One of the excursions we have wanted to take ever since we have lived in Central Oregon is to Richardson’s Rock Ranch, just north of Madras.  Somehow or another … it just never made it onto our calendar.  Now that we are moving, I insisted we make the drive.  The night before our departure, my mother called me to say that she would be joining us and that she would be bringing along my niece and nephew!  What a nice surprise!

We met at the ranch around 9:45 a.m. and checked in at the office.  From there, it was only 7 miles to the digging site (we selected an easier one since we were beginners) but the road was not maintained so it took us a good 20-30 minutes to get out there.   We began our quest immediately and were not discouraged.  Everyone found thundereggs … some even appeared to be discarded on the ground.  Our buckets were full within an hour, thankfully as it was getting quite hot already, and we made our way back to the office to weigh and cut open our thundereggs.

According to ancient Native American legend, when the Thunder Spirits living in the highest recesses of snowcapped Mount Hood and Mount Jefferson became angry with one another, amid violent thunder and lightning storms they would hurl masses of these spherical rocks at each other. The hostile gods obtained these weapons by stealing eggs from the Thunderbirds’ nests, thus the source of the name “Thundereggs.”

The Thunderegg was designated Oregon’s official state rock in 1965.  Today, Thundereggs are made into beautiful jewelry, especially bolo ties and pendants, pen stands, bookends, and decorator pieces. Their value ranges from about $1 per slice or half egg to well over $100 per slice or single cabochon.

A Thunderegg is not actually a rock. It is a structure, sometimes a nodule or geode, occurring in rhyolite, welded tuff, or perlitic rocks.  Scientists do not agree on the processes forming Thundereggs. Some insist that the characteristic and unique internal pattern of typical Thundereggs is due to expansion and rupture of rock by gases. Others claim the pattern is due to drying of a colloid or gel. Thundereggs range in size and weight from less than an inch and under one ounce to over a yard in diameter and over a ton in weight. Most eggs collected are between two and six inches in diameter.

If you are interested in teaching a geology unit in your homeschool, you may be interested in a 10 week unit I developed for the middle grades or logic stage.  Earth Logic: Our Dynamic Earth.  I have also created a Squidoo lens where I have organized numerous free online resources and have shared a short interactive quiz.  You can find the lenses here …  Geology Rocks:  A Homeschool Unit Study and Geology: How Well Do You Know Your Stuff?


  • christian stokes

    August 5, 2016 at 5:26 pm

    can you tell me if this is a thunder egg?

    • Eva Varga

      August 8, 2016 at 9:27 am

      Yes, these are what are commonly called thunder eggs. There is a wide variety of crystalline structures inside the “egg”.

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