( Fr- tonnerre oeuf; Ger- Donner-ei; Nor- tordue-egg; Rus )


A. Thunder egg. Section (width - 10 cm) that exhibits plume agate within gray chalcedony, from Saguache County, Colorado.  (© photo by Daniel E. Kile, United States Geological Survey)

B. Thunder eggs  from the Little Florida Mountains, southeast of Deming, Luna County, New Mexico (left, height - 9 cm).  The upper group of layers of the thunder egg on the right appear to have been deposited when the "egg" had a different horizontal positions than it did when its lower layers were deposited.  W.D. Lowry collection. (© photo by Dick Dietrich)

Thunder eggs are spheroidal nodules
that have the following characteristics:  1.knobby outer surfaces that are drab brown or gray;  2.border zones that consist largely of extremely fined grained feldspar and silica;  [and]  3.interiors (for lack of a better word) that typically exhibit roughly starlike or irregular overall shapes in equatorial, or near equatorial, sections. These nodules, as typified by thunder eggs from Oregon, "range in size and weight from less than an inch[< 2.5 cm] and under one ounce[< ~28 gm], to over a yard[> 90 cm] in diameter and over a ton[> 90 kg] in weight [, though] Most eggs collected are between two and six inches[5  - 15 cm] in diameter." (  The interiors of many thunder eggs<> consist largely of chalcedony (see CHALCEDONY entry) that may be virtually a single color (commonly bluish)  or just about any of the combinations of colors and patterns known for diverse agates (see AGATE entry), or a combination of those alternatives;   however, the interiors of some thunder eggs contain jasper, opal, and/or quartz etc. either in lieu of or along with the chalcedony.  And, the quartz commonly occurs as druses or larger crystals that protrude into cavities.   A few so-to-speak multiple thunder eggs -- i.e., masses that consist of inseparable masses that appear to be two or three (etc.) intergrown thunder eggs.

OTHER NAMES: According to Zeitner (1979, p.1260), thunder eggs "have been called nodules, agate filled nodules, eggs, [and] agate eggs" as well as thunder eggs. However, none of those names, so far as I have been able to determine, have been used widely, if at all, in either the literature or on the markeplace.  Contrariwise, the six terms, listed below, have been spread rather widely.

An additional note relating to nomenclature seems note worthy here:  The name thunderegg has sometimes been given to septaria from the Mackenzie River area of Canada.  Considering the widespread acceptance of the name thunder egg (or thunderegg) for the rock masses described in this entry, it is extremely unfortunate that the term has been applied to these septaria.  Every effort should be made to avoid perpetuating this usage even by reference;  indeed,  I apologize for doing so even in the context of this paragraph.  

  Thunder eggs are used:   A. to exhibit relationships between their rims and cores  or  B. to serve as the source of chalcedony, agate etc. recovered from their interiorsImportant "A" uses include:  Jewelry - pendants, brooches, bolo slides;  bookends (large thunder eggs can  be cut into eighths and then sold as four matching, or not-matching, sets);  knobs for doors and drawer pulls;  and display pieces.   And, "B" uses, which exceed "A" uses in number, include:  Jewelry - just about anything;  eggs, spheres, pyramids, wands and carvings; and miscellaneous pieces such as pen stands, candle sticks  and guitar picks.  A few spheres etc. have been fashioned to include both rims and interiors of thunder eggs;  in my opinion, most of these are hardly worth mentioning.

 Most thunder eggs occur as nodules within or weathered out of rhyolitic, rhyodacitic, and dacitic composition tuffs, at least some of which are ignimbrites (welded tuffs).   Chemical composition-wise, these tuffs have roughly the same compositions as granitic,  granodioritic and quartz diorite magmas, respectively.

