A geode is a rock, with a rough and insignificant looking exterior, that is either partially or completely filled with crystal. The geo prefix means earth, and the rough lumps on the outer surface are often spherical, resembling small planetoids.
Petrologists do not really have a clear understanding of how a geode forms. It is theorized that round geodes formed from bubbles in volcanic rock, while the crystals formed later, with the slow seeping of mineral-laden water into the bubble. Irregularly shaped geodes are perhaps formed in cavities created by ancient roots or burrows.
Any number of precipitating crystals can form inside a geode. Smooth crystal such as agate can entirely fill the cavity, and split agate geodes of various colors make attractive bookends for rockhounds.
Perhaps the most beautiful geode is a an amethyst geode, filled with deep purple amethyst crystals all pointing inward. Amethyst geodes are mined in Brazil and Uruguay, with Uruguay producing geodes of the deepest purples.
A number of sites claim to be the home of the world's largest geode; Put-in-Bay, Ohio and Jacob's Cave in Missouri are two locations within the US that claim the honor. A recent discovery in Spain dwarfs these sites, where a huge geode was discovered in 1999 in an abandoned silver mine. Some eight meters long (about 26 feet), the geode is lined with pure white gypsum crystals. Some of the crystal facets extend a meter or more (3 feet) into the hollow interior. It is thought that the crystals may have formed some six million years ago, when the Mediterranean Sea largely evaporated, leaving quantities of saline behind to form the seeds around which the giant crystals could form. The site, accessible only via a lengthy trek through the abandoned mine, is currently open only to scientists and geologists.