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Thunderegg Specimens
A thunderegg is a nodule-like geological structure, similar to a geode, that is formed within a rhyolitic
lava flow. 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 in the form of agate, jasper, or opal either
uniquely or in combination. Also frequently encountered are quartz and selenite 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 colors. 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 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.
Thundereggs are found all over the world wherever the conditions are right. The USA, Oregon especially,
remains one of the most famous thunderegg locations. Germany is also an important centre for thunderegg agates (especially
sites like St Egidien and Gehlberg). Other countries known for their thundereggs include some places in Africa, Poland, Romania,
Turkey, Mexico, Argentina, Canada, Australia and France.
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Nice thunderegg specimen from the classic location of Pony Creek, in Oregon. With a solid blue-grey agate center,
this thunderegg has been halved and polished. Approx. sizes: 47x45x23 & 46x45x24 Total weight: 101.8 grams
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Fascinating polished thunderegg half from the classic location of Succor Creek, Oregon. Approx. size: 49x49x28
Weight: 65.2 gm.
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