Thundereggs are fascinating nodule-like rocks that appear unassuming on the outside but reveal breathtaking beauty when cut open.
Often looking like rough, grayish or brown spherical rocks, a thunderegg’s true value lies in its interior – usually filled with agate, chalcedony, jasper, or even opal in striking patterns.
These formations have captivated rockhounds and mineral collectors for generations, not only for their colorful crystal-filled cores but also for the lore and legend surrounding them.
In this article, we’ll explore the geology of thundereggs (and how they differ from geodes), where they are found, their appeal and value to collectors, as well as the historical and cultural significance that makes them legendary.
How Are Thundereggs Formed?
Thundereggs are products of ancient volcanic activity and unique geologic conditions. They form within rhyolitic volcanic ash flows or tuffs, specifically in gas pockets or hollows created as the lava cools.
Here’s how a thunderegg comes to life: Silica-rich groundwater percolates through the porous volcanic rock, depositing layers of minerals into the empty cavity. Over time, the silica gels and crystallizes, filling the void with banded chalcedony (microcrystalline quartz) and other minerals.
Initially, a darker matrix of silica may line the cavity, followed by successive inward growth of lighter agate or chalcedony. The result is a solid nodule with a rough, often spherical exterior of hardened lava, and a hidden inner core of colorful cryptocrystalline quartz. Mineral impurities in the silica create the vivid colors and banding – for example, iron can impart reds and oranges, while other trace elements produce blues, yellows, or greens.
In terms of mineral composition, most thundereggs have a core of chalcedony or agate (which are forms of quartz), sometimes intermingled with jasper (an opaque form of quartz) or opal.
It’s not uncommon to find sparkling inclusions or pockets as well: some thundereggs contain clear quartz or amethyst crystals, and occasionally minerals like gypsum or calcite can form inside.
The hardness of these interiors is typically around 6.5–7 on the Mohs scale (the hardness of quartz), meaning they are quite tough. Externally, the thunderegg’s matrix (the outer rind) is usually the host volcanic rock (rhyolite or a similar siliceous rock), which can be gray, brown, or reddish and much less attractive than the polished interior.
Largest Known Thunderegg
Most thundereggs range from about 5–10 cm (2–4 inches) across – roughly the size of a baseball – but they can be much larger or smaller. In fact, some giants have been found well over a meter across. For instance, Oregon produced a thunderegg weighing 1.75 tons (the largest known), now housed at the Rice Northwest Museum of Rocks and Minerals.
How Old Are Thunder Eggs?
It would be near impossible to say how old any one thundereggs is, without extensive chemical aging techniques. Though to make a thunderegg, is a process which takes millions and millions of years.
It is estimated that the thunderegg containing rhyolite lava flows that are found across much of Oregon, the capital for thundereggs, are around 60 million years old. However, other thunderegg sites have been estimated at 10-20 million years old. Regardless, the time it takes for minerals to accumulate and deposit intricate and beautiful patterns inside of a hollow rock is a very slow one.
Thundereggs vs. Geodes – What’s the Difference?
It’s a common question: how do thundereggs differ from geodes? While both are beloved by collectors for containing crystals or mineral fillings inside, geodes and thundereggs have distinct definitions in geology.
A geode is any rock cavity (often roughly spherical) that became lined with crystals. Classic geodes usually have a hollow center; when cracked open, you find an open space with crystal points (often quartz or amethyst) pointing inward. Geodes form in various environments – for example, in sedimentary rocks like limestone or in volcanic bubbles – and the key is the hollow interior.
A thunderegg, on the other hand, is a specific type of nodule that forms in volcanic ash/lava and is typically solid inside (fully filled with agate/chalcedony rather than a large hollow)
In simple terms, every thunderegg is essentially an agate-filled nodule, whereas not every geode is a thunderegg.
Thundereggs often have more complex, star-like or radiating agate patterns because of the way the silica solution filled the cracked inner space of the “egg.”
If a thunderegg happens to have a small cavity in the center, it could be called a geode in the loose sense, but geologists reserve “thunderegg” for those formed in rhyolitic volcanic contexts
Read my article Thundereggs VS Geodes for a more detailed look at the differences between geodes and thundereggs.
Historical and Cultural Significance
Thundereggs carry a rich folklore and have made their mark in local culture, especially in places like Oregon where they’re emblematic. The very name “thunderegg” has legendary origins. According to a Native American legend of the Pacific Northwest, thundereggs were thought to be the eggs of thunderbirds – powerful mythical creatures – that were hurled by the thunder spirits during storms.
In the Warm Springs tribal legend, Mount Hood and Mount Jefferson (two prominent snow-capped mountains in Oregon) were home to rival thunder spirits who would throw these “eggs” at each other in anger, accompanied by flashes of lightning and claps of thunder.
Early settlers, upon hearing this tale, adopted the name “Thunder Eggs” for the strange agate-filled rocks found in the region. The name stuck, and to this day carries the echo of that thunderbird mythology.
Beyond the legend, thundereggs have become culturally significant in Oregon and elsewhere. As mentioned, Oregon named the thunderegg its State Rock in 1965, spurred by rockhounds and geology enthusiasts who wanted to recognize these treasures. This official designation not only acknowledged the abundance and beauty of Oregon thundereggs but also helped promote tourism – rock collectors from around the world visit Oregon for its thunderegg beds.
Towns like Nyssa, Oregon even host annual Thunderegg Days festivals, celebrating the local geology with rock and mineral shows, guided thunderegg digs, and community events. In Germany, thundereggs (Donnerkeile or “thunderbolts”) have been part of local mineral collections for centuries, often mistaken in folklore as lightning stones or petrified thunder.
