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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.
How Thundereggs Form
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
How Thundereggs Look
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
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 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.
For information on places to collect in central Oregon,
Madras Chamber of Commerce, 197 SE 5th St., Madras OR
97741, phone 541-475-2350;
Prineville Chamber of Commerce, 390 North Fairview,
Prineville, OR 97754, phone 541-447-6304,
Richardsonís Recreational Ranch, Gateway Route, Box 440,
Madras, OR 97741, phone 541-475-2680 or 541-475-2839. This
is a commercial site.
Lucky Strike Mine, PO Box 128, Mitchell, OR 97750 Phone:
541-462-3073, Email [email protected]
This is a commercial site.
In Southeastern Oregon contact the Bureau of Land Management
(BLM), 100 Oregon Street, Vale OR 97918, phone (541)
To find out more about Thunderegg Days (held for five days
around the first of June), in Nyssa, click
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 for 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.
Maps and publications about Oregonís Geology and its
geologic treasures are available from the Nature of the
Northwest Information Center, a partnership of the Oregon
Department of Geology and Mineral Industries and the USDA
Forest Service. Nature of the Northwest is open 9-5, Monday
through Friday at:
800 NE Oregon Street #5, Suite 177, Portland Oregon
97232, phone 503-872-2750, fax 503-731-4066.
of the Northwest
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