Our club was instrumental in the designation of 'thundereggs' as Oregon's official State Rock.
According to legend, prehistoric Indians found solid nodules near Mt. Jefferson and Mt. Hood in Oregon and thought that when the gods or spirits who inhabited the mountains became angry with one another they would hurl rocks/nodules at each other with accompanying thunder and lightning. Hence they called these nodules thundereggs (Shaub, 1979).
The following is an excerpt about thundereggs from 'Roadside Geology of Oregon' by David D. Alt and Donald W. Hyndman. This book is in the Roadside Geology Series published by Mountain Press Publishing Company and is a 'must-have' for anyone who want to know where to rockhound in Oregon. There is a handy index in the back which enables the reader to easily reference a specific area of interest.
Start of quote from book:
"Thundereggs, Oregon's official state rock, are unusual and distinctive agates found only in light-colored volcanic ash at a number of localities in Oregon and elsewhere. Like all agates, thundereggs form as circulating groundwater slowly fills cavities in the original rock with silica. So agates are not volcanic even though they are usually quite common in volcanic rocks, simply because those tend to have lots of open cavities and are good sources of dissolved silica.
Although each thunderegg is unique in its own way, they all have a few things in common. Most are about the size and shape of a tennis ball, and they usually have a brown or gray rind which is rough like a cauliflower and marked by a simple pattern of several more or less distinct ridges. Their insides are filled with some surprising combinations of quartz crystals and gray agate which is usually marked with a pattern and often with splashes of brilliant color." End of quote.
More random information about thundereggs:
* Thundereggs and geodes cannot be distinguished from one another until they have been sawed or broken apart.
* Thundereggs are also known as spherulites.
* Geodes are hollow or near-hollow, crystal-lined cavities found in igneous as well as in sedimentary rocks.
* Thundereggs are solid or near-solid nodules formed by magmatic and volcanic processes and are found only in volcanic rocks (Lofgren, 1971).
* Thundereggs range in diameter from less than 1/4 inch to greater than a foot.
Concretions, on the other hand, are neither geodes or thundereggs, but instead are accumulations of minerals cemented together to form hard masses in sedimentary rocks. Concretions can be hollow inside, and some concretions contain a loose 'nut'. Concretions can be any shape or size whereas geodes and thundereggs are typically rounded to slightly ovoid.
*Lofgren, G., 1971, Spherulitic textures in glassy and crystalline rocks: Journal of Geophysical Research, v. 76, no. 23, pp. 5635–5648.
*Shaub, B. M., 1979, Genesis of thundereggs, geodes, and agates of igneous origin: Lapidary Journal, v. 32, pp. 2340–2354, 2548–2566.
*Alt, David D. and Hyndman, Donald W. 1978, Roadside Geology of Oregon, Twelfth Printing, December 1994, p. 217.