Geodes are hollow rock masses that are lined with crystals that have grown toward the center of the cavity. Geodes are usually roughly spherical in shape, up to 12
in (30 cm) or more in diameter. Most frequently, the crystals growing within a geode are quartz, calcite, or fluorite, though occurrences of other minerals are found. These objects are prized by collectors for their well-formed crystals and outstanding beauty.
Geodes are typically characterized by an outer shell of chalcedony, a dense microcrystalline form of quartz. The hard outer shell of the geode can usually be separated from the enclosing rock material. Geodes are most often formed in limestone or volcanic rocks, though they are rarely found in mudstones. The mineralogy of the geode and the rock in which it formed are commonly different. Because chalcedony is often harder and more weather resistant than the host rock, the hollow spheres resist the effects of erosion and are left behind as the host rock is eroded. Collectors identify the geodes in the field based upon their shape, the characteristic chalcedony shell, and the lower density arising from the hollow center.
The initial requirement for the formation of a geode is the presence of a cavity in the host rock. In a volcanic rock, such voids are frequently a result of the release of gases from the molten lava. As the lava hardens, the gas bubbles are preserved as holes in the rock. In the cases of sedimentary rocks, such as limestone, a hole may develop as groundwater dissolves the rock itself or as a result of the decay of biologic material buried at the time of deposition of the sediments. Groundwater within the sediments then carries dissolved minerals, including silica, through the host rock and into the cavity. The chalcedony shell is formed first, only fully hardening after an extended period of time. Subsequent mineralized groundwater flow may then deposit additional layers of minerals within the void. These crystal growths form first on the walls of the shell and then grow toward the center, producing the distinctive crystalline interior of the geode. The formation of a geode with large crystals may require tens or hundreds of millions of years.
Some of the most spectacular geodes come from Brazil. Some of these may be as large as one meter in diameter and contain very large amethyst (purple quartz) crystals. Once found, the geodes are often cut open with a diamond-tipped saw blade and the cut surfaces of the sphere polished for display.