top of page

Tuff Nuts to Crack: A Thunder Egg Story

Figure 1: Bulk spillover stone in front of Richardson's Rock Ranch rock shop.
Figure 2: Roadside geology, indeed! Tumultuous ramparts of rock greet you at Richardson's Rock Ranch in Madras, OR.

Prepare yourself & hold on to your butt, because this is the epic tale of how we, Alex Jensen and I, went digging for balls of colored minerals in the ranchy, raunchy, rain-free, reflie*-riddled valleys of central Oregon [Figure 1]. Alex is getting his PhD in biology and was a roomie for the whole month of August. Lucky me, because housemates with good taste in extra curriculars are housemates to have. They move from stranger to former stranger to friend. Before long, they might let you share the addiction of rockhounding as their enthusiasm -or is it tolerance?- grows. Maybe then you can cut and run off into the desert for 26 hours, nine of which you might spend car-singing to killer tunage, and three digging in the August heat for thunder eggs! These angelic, mineral-filled globs of metaphoric rock can host colors, patterns, and gem material like you've never seen. Or mayhaps you have seen. I don't know you. I don't know what you've not seen. No -but really- some places even have precious stones in their thunder eggs! Not this place, but they are out there. This place -pause for exaggerated effect- is the fabled Richardson's Rock Ranch (RRR), located in scenic Madras, OR [Figure 2].

Thunder eggs are a type of geode. Just so you know. Just in case you opened this page expecting manifested greatness in blog form and got something else entirely, because I never simply stated that thunder eggs are basically solid geodes [Figure 3]. They form when roiling volcanic ash, called tuff, is intruded with mineral-rich water or molten minerals. Now, that is my understanding from months of asking around and being told slightly different variations on the definition of a geode versus a thunder egg. Frank, owner of The Crystal Habit rock shop, told me that the kind of tuff that makes up the matrix of these geodes varies by location but behaves the same across the board, and that the process for thunder eggs is slightly different than the process to create hollow geodes (probably why I kept getting different answers to the same question). Here is what my pocket rock guide, Rocks & Minerals of Washington and Oregon, Dan R Lynch & Bob Lynch, 2012, has to say about it:

Figure 3: A thunder egg from the Blue Bed at Richardson's Rock Ranch.

"Thunder eggs are a particularly interesting variety of agate that formed in a completely different geological period than most other agates. Most agate formed within vesicles (gas bubbles) in recently cooled lava (molten rock), such as basalt. Thunder eggs formed within hollow pockets in tuff, a type of rock formed by the solidification of volcanic ash. These pockets were made by the rapid expansion of hot gases, leaving a void that later filled in with a solution bearing silica, the quartz material that forms all agates. As the silica crystallized and layered to form agate, it also adhered to the surrounding tuff, forming a round shell of hard rock surrounding the agate core. Therefore, when thunder eggs are found, they appear only as round rocks. One might think this would make them impossible to find, but in most localities, the tuff in which the thunder eggs formed has decayed to a soft, muddy clay and the hard thunder eggs are conspicuous. Finally, some thunder eggs are hollow, making them geodes."

Now, I adore my pocket rock guide, it has shown me a beautiful side of my new state that would otherwise have gone neglected (probably). However, it doesn't always hit the nail directly on its head. For example, it lists snake skin agate as occurring exclusively in Oregon, and yet I have a handful of samples from the diatomaceous earth pits outside George, WA. That said, this description is fairly comprehensive regarding the thunder eggs we were apt to find last weekend. They are geodes, but they are not always exclusively agate, or so I am lead to believe by the rock market. In any case, these handsome fellas are spherical, lumpy masses of rock with interiors that come in a tantalizing array of layers and colors. In Figure 3, you can see another interesting facet of a thunder egg, its pressure cap. Frank is cutting many of the eggs we brought back from this trip, and while he prepped a geode to be cut, he explained: "There are two primary pressure caps on a thunder egg. You want to find them, and then cut perpendicular to that line, because the edges of the caps are where the material got in and created the bubble. Hopefully, when you do that you find a five-point star, or a nice big cavity with shapes and pictures looking back at you."

Figure 5: One of the owners of Richardson's Rock Ranch, after giving us our map and introductions for digging.