Geode

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.

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.

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah).

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.

Quartz -filled geode, shown from inside (top) and outside (bottom)
Reddish chalcedony geodes
Very large amethyst and regular quartz geodes