Thundereggs or thunder eggs are nearly spherical structures or rocky bodies filled with mineral masses or crystals in rhyolite volcanic lava flows, ash layers, and tuffs.
These spherical objects resemble nodules or filled geodes. However, they are filled, not hollow or partially filled like geode or mineral aggregate or lump that defines nodules.
They form when groundwater deposits minerals inside vesicles. Vesicles are cavities formed when solidifying lava traps gas bubbles formed as volatiles exsolve.
Most thundereggs are filled with chalcedony and sometimes jasper, agate, quartz, amethysts, and other minerals. Quartz and amethysts often occur as crystals.
The word thunderegg came from a Native American legend. According to the legend, these spherical rocky bodies are eggs of thunderbirds that lived in Mt. Jefferson and Mt. Hood, which Thunder Spirits threw at each other.
Other names for thundereggs are nodules, agate eggs, agate-filled nodules, agate-geodes, volcanic geodes, star agate, etc. Also, they often have different local names in various places.
Lastly, thundereggs are Oregon’s state rock designated on March 30, 1965. They are most abundant in this US state. However, they occur in other states and countries too.
What do thunder eggs look like?
Thundereggs are usually fist or base-ball-sized spherical bodies. Most collected are 2-6 inches (5-15 cm). However, their sizes can range from half an inch to three feet (about 1.3 to 91 cm) and can weigh over a ton.
The world’s largest known thunder egg is in Rice Northwest Museum of Rocks and Minerals in Oregon. It weighs a whooping 1.75 tons.
On the outside, unopened thundereggs are nearly spherical, rough, or knobby and have a grayish-to-brown hue.
This exterior appearance is nondescript, with mostly a rhyolitic rind. It is very easy to ignore them. However, their marvel lies inside, seen when half-cut.
Thunder eggs are colorful and stunningly attractive inside, with a fine-grained silica and feldspar grains layer that goes towards the outer border.
Most thundereggs are filled with chalcedony, often bluish, but can have white to gray, bluish-gray, or brown shades.
However, opal, agate, jasper, and sometimes quartz can fill these spherical rocky bodies alone or with chalcedony.
And on rare occasions, they can have quartz or amethyst druses. Druses are small minerals that project inward to a cavity.
Furthermore, those from Oregon may display a 5-point star when cut into cabochon.
Depending on what fills these spherical bodies, you can have the following varieties or kinds:
1. Thunder egg agate
Agate-filled thunderegg is colorful with moss, plume, and sagenite inclusions and repeated stipes or concentric banding. It can occur alone or with opal, quartz, chalcedony, etc.
Agate is a banded variety of chalcedony in many amazing colors, including blue, green, red, purple, orange, pink, yellow, white, or black.
2. Opal thundereggs
Opal-filled thunder eggs are also colorful. They can be colorless or red, yellow, brown, orange, white, blue, black, pink, etc. Some exhibit the unique flashing or changing rainbow color, a phenomenon known as the play of color.
Owyhee River in Idaho is particularly famous for blue opal thunder eggs.
3. Amethyst thunder egg
This variety has a purplish or violet interior due to the amethyst crystals filling it. However, it often has other minerals like quartz, chalcedony, or agate.
Where to find thundereggs
Thundereggs mostly occur in weathered or decomposed rhyolite lava, tuffs, or ash layers.
However, they can occur in tuff, lavas, or ash of silica-rich volcanic rocks like dacite and rhyodacite, including in ignimbrite.
Ignimbrites are tuffs or volcanic rocks formed from pyroclastic flow deposits.
How to find thunder eggs
As an enthusiastic rockhound, the prospect of finding thundereggs depends on correctly identifying the prospective areas. These areas have weathered or decayed silica-rich or rhyolitic lava flows, tuff, or ashes.
Armed with your rock hammer and chisel, you should dig searching for nearly spherical, bumpy rock bodies with a brownish or grayish hue.
