Geode
Crystalline fillings of a rock cavity
Rocks containing fractures or larger spaces such as caves usually allow the flow of ground water. A large cavity partially or completely filled with minerals is called a geode. As ground water flows from place to place, it dissolves chemicals in rock and deposits them elsewhere. Silica—SiO2 (quartz in its crystalline form) — abundant and slightly soluble in water, is frequently the mineral deposited in geodes. Deposition occurs from the outside inward, causing a banded appearance. Banded silica is called agate.
The specimen in this exhibit is from the collection of the late Dr. Kenneth Ohme of Mitchell, Nebraska.