# Geode > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Geode > Markdown URL: https://mediated.wiki/source/Geode.md > Source: https://en.wikipedia.org/wiki/Geode > Source revision: 1342170855 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Hollow formation inside a rock For other uses, see [Geode (disambiguation)](/source/Geode_(disambiguation)). Not to be confused with [Geoid](/source/Geoid). [Quartz](/source/Quartz)-filled geode, shown from inside (top) and outside (bottom) A **geode** ([/ˈdʒiː.oʊd/](https://en.wikipedia.org/wiki/Help:IPA/English); from [Ancient Greek](/source/Ancient_Greek_language) *γεώδης (*geṓdēs*)* 'earthlike') is a [geological](/source/Geology) secondary formation within [sedimentary](/source/Sedimentary_rock) and [volcanic rocks](/source/Volcanic_rock). Geodes are hollow, vaguely spherical rocks, in which masses of [mineral](/source/Mineral) matter (which may include crystals) are secluded. The [crystals](/source/Crystal) are formed by the filling of [vesicles](/source/Vesicular_texture) in volcanic and [subvolcanic rocks](/source/Subvolcanic_rock) by minerals deposited from [hydrothermal fluids](/source/Hydrothermal_fluid); or by the [dissolution](/source/Solvation) of syngenetic concretions and partial filling by the same or other minerals [precipitated](/source/Precipitation_(chemistry)) from water, [groundwater](/source/Groundwater), or [hydrothermal](/source/Hydrothermal) fluids. ## Formation Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in [igneous](/source/Igneous) and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in [basaltic](/source/Basalt) [lava](/source/Lava); or, as in the American [Midwest](/source/Midwest), in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved [silicates](/source/Silicate) and/or [carbonates](/source/Carbonate) are deposited on the inside surface. Over time, this slow feed of mineral constituents from [groundwater](/source/Groundwater) or hydrothermal solutions allows crystals to form inside the hollow chamber. [Bedrock](/source/Bedrock) containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.[1] ## Coloration Reddish [chalcedony](/source/Chalcedony) geodes Geodes and geode slices are sometimes dyed with artificial colors.[2] 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.[3][4] 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.[5][4] 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.[4][5] ## Occurrence Geodes are found where the geology is suitable with many of the commercially available ones coming from [Brazil](/source/Brazil), [Uruguay](/source/Uruguay), [Namibia](/source/Namibia), and [Mexico](/source/Mexico).[1] Large, [amethyst](/source/Amethyst)-lined geodes are a feature of the basalts of the [Paraná and Etendeka traps](/source/Paran%C3%A1_and_Etendeka_traps) found in Brazil and Uruguay. Geodes are common in some formations in the United States (mainly in [Indiana](/source/Indiana), [Iowa](/source/Iowa), [Missouri](/source/Missouri), western [Illinois](/source/Illinois), [Kentucky](/source/Kentucky), and [Utah](/source/Utah)). Geodes are also abundant in the [Mendip Hills](/source/Mendip_Hills) in [Somerset](/source/Somerset), [England](/source/England), where they are known locally as "potato stones".[6] The term geode generally describes hollow formations. If the rock is completely solid inside, this would be classified as a [nodule](/source/Nodule_(geology)) or [thunderegg](/source/Thunderegg).[7] ## Crystal caves Very large [amethyst](/source/Amethyst) and regular quartz geodes 'Crystal cave' is both an informal term for any large crystal-lined geode and also used for specific [geoheritage](/source/Geoheritage) locations such as the [Crystal Cave (Ohio)](/source/Crystal_Cave_(Ohio)), discovered in 1887 at the Heineman Winery on Put-In-Bay, Ohio, the [Cave of the Crystals](/source/Cave_of_the_Crystals) (Mexico), and the [Pulpi Geode](/source/Pulpi_Geode), discovered in 1999 in Spain. In 1999, a mineralogist group discovered a cave filled with giant [selenite](/source/Selenite_(mineral)) (gypsum) crystals in an abandoned silver mine, Mina Rica, near [Pulpi](/source/Pulpi), Province of Almeria, Spain. The cavity, which measured 8.0 by 1.8 by 1.7 metres (26.2 ft × 5.9 ft × 5.6 ft), 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](/source/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.