geodes

Geodes are spherical to oblong rocks that contain hollow cavities lined with crystals. They typically appear dull and solid on the outside, but when opened, they reveal crystalline interiors.

Geodes form when mineral-rich groundwater fills cavities in rocks such as volcanic vesicles or sedimentary concretions. Over time, minerals precipitate from the solution and crystallize on the cavity walls, creating a sparkling lining that can take the form of quartz varieties or calcite.

Interior crystals are commonly quartz (including amethyst and citrine varieties) and calcite; other minerals such as celestite, agate, opal, or gypsum may be present. The crystals grow inward from the walls, sometimes creating a hollow center.

Geodes range in size from a few centimeters to more than a meter in diameter. Their exteriors are typically rough and unremarkable rock, while the interior can be a dramatic crystalline chamber.

Geodes occur worldwide, most often in volcanic rocks like basalt or rhyolite, or in sedimentary basins where gas pockets or cavities form. Notable localities include the Keokuk region of Iowa, United States, and geode-rich deposits in Brazil and Uruguay; geodes are also found in parts of Mexico, Spain, Portugal, and Germany.

People collect geodes for lapidary use, education, and decorative displays. They are cut open and sometimes polished to reveal the crystal-lined interior. A hollow interior is the distinguishing feature; rounded nodules without hollow interiors are generally called concretions rather than geodes.