Characteristics of Geodes

Exterior

• While most pieces of rock are irregularly shaped, rock hounds know to look for the roughly spherical shape of a typical geode. The hard outer shell of chalcedony, a type of crypto-crystalline quartz, may be fairly smooth or it may be covered with lumpy masses resembling cauliflower. Either way, this hard shell remains intact as the softer rock surrounding it weathers away, leaving the geode visible at the earth's surface. The smallest geodes are about an inch in diameter, though most fall in a range of 2 to 6 inches.

Interior Hollows

• From the outside a geode often looks like nothing more than a round rock. However, when it is cracked or sawed open the interior of this geologic wonder sets it apart. At some point in a geode's formation the interior becomes hollow. Geologists have yet to agree on how this happens, and there may be different processes at work in different locations. Common theories suggest that in sedimentary rock the hollow may be left after organic matter decomposes, while geodes in volcanic rock might form around gas bubbles or clumps of hot mud that shrink as they cool.

Crystals

• Over time, mineral-rich water seeps through a geode's outer shell into the hollow. When temperature and pressure are right the minerals slowly build into crystals that may eventually fill the hollow space. The type, color and purity of geode crystals depends on the mineral content of the surrounding rock, so most geodes found in a particular area will have the same type of crystals. Calcite crystals are the most common, with quartz crystals also appearing frequently---including highly prized purple amethyst crystals.

Geode Locations

• Geodes tend to be found in fields, or areas where they are common, because the set of conditions that create one geode usually occurs over a wide area creating many more geodes. Volcanic geodes appear in locations where ancient eruptions of lava and steam took place, such as the deserts of Southern California and Utah. On the other hand, sedimentary geodes formed in carbonate-rich soil that became limestone or dolomite, including the Keokuk geodes found in Iowa.