Thunder egg is a nodule-like rock, similar to a filled
geode, that is formed within volcanic ash layers.
  credit: Bill The Eggman

Geodes are geological secondary structures which occur in certain sedimentary and volcanic rocks. They are themselves of sedimentary origin formed by chemical precipitation.
Inside and outside views of the geode

Geodes differ from vugs in that the former were formed as early, rounded, structures within the surrounding rock and are often removed intact, whereas vugs are irregularly shaped pockets, voids or cavities within a formation, often along a vein or in breccia. Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that has accreted around the nodule nucleus.
Thunder egg agate geode. from New Mexico. by Bill The Eggman

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 basaltic lavas; or, as in the American Midwest, in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates 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.

Large Vugs of amethyst crystals were everywhere inside the mine
photo: lifeofsaturdays.com

Geode banding and coloration is the result of variable impurities. Iron oxides will impart rust hues to siliceous solutions. Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, or jasper banding or crystals such as calcite, dolomite, celestite, etc. 
Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered. 
Massive Amethyst Geode From Uruguay

Geodes commonly have a chalcedony (cryptocrystalline quartz) shell lined internally by various minerals, often as crystals, particularly calcite, pyrite, kaolinite, sphalerite, millerite, barite, celestite, dolomite, limonite, smithsonite, opal, chalcedony and macrocrystalline quartz, which is by far the most common and abundant mineral found in geodes.

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah). They also are common in Brazil, Uruguay, Namibia, and Mexico. 

Amethyst Geode inside Limestone rocks!

Geodes are found mostly in basaltic lavas and limestones.

The world's largest known crystal cave or vug is Crystal Cave, a celestine geode 35 feet (10.7 m) in diameter at its widest point, located near the village of Put-in-Bay, Ohio, on South Bass Island in Lake Erie. 
Empress of Uruguay

The largest amethyst geode in the world is The ‘Empress of Uruguay’ Over three meters tall (that’s eleven feet) and weighing two and a half tonnes. It is located here in Atherton, North Queensland, Australia. 

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