NOTEWORTHY LOCALITIES:   First, localities from which I have seen good specimens:  Oregon -- northeast of Madras in Jefferson and Wasco counties, northeast of Prineville, Crook County, and south of Nyssa, Malheur County;    Colorado -- Saguache County;   and   New Mexico -- Rock Hound State Park, southeast of Deming, Luna County.    Noted in a few publications (see REFERENCES) are:  California -- e.g., the Berkeley Hills and Mojave Region;  Idaho -- e.g., Weiser, Twin Falls and American Falls;  Nevada -- e.g., Virgin Valley, Beatty, Coyote Springs and Duckwater;   British Columbia, Canada -- south of "Black Dome," which is southwest of Gang Ranch, Cariboo District;  Australia -- e.g., Thunderbird Park, Tamborine Mt., Queensland,  near Doon, New South Wales,  and along Snowy River, Victoria;  France - L'Esterel, Frejus, near Cannes (called lithophysae);   Germany -- St. Egidien, Saxony;  Poland (southwestern) -- e.g., Nowy Kosciol.  Also, I have seen reports of thunder eggs from Africa, Brazil, Formosa, India, Mexico and Uruguay, with no specific localities given, plus I feel sure that I have forgotten.

REMARKS: Thunder eggs were named for occurrences in Oregon, where "According to ancient American legend, when the thunder spirits living in the high recesses of snow-capped Mt. Hood and Mt. Jefferson became angry with one another, amid violent thunder and lightening storms, they would hurl these spherical rocks at each other. The hostile gods obtained these weapons by stealing eggs from the thunder birds' nests,  thus the source of the 'Thunderegg.' " (   It is interesting along this line that according to Amerindian legend, thunderbird eggs were the progenitors of Chinooks  (see, for example, stuproj/ametf99/wenzl/chinook.html ).

Much research needs to be done -- including at least tangential consideration of certain concretions (e.g., septaria),  some geodes and probably even some amygdaloids -- before any definitive origin(s) can be accepted as proved for thunder eggs.  Both field work and laboratory investigations must be involved:  For example, field relations -- such finding out if it is really true that "Each thunderegg is different, yet there are enough similarities that a practiced eye can say which bed an individual egg came from." (Zeitner, 1979) -- are needed because of the bearing such relations have on considerations dealing with geological and geochemical controls that obtained during different stages in their formation.  Laboratory investigations need to be extended by utilizing state of the art equipment and the ever growing catalogue of  information, which is becoming more and more readily accessible through continually updated data bases.

Currently, In lieu of summarizing the diverse origins previously suggested for thunder eggs, the pertinent part of an answer the U.S. Geological Survey gives to the question  "What is a thunderegg?"  follows:  "Thundereggs were not, as believed by some people, ejected from volcanoes, but formed in very soft and friable volcanic ash beds. Solutions containing silica permeated the cinders until favorable points for chalcedony deposition were achieved. Aggregations of chalcedony were deposited, but before the material could fully solidify the center of the concretion split apart, possibly because of shrinkage, permitting the later introduction of additional materials [the interiors]. The resulting star-shaped centers of chalcedony may be in the form of agate, jasper, or in some cases different varieties of opal."  However, for what it may mean, it should be noted that the USGS attributes the source for this statement not to any of its scientists or nvestigations but to "Gemstones, An Overview of Production of Specific U.S. Gemstones: U.S. Bureau of Mines Special Publication 14-95"  (see  web site)  

The feature illustrated by the right-hand mass in Figure B has been of great interest to W.D. Lowry, my colleague when we were both at Virginia Tech,  and me for several years. Currently, we think it indicates tilting while solutions were depositing silica (etc.) in the interiors of these masses  -- i.e., we think the horizontal position of these masses was changed one or more times during prolonged, possibly interrupted deposition of the chalcedony within them. If this conclusion is correct, this feature provides an interesting geological indicator of former earth movements as well as prolonged, and perhaps interrupted, deposition.

Thunderegg, spelled as one word, has been the official state rock of Oregon since March 29, 1965.

SIMULANTS:  None that I have seen or seen described.

REFERENCES: Kile, 2002; Staples, 1965; Zeitner, 1979.

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Last update:  29 October 2005
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