[8] ## See also - [Minerals portal](https://en.wikipedia.org/wiki/Portal:Minerals) - [Bristol Diamonds](/source/Bristol_Diamonds) - [Coso artifact](/source/Coso_artifact) - [Lithophysa](/source/Lithophysa) - [Septarian nodule](/source/Septarian_nodule) - [Thunderegg](/source/Thunderegg) ## References 1. ^ [***a***](#cite_ref-:0_1-0) [***b***](#cite_ref-:0_1-1) ["Geodes: The rocks with a crystal surprise inside!"](https://geology.com/articles/geodes/). *geology.com*. Retrieved 2017-12-30. 1. **[^](#cite_ref-2)** ["Geodes"](https://geology.com/articles/geodes/). geology.com. 1. **[^](#cite_ref-3)** ["Guide to Geodes"](https://online.maryville.edu/education-degrees/guide-to-geodes/). *Maryville University Online*. Retrieved 2026-01-25. 1. ^ [***a***](#cite_ref-:1_4-0) [***b***](#cite_ref-:1_4-1) [***c***](#cite_ref-:1_4-2) Schott, Sachiko (2018-03-13). ["What Colors Are Geodes Naturally?"](https://www.sciencing.com/colors-geodes-naturally-7456633/). *Sciencing*. Retrieved 2026-01-25. 1. ^ [***a***](#cite_ref-:2_5-0) [***b***](#cite_ref-:2_5-1) Andrei, Mihai (2024-03-29). ["Geodes: Nature's Hidden Treasures"](https://www.zmescience.com/feature-post/natural-sciences/geology-and-paleontology/rocks-and-minerals/geodes/). *ZME Science*. Retrieved 2026-01-25. 1. **[^](#cite_ref-6)** ["The Quarries"](http://www.dulcote.com/the-quarries.html). *Dulcote, Somerset*. Retrieved 2017-12-30. 1. **[^](#cite_ref-7)** ["Definition of Geode"](https://www.mindat.org/glossary/geode). *Mindat*. Retrieved 2023-05-22. 1. **[^](#cite_ref-8)** ["Geoda de Pulpi"](https://www.geodapulpi.es/Servicios/cmsdipro/index.nsf/informacion.xsp?p=Geoda&documentId=6964084B70F2F7E5C1258432003E19F5). Retrieved 1 September 2022. ## Further reading - Pough, Fredrick H. *Rocks and Minerals*, [ISBN](/source/ISBN_(identifier)) [0-395-91096-X](https://en.wikipedia.org/wiki/Special:BookSources/0-395-91096-X) - Middleton, Gerard V. (2003). *Encyclopedia of Sediments and Sedimentary Rocks*. Springer, [ISBN](/source/ISBN_(identifier)) [978-1-4020-0872-6](https://en.wikipedia.org/wiki/Special:BookSources/978-1-4020-0872-6), p. 221 (*[restricted online copy](https://books.google.com/books?id=r_2Ia8wOBlgC&pg=PA221)*, p. 221, at [Google Books](/source/Google_Books)) - Keller, Walter David (1961). *The Common Rocks and Minerals of Missouri*. University of Missouri Press, [ISBN](/source/ISBN_(identifier)) [978-0-8262-0585-8](https://en.wikipedia.org/wiki/Special:BookSources/978-0-8262-0585-8), S. 67 (*[restricted online copy](https://books.google.com/books?id=haCztne87E4C&pg=PA67)*, p. 67, at [Google Books](/source/Google_Books)) - Witzke, Brian J. [*Geodes: A Look at Iowa's State Rock*](https://web.archive.org/web/20100202195106/http://www.igsb.uiowa.edu/browse/geodes/geodes.htm). Iowa Geological Survey - [*Geodes*](https://www.uky.edu/KGS/rocksmineral/rm-rocks-geodes.php) Kentucky Geological Survey (University of Kentucky) ## External links Wikimedia Commons has media related to [Geodes](https://commons.wikimedia.org/wiki/Category:Geodes). - [Indiana geode specimens, facts and stories](http://www.americangeode.com) - [Video of a geode cracking using industrial soil pipe cutter](http://www.americangeode.com/cracking.php) - [Australian Museum Fact sheet](http://www.amonline.net.au/factsheets/geodes.htm) - [Utah Geode Beds](https://web.archive.org/web/20060430070906/http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm) - Illinois State Geological Survey. ["Geodes – Small Treasure Vaults in Illinois"](https://web.archive.org/web/20070927100407/http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf) (PDF). Archived from [the original](http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf) (PDF) on 2007-09-27. --- Adapted from the Wikipedia article [Geode](https://en.wikipedia.org/wiki/Geode) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Geode?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.