The term geode is derived from the Greek ,meaning the shape of the Earth. This is actually a rather accurate geometrical description since geodes are typically oblate spheroids ranging from a few millimeters to over tens of meters in diameter. Most geodes are easy to identify in the field, because their spherical shape tends to set them apart from any of the surrounding rocks.

In addition to being spherical, geodes are defined as a rock cavity lined with crystalline material. More importantly however, geodes must possess discrete outer layers of mineral deposition which, upon weathering, preserve the geodes as distinct entities. Geodes may be either hollow or completely filled with crystallized mineral material deposited from saturated solutions over millions, and in some instances hundreds of millions of years. In many cases it is possible to recognize numerous layers of growth within individual geodes.

In general, crystal growth proceeds inward toward the center of the cavity, thus the most recent crystallization is that nearest the center. More often the process of crystallization has gone to completion, and the cavity is filled with interlocking crystals - giving rise to heavier, solid geodes. In rare occurrences however, the cavity maybe hollow and partially filled with a complex intergrowth of large and small crystals.

Geodes typically form in one of two types of environment, late stage magmatic (volcanic) systems or in active sedimentary environments. In the magmatic systems geodes are found associated with volcanic landforms. The super-saturated fluids are typically silica rich and tend to deposit various forms of quartz. Geodes formed in sedimentary environments may be more diverse in both shape and mineral content. The watery solutions are cool and are either marine in composition or are carbonate-rich groundwater.

Typical minerals found in geodes include quartz and chalcedony, calcite, selenite, marcasite and pyrite, barite, sphalerite, strontianite, pyrolusite, goethite and kaohnite. You will find on display in this exhibit examples of geodes from both volcanic and sedimentary environments. See if you can find examples that contain each of the above minerals. Geodes are unique examples of nature, and like fingerprints, no two are exactly alike.