Geode, a hollow, rounded mineral body. A typical geode contains an outer layer of chalcedony, or fine-grained quartz, lined on the inside with crystals of quartz, calcite, or some other mineral. Most geodes are found in beds of limestone and measure between 1 inch (2.5 cm) and 12 inches (30 cm) in diameter. Geologists think a geode begins as a cavity in rock formed from ocean sediments. According to this theory, silica trapped within the cavity forms the outer layer of the geode, and the geode's inner crystals form at a later time, when mineral-bearing groundwater seeps into the geode structure.