Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Caitlin McPhillips
Hunting for Thundereggs
From the outside, lithophysae, or thundereggs as they are commonly called, are
odd. Spherical in shape, the thunderegg is often plain colored usually russet to
gray with a knobby, almost bubbly appearance.
However, once you “crack” open the rock, the wonders that can be found inside
never cease to amaze.
From chalcedony and manganese to calcite and opal, the insides of the thunderegg
vary from location to location, but to understand what the thunderegg really is,
you have to understand the process by which it forms.
Scientists do not typically agree on the exact process by which thundereggs form; however, it is generally accepted that the eggs themselves are volcanic bombs ranging in size from little more than a centimeter in diameter to more than a meter in diameter and can weigh anywhere from a small amount of grams to over 1.75 tons.
Thundereggs only occur in rhyolitic lava flows and can be found in varying concentrations and compositions amongst the different layers of deposited volcanics.
The above diagram shows the different layers of volcanics and the different concentrations of thundereggs found in the layers.
The tailings are piles of debris where thundereggs can be found buried in the cleared sections of earth.
1.-2. The top of the lava flow, contains perlite, breccias and a pumaceous
zones. Very few if any thundereggs are found in this area.
3. Black “glassy” perlite contains thundereggs with cavities filled to about half way.
4. This transitional zone shows the decomposition of perlite into the completely altered clay stages.
5 & 6. These layers
have the highest concentrations of well formed thundereggs.
7. This layer is of a rhyolitic bedrock.
8 & 9. Rhyolite bedrock rests on ash layers.
Volcanic bombs occur when a volcano erupts, throwing massive amounts of viscous
molten rock into the air. By the time this material impacts, it has cooled to
the point where it is mainly, if not all the way, solid. As more and more of the
material piles up, it forms layers of bombs in varying densities with pockets of
spaces scattered throughout. Over time, gasses locked in the material seep out
creating pockets in the centers. The gasses radiate out from the centers and
create the circular voids called spherulites. Eventually, water begins to
trickle in carrying and depositing minerals in the cavities.
Sometimes these cavities completely fill, such as in the picture to the above left, and sometimes calcite crystals, above left, form intricate growths in the cavity instead of solidly filling it.
The cavities of the thundereggs typically fill with silicate minerals like the
quartz family’s agates, chalcedonies, and jaspers. Other minerals, such as
cinnabar, can create inclusions into the agate (such as found in some East
Most often, the pattern formed in the thunderegg core is banded with different colors. This can occur only when the rock formation stays in one place, however, if the surrounding rocks and formations are shifted for some reason, the formations within the pockets will change directions and shape. This is called a “tilt” or “slip” angle egg.
So now that you know what a thunderegg is, how it formed and what it looks like, can you just walk out and pluck one out of the dirt?
Thundereggs are found all over the earth in rhyolitic layers of rock; however, finding them is not as easy as it sounds.
In the United States, thundereggs are typically found in an arcing belt from Texas to New Mexico and up into northern Arizona, through Utah, Nevada, Idaho and Oregon.
Other deposits can be found in countries where conditions of the Rhyolite are
just right, such as in Germany, Poland and France. Some thundereggs have been
found in Africa and Madagascar as well.
The closest deposits of thundereggs here in South East Arizona are about 15 miles south of Deming, New Mexico in Rockhound State Park as well as the Baker mine near Columbus N.M.
Fondly calling themselves “rockhounds,” many mineral enthusiasts spend
day-trips, long weekends and vacations in spots like Rockhound State Park in New
Mexico digging up these little beauties.
Other good sites to pick into can be found in Oregon such as the Lucky Strike
Mine, Priday Ranch, Friend Ranch, and Buchanan Ranch. Woodward Ranch in Texas
and Powell County in Kentucky also have the right conditions for thundereggs
along with some areas near Tampa Bay, Florida and Coyote Hole Nevada.
When you’ve chosen your site it’s time to start digging, but what will you need? On expeditions like these, it’s ALWAYS a good idea to do research on the site you’ve chosen before you go out to dig. The digging part of the process can only be made easier if you know what it is that you’re looking for. In addition to this knowledge, you will also need basic mining tools such as a good sturdy pick, a shovel or two, some buckets, a chisel and hammer, and a trowel. (Always keep in mind the safety requirements for being outdoors!! Specifications such as outdoor clothes and shoes as well as extra water during a hot summer and sunscreen are based on the environment you’re digging in and are important to keep in mind!)
When you find your thunderegg buried in the rock, you can chisel it carefully straight out of the rock face, but before you completely remove the egg, mark the way it was sitting in the rock face, the best tool to help you here is a felt tip marker, because this will help you determine how to cut it.
When you get your wonder home, if a large saw, like a tile cutting saw, is not
available to you, you could always try finding a local gem and mineral show and
talking to the people there, maybe one of them will have a saw, if not they more
than most likely know where you can find one to use.
From digging it out of the ground to cutting and polishing, the process of
learning about and admiring the thunderegg is as awe inspiring as it is a
thrilling adventure. Be aware, as this author has become, that seeking out
these treasures is not always easy, but it always fun ; and who knows, maybe
you’ll end up a rockhound hunting down thunder.