A geode (; ) is a

geological Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...

secondary formation within

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

and

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

s. Geodes are hollow, vaguely spherical rocks, in which masses of

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

matter (which may include crystals) are secluded. The

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s 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 Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

, or

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

fluids.

Formation

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

; or, as in the American

Midwest The Midwestern United States (also referred to as the Midwest, the Heartland or the American Midwest) is one of the four census regions defined by the United States Census Bureau. It occupies the northern central part of the United States. It ...

, in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

s and/or

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

s are deposited on the inside surface. Over time, this slow feed of mineral constituents from

or hydrothermal solutions allows crystals to form inside the hollow chamber.

Bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

Coloration

Geodes and geode slices are sometimes dyed with artificial colors.

Occurrence

Geodes are found where the geology is suitable with many of the commercially available ones coming from

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

Uruguay Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the A ...

Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...

, and

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

. Large,

amethyst Amethyst is a Violet (color), violet variety of quartz. The name comes from the Koine Greek from - , "not" and (Ancient Greek) / (Modern Greek), "intoxicate", a reference to the belief that the stone protected its owner from Alcohol into ...

-lined geodes are a feature of the basalts of the

Paraná and Etendeka traps The Paraná-Etendeka Large Igneous Province (PE-LIP) (or Paraná and Etendeka Plateau; or Paraná and Etendeka Province) is a large igneous province that includes both the main Paraná traps (in Paraná Basin, a South American geological basin ...

found in Brazil and Uruguay. Geodes are common in some formations in the United States (mainly in

Indiana Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...

Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...

Missouri Missouri (''see #Etymology and pronunciation, pronunciation'') is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it border ...

, western

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. It borders on Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash River, Wabash and Ohio River, Ohio rivers to its ...

Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...

, and

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

). Geodes are also abundant in the

Mendip Hills The Mendip Hills (commonly called the Mendips) is a range of limestone hills to the south of Bristol and Bath, Somerset, Bath in Somerset, England. Running from Weston-super-Mare and the Bristol Channel in the west to the River Frome, Somerset ...

Somerset Somerset ( , ), Archaism, archaically Somersetshire ( , , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by the Bristol Channel, Gloucestershire, and Bristol to the north, Wiltshire to the east ...

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

, where they are known locally as "potato stones". The term geode generally describes hollow formations. If the rock is completely solid inside, this would be classified as a

nodule Nodule may refer to: * Nodule (geology), a small rock or mineral cluster * Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, ...

or thunderegg.

Crystal caves

Amethyst and drusy quartz cathedrals in Crystal Mountain Museum

'Crystal cave' is both an informal term for any large crystal-lined geode and also used for specific

geoheritage Geoheritage (a blend of ''geological'' and ''heritage'') is the geological aspect of natural and cultural heritage. A geosite is a particular geological heritage asset. It is a heritage category comparable to other forms of natural heritage, such ...

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 Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...

. The cavity, which measured , 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 Geotourism is tourism associated with geological attractions and destinations. Geotourism (tourism with a geological base) deals with the abiotic natural and built environments.Sadry, B.N.(2009)''Fundamentals of Geotourism: with special emphasis ...

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.

References

External links

Indiana geode specimens, facts and storiesVideo of a geode cracking using industrial soil pipe cutter
*{{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 Rocks Mineralogy Petrology