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This article is about a type of rock; for the rock and roll band, see Thunderegg (band)

A thunderegg (or thunder egg) is a geological structure, similar to a geode, that is formed within a rhyolitic lava flow. Thundereggs are not actually rocks, but rather geologic nodules which occur within other igneous rocks.


Appearance and composition

  Thundereggs are rough spheres, most about the size of a baseball. They contain centers of chalcedony in the form of agate, jasper, or opal.[1] Thundereggs look like ordinary rocks on the outside, but slicing them in half may reveal intricate patterns and colors, which make them valuable in jewelry.


According to Pacific Northwest native legend, the Thunder Spirits lived at the highest reaches of Mount Hood and Mount Jefferson, and when they became angry at each other, they threw these spherical rocks at each other.

State rock designation

Despite not being geologically classified as a rock, on March 30 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly.[2][3][4] While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties.[5]


  1. ^ America's Volcanic Past - Oregon. USGS Cascades Volcano Observatory. Retrieved on 2007-06-08.
  2. ^ Oregon Symbols. SHG Resources. Retrieved on 2007-09-17.
  3. ^ Chapter 186 — State Emblems; State Boundary 2005 Oregon Revised Statutes
  4. ^ Rock Hounding. Nature of the Northwest. Retrieved on 2007-09-17.
  5. ^ Oregon Almanac. Oregon Blue Book. Retrieved on 2007-09-19.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Thunderegg". A list of authors is available in Wikipedia.
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