A geode is a hollow and somewhat spherical rock. The inside of a geode is commonly infilled with concentric or radial crystals. The crystals occur when the inside of the rock is permeated by mineral-rich solutions.
Geodes occur within sedimentary and volcanic or igneous rocks. The minerals within the geode were deposited from hydrothermal fluids. Geodes can form in gas bubbles in igneous rocks. When the rock around the bubble hardens the cavity is formed, then minerals such as silicates and carbonates and deposited on the inside surface.
Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.
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, or celestite.
There is no easy way of telling what the inside of a geode will look like until it is cut open or broken apart. However, geodes from a particular area are usually similar in appearance.
Geodes are common in some rock formations in the United States, mainly in the Midwest. They also are common in Mexico, Brazil, and Namibia.