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Roger Weller, geology instructor

Shawn Engle

Physical Geology
Spring 2005


            Geodes are somewhat of a mystery.  Not only is it impossible for one to know exactly what’s on the inside but also the formation of geodes is still scientifically unconfirmed.  There are many offered explanations that most accept as possibilities and are tailored to the type of geode found and its location.   Due to the estimated amount of time it takes for geodes to form, the unpredictability of numerous geological variables and physical forces that nature has at its disposal, simulating the same conditions in order to produce a geode is seemingly impossible to accomplish in one lifetime.  However, each geode offers a unique glimpse of the spontaneous beauty nature can generate with the earth’s minerals. 

            In order to attempt to formulate a hypothesis of how geodes are formed, an assessment of their basic physical properties would be a good start.  Geodes are spherical hollow rocks containing crystalline structures that vary in chemical compound, size, color, density and internal shape.


Picture is property of Shawn Engle for inquires e-mail to


But as in nature there are differences and exceptions even between similarities.   The color of a geode is determined by the impurities the crystals contain, and the geode sizes range from an inch to several feet.  As majorities of geodes live up to there linguistic Latin roots as being “earth like” or spherical the geodes found in Brazil are elongated.


Photo property of ZEE’S Minerals used with permission from Colin Cople.


Quartz is most common mineral found in geodes but numerous other types of minerals can fill these cavities.  Some of the most sought after geodes boast hexagon shaped crystals that line the interior walls of the hollowed center, ranging from a centimeter in length to several feet depending on the internal shell size and the amount of time it remained undisturbed.  If these types of geodes are allowed to continue their growth the hollow center will eventually be entirely filled with quartz or agate.  Other geodes get completely filled with fine-grained minerals that don’t form a hexagon shape.  These form a single solid or banded crystalline structure.


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Geodes that contain crystalline structures have a round hard covering consisting of extremely fine-grained quartz or silica called chalcedony.  With out this layer the vulnerable crystallized minerals wouldn’t be able to withstand earth’s weathering process.  Yet in some locations there are other minerals that rank high enough on “MOH’s scale of hardness” to resist aspects of weathering and suffice as a protective shell allowing crystals to grow inside.  In some cases the original sediment or igneous material that hasn’t weathered away adheres to this layer of hardened silica.  This is due to cementation and/or compaction.  The pre-existing sediment/country rock becomes harder than its surroundings and weathers out with the geode then eventually weathers’ away entirely.  Because this material is clinging to the harder shell it will emulate the symmetrical shape making it easy to identify.    

The proposed process of how geodes are created is divided into two categories depending on whether they lie in sediment or igneous rock beds.  Geodes that are found in sediment such as dolomite, calcite and limestone are assumed to form from either hollow’s made by roots, decomposed animals, abandoned animal burros and or concretions.  Another suggestion involves cavities that are washed out by ground water later dry out and harden when the water table drops.  These hollows remain full with mineral rich water or get filled again if the water table rises.  This water then with the constant drying out then re-depositing of mineral water forms the crystalline structures over a very long period of time.  Another way these crystals form is through a leaching process.  As the mineral rich ground water permeates the hollow, it begins to form the chalcedony shell through silica deposits.  After continual permeation the remaining deposits eventually form hexagons shaped crystals and entirely fill the hollow if allowed to grow.  On occasion some geodes that are completely filled have an inner layer of agate surrounded by the hexagon crystals.  This is thought caused by a silica gel that got through the chalcedony and later dried.  

If the geodes are found in igneous rock beds the hollows are usually explained as being expanded gas bubbles, volcanic bombs or igneous mud that contained steam and shrank as it dried. Lithophysae or thundereggs as they are commonly called are a type of geode found in igneous rock and have kind of star shaped mineral fillings when cut. .

Pictures are property of Shawn Engle for inquires e-mail to


One theory of how the crystals are formed is silica rich water interacts with cooling lava creates hexagon crystals or  solid chalcedony or agate.  

Pictures are property of Shawn Engle for inquires e-mail to




Pictures are property of Shawn Engle for inquires e-mail to


Other theories most agree on are consistent with the deposition of silica gel and fluctuating water table.  Permeation is also an accepted explanation of crystal formations.

Geodes are found in specific areas throughout the continents of the Western Hemisphere and parts of Australia. It’s almost certain where one is found there is more to be discovered close by.  This is because these specific types of environment geodes grow in produce multiple specimens.  Those areas are called geode fields and though geodes differ in physical traits, depending on the fields’ underground mineral content similarities between neighboring geodes do occur.  For instance one might collect a few that have small quartz crystals you cant see through then walk twenty five yards and gather others that have larger, clearer crystals.  Although the crystals are different in size and translucency they are still made of quartz.  

In Mexico a dominantly mined geode is the “coconut geode” or “chyoas” which contain a plethora of less prominent secondary minerals and mainly consist of quartz.


Pictures are property of Shawn Engle for inquires e-mail to


Yet the color of quartz varies from crystal clear, to amethyst (purple quartz).  In Brazil geode fields are mined for their inner contents of agate while other mining locations contain large (Expensive) geodes filled with some of the best specimens of amethyst.


Photo property of ZEE’S Minerals used with permission from Colin Cople.


A particularly famous location celebrated for its abundance and diverse geodes is Keokuk, Iowa.  Keokuk boasts more than 300 rock formations in a 100-mile radius around the city where geodes can be found and around 21 different minerals that fill them.   One of the largest geodes is located in Maine and named crystal cave.  Fields are also located in Oregon, Iowa, New Mexico, Mississippi, and many other states. 

            If one goes in search of geodes and finds a few the next step is cutting them open.  Some use chisels and a hammer while others use a wet saw.  From personal experience a wet saw is preferable and the type of cutting depends on the type of geode in possession.   The best way to decide on the angle to cut is cross your fingers and hope it yields the best result.  If the types of geode are yielding hollow cavities lined with crystals I would recommend tumbling the geode around the blade until a holes are made around most of the chalcedony shell.  Then get a screwdriver or chisel and pop it open, if it doesn’t open then continue to tumble and cut at the shell eventually it’ll pop open.  However if the geode is most likely entirely filled, chose an angle and drive it straight through the saw.  After the cut polishing is an option but not necessary because when moisture is applied it looks polished.

Despite the fact that most people agree with the proposed formation processes, there are still specimens and factors that defy these suggested explanations.  For instance the giant geode in Maine that one can literally walk into or why western continents and Australia are virtually the only place geodes have formed.  If there is volcanic activity and depositing of sediments across the seas from the United States why aren’t these same natural conditions creating geodes.  Until proof of the actual process is found we can only speculate and admire the beauty of these fascinating treasures.


                                                      References[bot3/geobit3.html , accessed April 15 2005. , accessed April 15 2005. , accessed April 15 2005. , accessed April 15 2005. , accessed April 15 2005. , accessed April 25 2005.

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