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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...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 sometime 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 ... have inclusions, pattern growth, or crystals. Sometimes 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...

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. In some eggs that have a filling of opal it is often fluorescent because of a low content of uranium. One collecting site in Oregon has carnelian filled eggs. (WOW—ed) Others have filling colored red with cinnabar from pink to intense red. Some have pastel jaspers. Others have opal fillings that may exhibit many different beautiful colors and a small percent have 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 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.

Where to Find Thundereggs in Oregon

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 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.

For information on places to collect in central Oregon, contact

In Southeastern Oregon contact the Bureau of Land Management (BLM), 100 Oregon Street, Vale OR, phone (541) 473-3144

White Fir Springs

Thundereggs from this spot contain both Agate and Jasper filled cores. The cores also have the shape of a diamond (biconic). Head east from Prineville on US 26 to MP 41. Turn left on 3350 and head up the road about 5 1/2 miles ... don't turn on any of the spur roads. Once you reach a sign saying Chamber of Commerce Claim you are at the jasper core locality. For agate filled thundereggs turn right on the road just before you reach the jasper deposit.