour in a geode reflect the chemical composition of the original solution and the conditions of crystal formation.<ref name=":1" /><ref name=":2" />
== 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 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 (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 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. 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