A thunderegg is a type of rock. They are like a geode but formed in a rhyolitic lava flow. They are rough spheres, most about a big as a baseball. They look uninteresting on the outside, but when sliced in half may be stunning.

They are found only in areas of volcanic activity.

It is the state rock of the American state of Oregon. According to native legend, the Thunder Spirits lived at the highest reaches of Mount Hood and Mount Jefferson, and when they became angry at each other, they threw these shperical rocks at each other.

They are of value in making jewelry.

External links

• naturenw.org on thundereggs
• USGS on thundereggs