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Home > Geology > Rock Hounding > Thundereggs
  Rock Hounding
Gold Panning
Oregon Sunstone



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."




Oregon deposit recognized, 1893
Dr. George F. Kunz, Tiffany's famed gem authority, estimated as much as $20,000 worth of opal-filled eggs from one Oregon deposit had been marketed in 1892.

Thunderegg designated Oregon's official state rock, 1965

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, sometimes a geode, occurring in rhyolite, welded tuff, or perlitic rocks. However, without question, the Thunderegg is by far the most popular "rock" in Oregon.

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 desiccation (drying) of a colloid or gel. Whatever the process, after the cavity that contains the egg is formed, further development is extremely variable in the amount of time needed to complete the egg, degree and type of infilling, and physical characteristics.

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.





Typically, an egg has a russet-colored outer shell that is often knobby and often has a characteristic ribbed pattern. Frequently, the inside of the outer shell has a relatively thin intermediate or transitional lining. This is sometimes composed of an iron or manganese compound, often with a thin coating of opal or chalcedony. Sometimes only opal or chalcedony is apparent. Finally, the center of an egg is usually filled with chalcedony or opal and may or may not have inclusions, pattern growth, or crystals. In some variants, the egg may be hollow or may have a thin layer of chalcedony coating the interior.

This layer sometimes is topped with a coating of small quartz crystals. Growths of algalike tubes, or plumes, or "moss" of manganese or iron compounds or of clay may be free standing or partially or wholly embedded in chalcedony. Some eggs with plumes ("flowers") in chalcedony are among the most valuable specimens. Several zeolites have been observed or reported in Thundereggs; clinoptilolite is fairly common, and mordenite, natrolite, and mesolite have also been reported.

Thundereggs are sometimes found with fortification banding just inside the shell, then an area of horizontal layering, with the remaining central area filled with clear chalcedony or inward-pointing quartz crystals. Banding and layering vary in color, thickness, and content. Some layers are composed of a fibrous cristobalite (lussatite). Other eggs have a partial botryoidal filling of an opal form of low cristobalite. This opal is often fluorescent because of a low content of uranium salts. One collecting site in Oregon has eggs filled with carnelian. At another, the filling may contain cinnabar, which colors it pastel to intense red. Some eggs are filled with pastel jaspers. Others may have any one of a variety of opal fillings that may be opaque blue, opaque red, translucent pastel blue, translucent yellow, translucent red, white, or colorless. Some of the opal can be faceted, and a small percentage is true precious opal.

Some eggs have well-developed calcite crystals encased in chalcedony, and others contain pseudomorphs of chalcedony after calcite. Some eggs have layering that is fanned from one edge, because the egg was rotated by earth movement while the filling was being deposited. This and other features suggest that the complete development of some eggs may have taken considerable time, and the filling-in of the egg may have recorded a series of geologic events. Some eggs contain brecciated rock fragments, while others show faulting, offset, and healing. One of the most unusual Thunderegg variants is up to 3 feet long and 2 to 3 inches in diameter and looks much like a fat gray worm. In some areas, it is common to find the characteristic chalcedony core weathered out of its shell. If a complete egg is sawed in the right orientation, one or more conduits through which filling materials flowed may be found. The beauty and complexities of many of the cut and polished eggs explain why Oregon rockhounds have long been fascinated by Thundereggs.





Thundereggs can be collected at many sites in Oregon. Some localities occur in beautiful forested hill country, others in dry, desertlike terrain. Some are "free sites," while others are "fee sites." As Thundereggs have been collected in Oregon for fifty years, collectors on "free sites" must expect to dig and work for the eggs. Proper equipment, including shovel, pick, and bar, makes the job much easier. The "fee site" will almost always have some preparatory work (overburden removal) done. Eggs often may be purchased and equipment rented at the site office. Conditions change, so collectors should contact sites for current fee status and appropriate authorities for permission to dig.

The sites are grouped into four major collecting areas.
Addresses and phone numbers are those you should contact for information.

Madras-Prineville area - free sites
White Fir Spring
Whistler Spring
Lookout Mountain Ranger District
3160 NE 3rd
Prineville, OR 97754
(541) 416-6500

Maury Agate Beds
Lookout Mountain Ranger District
3160 NE 3rd
Prineville, OR 97754
(541) 416-6500

Ochoco Agate Beds
Mt. Hood Rock Club Claim
Dick Parks (360) 892-3716

Madras-Prineville area - fee sites
Richardson's Recreational Ranch
6683 NE Hay Creek Rd.
Madras, OR 97741

Lucky Strike Thundereggs
P.O. Box 128
Mitchell, OR 97750
(541) 462-3073
Summer only

General information
Prineville Chamber of Commerce
102 NW Second Street
Prineville, OR 97754
(541) 447-6304

Judy Elkins Gemstones
972 South Main
Prineville, OR 97754
(541) 447-5547

Burns area
Buchanan (fee site)
Highland Rock and Gem Shop
1316 Hines Boulevard
Burns, OR 97720
(541) 573-5119

Lakeview area
Crane Creek
High Desert Craft Rock Shop
244 North M Street
Lakeview, OR 97630
(541) 947-3237

Southeastern Oregon area
Succor Creek
Nyssa Chamber of Commerce, Thunderegg Days
105 Main Street
Nyssa, OR 97913
(503) 372-3091

U.S. Bureau of Land Management
100 Oregon Street
Vale, OR 97918
(541) 473-3144

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