Geodes are egg-shaped stones that have a hollow cavity in the centre. When the hollow cavity is completely filled with crystals, it is called a nodule. Most geodes remain hollow, with crystals pointing in toward the centre. Geodes are usually made up of chalcedony and quartz. In rare cases, the crystals may be jasper or amethyst and are highly prized.
Geodes can form from volcanic or sedimentary rocks. Gases form bubbles in volcanic rocks and get trapped. These bubbles form the hollow centre where the crystals grow. Geodes in sedimentary rocks form around animal burrows, tree roots, or mud concretions. Regardless if it is sedimentary or igneous, hot groundwater passes through the outer shell and into the hollow cavity. The hot water transfers silica to the cavity. This silica cools and forms crystals. The silica cools at different rates, forming layers of crystals on the inside of the geode.
Many of the geode rock beds in the United States are located in desert areas. Geodes can be found throughout the world where there are volcanic ash beds or areas of concentrated limestone. Volcanic ash and limestone contain high concentrations of silica and calcite. The silica compresses to form the hard chalcedony shells that are characteristic of geodes. Geode beds are located in Southern California, Illinois and parts of Australia. Brazil boasts some of the largest and most highly prized amethyst geode beds in the world.
Prospecting for geodes requires some tools. If you go, make sure that you are prepared for the heat of the desert areas. Be prepared with water, a hat and lightweight clothing. You should also have a shovel and a rock hammer. You will also need buckets to carry the geodes. Be prepared with a first aid kit in case of an emergency.
Hauser Geode Beds
The Hauser Geode Beds are located in Southern California in the Imperial Valley, southwest of Blythe. Geodes located there are anywhere from two inches to five inches across. These geodes have formed from volcanic ash beds. Many of the geodes found at the Hauser Geode Beds are lined with black calcite and amethyst.
The Cinnamon Geode Beds are about 16 miles west of Blythe, California. The geodes are larger than what can be found in the Hauser Geode Beds, ranging from two to 10 inches across. The geodes have smaller crystals and many are only partially lined. Many have a beautiful cinnamon brown quartz druse for which the beds were named.