These bodies lie close to the surface, i.e., are usually shallowly covered or embedded in the decayed soft clayey mud, making them easy to notice.
How do they form?
Thundereggs form when fluids or groundwater with dissolved silica and other minerals percolate into vesicles in rhyolite lava flows or tuffs.
As these fluids pass through the vesicles, they deposit the dissolved minerals to form the filling masses or crystals. The various colors come from impurities that dye these minerals masses/crystals or the characteristic look of the minerals.
However, some unusual thundereggs in Buchanan in Oregon point to a different origin, which is spherulitic. Some have a reddish color from cinnabar. It is possible that many small spherulites raptured and were filled by chalcedony.
Where are thunder eggs found?
In the United States, thundereggs are widespread in Oregon and New Mexico. They also occur in Colorado, Washington, Arizona, Idaho, California, and Nevada, among other places.
If you are a rockhound in the US, some of the prospective areas include:
1. Oregon
In Oregon, thundereggs occur mostly in central and eastern Oregon, such as Jefferson, Wasco, Malheur, Crook, and Wheeler counties.
Specific locations include Richardson’s Rock Ranch, Polka Dot Agate Mine /Canyon Rim, Ochoco National Forest, and Succor Creek Canyon.
2. New Mexico
Common places in New Mexico with thunder eggs are Baker Ranch, Lava Cap Claim and Lava Cap Claim, and Deming in Lunar County and the South Canyon area in Doña Ana County.
3. Washington
In Washington, thunder eggs occur in Little Naches River in Yakima County, First Creek in Kittitas County, and Green River in King County. Also, they are in Deer Creek Drainage in Snohomish County and Succor Creek on the edge of Owyhee County.
4. Colorado
Some places with thundereggs in Colorado include Del Norte and Old Woman Creek in Rio Grande County and a few places in Saguache County
5. More places
- Arizona: Aguila in Maricopa County and KOFA mountains in Yuma and La Paz counties
- Idaho: Rabbit Springs in Twin Falls County, Weiser in Washington County, American Falls in Power County.
- California: Mojave region in Kern County and Berkeley Hills in Alameda County
- Nevada: Coyote Springs in Lincoln and Clark Counties, Beatty and Duckwater in Nye County, and Virgin Valley in Humboldt County
Locations outside the US with thundereggs include Tamborine Mountain, Thunderbird Park in Queensland, Snowy River in Victoria, and around Doon in New South Wales in Australia.
Also, they occur in Nowy Kosciol in Poland, Esterel massif in France, Mount Hay and Black Dome in Canada, and St. Egidien and Gehlberg in Germany.
Other countries are Romania, Ethiopia, Uruguay, Argentina, Brazil, Turkey, Mexico, and Formosa in Taiwan.
Thundereggs uses
Collectors price the beautiful and colorful thundereggs. Many people sell them online.
Some of the uses of thundereggs include jewelry to make bookends, brooches, pendants, and bolo slides.
Also, they make door or drawer knobs, while others are cut into halves or cabochons and used as ornamental or display pieces.
Large pieces are a source of the minerals they contain, like agate, opal, or chalcedony. These minerals make an array of jewelry.
Also, they make ornamental items like spheres, pyramids, eggs, wands, towers, carvings, etc., and other items such as pen stands, candle sticks, picks for guitars, etc.
Frequently Asked Questions (FAQs)
Yes. They are rare and only found in a few places with rhyolite or silica-rich lava flows, tuff or ash layers like rhyolite, and sometimes dacite or rhyodacite that covered land about 60 million years ago.
Thundereggs are filled, not hollow or partially filled, and occur in vesicles in silica-rich lava flows, tuffs, or ash layers like rhyolite. In contrast, geodes are hollow or partially filled and commonly occur in limestone beds but can occur in volcanic rocks and shales. Also, geodes cavities form in other ways, not only from vesicles.