Spherical rocks with hollow cavities lined with crystals
What Is A Geode?
Geologists have long been challenged to explain how geodes, those mysterious spherical rocks, are formed. Geodes are a variable phenomenon, which means that they may be created in various different ways. The term geode is derived from the Greek word Geoides which means "earthlike."
A geode is a round rock which contains a hollow cavity lined with crystals. Rocks which are completely filled with small compact crystal formations such as agate, jasper or chalcedony are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.
How Are Geodes Created?
Geodes begin as bubbles in volcanic rock or as animal burrows, tree roots or mud balls in sedimentary rock. Over time, the outer shell of the spherical shape hardens, and water containing silica precipitation forms on the inner walls of the hollow cavity within the geode. The silica precipitation can contain any variety of dissolved minerals, the most common being quartz, but amethyst and calcite are also found.
Over a period of thousands of years, layers of silica cool, forming crystals of different minerals within the cavity. Different types of silica cool at varying temperatures, thus creating layers of different types of mineral crystals.
Each geode is unique in its composition which can only be truly discovered when it is cracked open or cut with a rock saw. The size and formation of the crystals and different shades of color within the crystals make each geode special. The rough exterior of the geode gives no indication of the secrets held within its core. The anticipation never fades for those who curiously collect buckets full of round geodes and eagerly expose the secrets of each individual sphere-shaped rock. The most prized contain rare amethyst crystals or black calcite.
Where Can You Find Geodes?
Geodes are found throughout the world, but the most concentrated areas are located in the deserts. Volcanic ash beds, or regions containing limestone, are common geode locations.
There are many easily accessible geode collecting sites in the western United States, including in California, Arizona, Utah and Nevada. The state of Iowa also has geodes, in fact, the geode is their state rock.
The finest concentration of geode sites in Southern California is in Riverside and Imperial counties. The most famous of these sites is called the Hauser Geode Beds, which are located at Wiley Well in the northern region of Imperial Valley, CA. In the same area there are also the North Black Hills Geode Beds and the Cinnamon Geode Beds.
Desert Environment & Geology Index
Things To Do: Rockhounding
Hauser Geode Beds
N. Black Hill Geode Beds
Cinnamon Geode Beds
Potato Patch Thundereggs
The Bradshaw Trail
Opal Hill Mine
Things To Do: Rockhounding
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