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thunderegg, thunderegg rock
A thunderegg or thunder egg is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers1 Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a meter across They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal,1 either uniquely or in combination Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours A characteristic feature of thundereggs is that like other agates the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them

Thunderegg is not synonymous with either geode or agate A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth A thunderegg on the other hand is a specific geological structure A thunderegg may be referred to as a geode if it has a hollow in it see illustration of Gehlberg specimen, but not all geodes are thundereggs because there are many different ways for a hollow to form Similarly, a thunderegg is just one of the forms that agate can assume


  • 1 Occurrence
  • 2 Formation
  • 3 State rock designation
  • 4 Legend
  • 5 Images
  • 6 See also
  • 7 References
  • 8 External links


Thundereggs are found globally wherever conditions are right In the USA, Oregon is one of the most famous thunderegg locations Germany is also an important center for thunderegg agates especially sites like St Egidien and Gehlberg Other places known for their thundereggs include Ethiopia,2 Poland,3 Romania, Turkey, Mexico, Argentina,4 Canada, Mount Hay5 and Tamborine Mountain,6 Australia and the Esterel massif,7 Francecitation needed


Thundereggs are found in flows of rhyolite lava They form in gas pockets in the lava, which act as molds, from the action of water percolating through the porous rock carrying silica in solution The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony The various colors come from differences in the minerals found in the soil and rock that the water has moved through8

State rock designationedit

On March 30, 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly91011 While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties12 The world’s largest thunderegg, a 175 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Oregon13


Native American legend reportedly considers the rocks to be the eggs of the thunderbirds which occupied Mount Hood and Mount Jefferson Oregon Thunder Spirits on the mountains hurled the "eggs" at each other11


See alsoedit

  • Lithophysa


  1. ^ a b "America's Volcanic Past - Oregon" USGS Cascades Volcano Observatory Retrieved 2007-06-08 
  2. ^ Rix, David "Thundereggs from Ethiopia" Eibonvale Thunderegg Gallery Retrieved 19 May 2017 
  3. ^ Rix, David "Thundereggs from Poland" Eibonvale Thunderegg Gallery Retrieved 19 May 2017 
  4. ^ Rix, David "Thundereggs from Argentina" Eibonvale Thunderegg Gallery Retrieved 19 May 2017 
  5. ^ "Mount Hay" Dig the Tropic: Outback to the Reef Capricorn Enterprise Retrieved 19 May 2017 
  6. ^ "Tamborine Mountain" Sydney Morning Herald The Sydney Morning Herald 15 August 2008 Retrieved 19 May 2017 
  7. ^ Richard, Jean-Jacques 1 February 2009 "Les Oeufs de Tonnerre" Bijoux et pierres precieuses in French Retrieved 19 May 2017 
  8. ^ Thunderegg Oregon State Rock, StateSymbols USA
  9. ^ "Oregon Symbols" SHG Resources Retrieved 2007-09-17 
  10. ^ Chapter 186 — State Emblems; State Boundary 2005 Oregon Revised Statutes
  11. ^ a b "Rock Hounding" Nature of the Northwest Retrieved 2007-09-17 
  12. ^ "Oregon Almanac" Oregon Blue Book Retrieved 2007-09-19 
  13. ^ Christie, Tim March 26, 2007 "Rock hounds check out goods at 18th annual Gem Faire" The Register-Guard Eugene, Oregon 

External linksedit

  • Geode Kid's Collection in Deming New Mexico on Mindat
  • Geologic Snowflake Hunters Flock To Oregon For Thundereggs Documentary produced by Oregon Field Guide

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Thunderegg Information about


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