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From Wikipedia... a site I love to hate: "Geodes (Greek γεώδης - geōdēs, "earthlike") are geological rock formations which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs with internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Other geodes are completely filled with crystal, being solid all the way through. These types of geodes are called nodules.

Formation

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks, while the more general term '"vug"' is applied to cavities in fissures and veins. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as in the American Midwest, 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. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

Coloration

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. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart. However, geodes from any one locality usually have a more restricted variety of interior mineralization.

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.

Prevalence

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico. A large geode was discovered in Put-in-Bay, Ohio in the early 20th century. It is known as Crystal Cave, and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a Geode State Park.

Old school Geologist call them “Vugs” and the material pictured in Cages home is typically found in Brazil. In that size it could be as cheap as $2.00 per pound but the values increase if it has large, clean and deep colored crystals. Most vugs this size don’t survive mining and moving without breaking into pieces which are then glued back together. When I was a teen, my father used to buy 20 and 40 ton containers of Amethyst vugs from Brazil and I would spend months cleaning and grading them. I bet Cage paid way too much for that monster and it ends up outside as part of the landscaping. The color also fades in daylight so that’s not a good move either.