Editing Geode (section)

== 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>