Historically, indigenous uses of thundereggs beyond the naming legend are not well documented – they were not particularly used as tools or weapons (since other stones like obsidian or flint were more practical). However, it’s easy to imagine that polished halves could have been traded or admired by those who found them. In modern times, thundereggs have primarily a legacy of scientific and hobbyist interest.
Geologists study them to understand the volcanic processes and mineral deposition. Collectors treasure them and often pass them down through generations or donate spectacular specimens to museums. In a way, each thunderegg holds a piece of geologic history – a tiny preserved geochemical environment from millions of years ago – as well as the cultural history through the legends and local pride they’ve inspired.
Where To Find Thunder Eggs
Thundereggs can be found in localized areas across the world where optimal conditions exist. Some of the best known thunderegg spots in the world are located in Germany and in Oregon. The thunderegg is even the state rock of Oregon!
Where To Find Thunder Eggs In Oregon
To find the state rock of Oregon, many free sites along with some pay-to-dig sites exist most densely in the central and southeastern regions of the state. Some of the pay-to-dig sites on private lands hold the highest quality and most coveted thundereggs.
A quick internet search should yield many different ranches that allow thundereggs to be dug and collected for a small fee. For novice collectors, this can often be the best way to get into finding these unique rock formations in the state best known for them.
If seeking out thunderegg sites on public land is more your style, there are many free collection locations on public lands. When collecting rocks on public lands, most groups hope others will subscribe to a code of ethics to preserve the lands and keep rock collection sites from closing down from negligent abuse.
Such practices include no trespassing, no explosives or mechanized equipment, only excavating to 4 feet and being sure to fill in the holes, collecting whats needed and nothing more, and packing out all that you may pack in.
~Southeastern Oregon~
Harney County in southeastern Oregon is host to a few public land sites managed by the Bureau of Land Management that are open to collecting. A few hours south of the town of Burns, there are several adjacent sites that rock collectors can not only find thundereggs, but also geodes, petrified wood, jasper, and agate.
On the east side of Steens mountain, near Mann Lake, collectors can use shovels and rock hammers to search for and collect the lot of these rocks and minerals. South of Mann Lake, rock hounders also flock to the areas around Pike Creek to collect thundereggs.
Further to the south, Borax Lake is also host to petrified wood, agate, and a lesser amount of thundereggs. Being public lands managed by the BLM, there are disperesed camping sites and some developed campgrounds throughout the area to serve as a basecamp for your rockhounding excursions.
~Central Oregon~
In central Oregon in Crook County, the Ochoco National Forest is a favorite area among rock collectors to look for thundereggs. Northeast of the town of Bend, there are several collection sites on this National Forest; some being private pay-to-dig sites, but also a few free public land sites.
White Rock Springs and White Fir Springs are two nearly adjacent sites on public lands managed by the U.S. Forest Service. Both of these sites contain not only jasper thundereggs, but also some different types of agate. These sites are accessed via state highway 26 east of Prineville, Oregon. At milepost 41, Wildcat Mountain Road (NF 3350) heads to the north and both of the sites are within a few miles and are well signed.
Read More: Rockhounding In Oregon (Ultimate Guide)
Where To Find Thunder Eggs In Washington
Despite Oregon being the hot spot of the western U.S. to find thundereggs, other states, including Washington, contain concentrations of thundereggs. Like Oregon, in the state of Washington you also may find a mixture of pay- to-dig sites along with sites located on public lands free and open to responsible collection.
The Little Naches River bed has several sites with high densities of thundereggs and is located on public lands. Located northwest of the town of Yakima, this is a well publicized site open to public rock hounding and collection. Along with thundereggs, a variety of crystals are also regularly found in this area.
While a few sites exist that are well known and sought out sites by collectors, to a curious rockhound, these thundereggs can be found just about anywhere in this area. Some of the favorite and more popular sites include dig sites that have previously upturned earth. These digging sites are fairly noticeable due to all the disturbance and contain ample parking around them.
Other Locations To Hunt For Thunder Eggs
It is without question that Oregon and close behind, Washington, remain the hotspots for thundereggs within the U.S. With a little bit of research through books and the internet, you can find thundereggs all over the state at diverse sites. However, there are still other states that one can find and even collect thundereggs in.
California
The Black Agate Hills Geode Bed in the southern California desert is a site that holds some unique thunderegg formations. In this lower Sonoran Desert landscape, rock-hounders are gifted with a unique variety of thunderegg. This site is not only known for producing on-average, larger thundereggs; but also containing cores that are usually a bold and rich with contrasting light and dark deposits.
There are several already dug-up areas where at this site so you can dig around in these areas without feeling to shy about it. For those reluctant or lazy to dig, foraging on the ground surface can also be productive. Getting to this site can requires following a list of directions that can be found online.
New Mexico
Along the rocky slopes for the Florida Mountains in southern New Mexico, near the town of Deming, recreators and rock-hounders flock. Rockhound State Park is an obvious attraction to rock and mineral enthusiasts and is also a popular campground during the spring, winter, and fall seasons. This is one of the few state parks where collection of rocks and minerals is actually supported.
While jasper is the most prevalent thing to collect at this site, this state park is also home to: geodes, pearlite, quartz, and of course; thundereggs. While the site receives quite a bit of visitors, constant erosion is continually unearthing new treasures. After heavy rains can be a good time to see what new rocks and minerals have been revealed.
Final Thoughts
Thundereggs are one of those geological finds that are just really intriguing. I’m not sure if it’s because they can only be found in very specific locations around the world, or if it’s because you just never know what kind of formation will be found inside the thunderegg.
Whatever the reason, thundereggs are a natural wonder that has intrigued me since I was a little kid, and they continue to intrigue me.
Have you had a chance to get your hands on a thunderegg? Or even better yet, have you had a chance to go out and dig and cut your own thundereggs? If not, don’t pass up on the opportunity to do so!
