What the species is and where it came from
A geode is a geological secondary formation within sedimentary and volcanic rocks. It is a hollow, vaguely spherical rock in which masses of mineral matter, which may include crystals, are secluded. The word comes from the Ancient Greek geodes, meaning earthlike.
Geodes are found where the geology suits. Many commercially available ones come from Brazil, Uruguay, Namibia and Mexico, and large amethyst-lined geodes are a feature of the basalts of the Paraná and Etendeka traps in Brazil and Uruguay. They are common in parts of the United States, mainly Indiana, Iowa, Missouri, western Illinois, Kentucky and Utah. In the Mendip Hills in Somerset, England, they are abundant and known locally as potato stones.
How it works, in terms someone would actually use
A geode can form in any cavity, though the term is usually kept for more or less rounded formations in igneous and sedimentary rocks. In igneous rock the cavity can be a gas bubble, such as a vesicle in basaltic lava. In the American Midwest it is a rounded cavity in sedimentary rock. Once the rock around the cavity has hardened, dissolved silicates or carbonates are deposited on the inside surface.
Mineral matter reaches the cavity from groundwater or hydrothermal solutions, and the slow feed lets crystals form inside the hollow chamber. A geode can also begin when a concretion dissolves and the space is partly filled by the same or another mineral precipitated from water. Bedrock containing geodes eventually weathers and decomposes, which leaves them at the surface if they are made of resistant material such as quartz.
What it gets right
The hues inside a geode depend on the minerals and trace elements present while the crystals formed. Iron oxide and cobalt can give reds, yellows or purples, titanium can give blues, chromium and nickel may give greens, and manganese can give pink tones. Many geodes contain banded agate or chalcedony, where changes in the chemistry of the solution over time produce layers of colour.
What it does not
A geode is hollow. If a rock is solid inside, it is classed as a nodule or thunderegg. Colour on the market is also not always natural: some geodes are artificially dyed, often in bright colours such as electric blues or pinks that would not form naturally.
What it changed
Some large crystal-lined geodes are geoheritage sites. Crystal cave is an informal term for any large crystal-lined geode, and it is also used for specific geoheritage locations, among them Crystal Cave in Ohio, found in 1887 at the Heineman Winery on Put-In-Bay, the Cave of the Crystals in Mexico, and the Pulpi Geode in Spain.
A group of mineralogists found the Pulpi cave in 1999 in an abandoned silver mine, Mina Rica, near Pulpi in the province of Almeria. It is filled with giant selenite crystals, measures 8.0 by 1.8 by 1.7 metres, and was then the largest crystal cave found. Its entrance was blocked with five tons of rock, with police posted to deter looters. In summer 2019 it opened as a tourist attraction, with small groups visiting with a guide.
Who it is for, and who it is not
It is for anyone who wants to see crystals grown in a rock cavity. It is not for buyers who want certainty about colour, since dyed specimens are sold.
Is it worth your time
Yes. A geode holds slow mineral growth and colour chemistry in one rock you can hold, and the Pulpi Geode can be visited with a guide.
