The word geode is derived from the Greek word Geoides which means
"earthlike." A geode is a sphere shaped rock which contains a hollow
cavity lined with crystals. Geodes are created in the hollow areas of
soil such as animal burrows or tree roots. They are also formed in
the bubbles in volcanic rock. Over time, dissolved minerals seep into
a hollow area and harden into an outer shell creating the geode. The
minerals continue to form on the inside walls of the shell, growing
towards the center. The most common dissolved mineral is quartz, but
amethyst and other minerals are also found. A geode which is completely
filled with crystals is called a nodule. Agate-filled nodules are
called thunder eggs. Geodes are found throughout the world, and many
are concentrated in deserts. Volcanic areas are a common place to find
geodes. In the United States they can be found in California, Arizona,
Utah, Nevada, Illinois, Missouri, Kentucky, and Iowa. In fact, the geode
is Iowa's state rock. Geodes can form in any cavity that is buried. These
can be gas bubbles in igneous rocks, pockets under tree roots, vesicles
in lava after a volcanic eruption, or even in animal burrows. Over time,
the external wall of the cavity hardens, and dissolved silicates and/or
carbonates are deposited on the inside surface. Over time, this slow feed
of mineral constituents from groundwater or hydrothermal solutions allows
crystals to form inside the hollow chamber. Then the geode makes its way
back to the surface through normal geologic processes. Geodes are common
in some formations in Indiana, Iowa, Missouri, Kentucky and Utah (in the
United States). They also are common in Brazil, Namibia and Mexico. A
large geode roughly the size of a minivan was discovered in Put-in-Bay,
Ohio in the early 20th century. It is known as Crystal Cave, and tours are
possible during the summer. A thunderegg is a type of rock similar to a
geode but formed in a rhyolitic lava flow and found only in areas of
volcanic activity. Thundereggs are rough spheres, most about as big as a
baseball. They look uninteresting on the outside, but slicing them in half
may reveal highly attractive patterns and colors valuable in jewelry.
According to native legend, the Thunder Spirits lived at the highest
reaches of Mount Hood and Mount Jefferson, and when they became angry
at each other, they threw these spherical rocks at each other. It is
the state rock of Oregon. (from Wikipedia, the free encyclopedia)  The
hostile gods obtained these weapons by stealing eggs from the
Thunderbirds' nests, thus the source of the name "Thundereggs."