But, before we go any further, we want anyone who finds a really nice geode to know that museum-quality geodes are sold in the collector market for prices that will astound the average person. The Arkenstone, a business that specializes in museum-quality mineral specimens, usually has a few nice geodes for sale. At the time…

An unopened geode has the appearance of an uninteresting rock. They become much more interesting when they are opened and their internal crystals and agate bands become visible. And even though every crystal-lined geode is a wonder of nature, there are many things that can be done to make it into a more marketable product…

Geodes are given a variety of names. The word “geode” is often preceded by the name of the mineral material which has filled the geode. “Agate geode” and “amethyst geode” are examples. The word “geode” might also be preceded by a geographic or stratigraphic name. “Keokuk geode” and “Brazilian geode” are examples.

Geodes in sedimentary rocks are usually found in limestones, dolomites, and calcareous shale. In these deposits a gas-filled void can serve as the opening for geode formation. Shells, tree branches, roots and other organic materials often decay away to leave a void for the formation of mineral materials. These cavities can be filled with quartz, opal, agate or…

The most widely known and sought-after geodes are those that formed in areas of volcanic activity. Voids in basaltic lava flows often are infilled with agate, quartz, opal and other material delivered by hydrothermal water or groundwater. Some voids are spaces occupied by gases that failed to escape the lava flow before its surface crusted over. Where…

Geodes are not found randomly here and there. Instead they are usually found in large numbers in areas where the rocks have formed in a special geochemical environment. Most geodes localities are in A) stratified volcanic deposits such as basalts and tuffs; or B) stratified sedimentary carbonate deposits such as limestones and dolomites. A diversity of other environments yield a small…

Most geologists enjoy geodes. However, the general public has a love affair with them. They are delighted and amazed that an uninteresting rock can contain a beautiful cluster of gemmy crystals, or a colorful lining of banded agate, or both of those in the same cavity. Broken open, or sawn and polished, people who have…

Geodes are spherical to subspherical rock structures with an internal cavity lined with mineral materials. They have a durable outer wall that is more resistant to weathering than the surrounding bedrock. This allows the geode to survive intact when the surrounding bedrock weathers away. The mineral lining the cavity is often a scintillating druse of tiny quartz crystals underlain by multiple…