Half a thunderegg!

A Thunderegg is a geological structure similar to a geode but with unique characteristics. Here's an overview:

Formation: Thundereggs form within rhyolitic volcanic ash layers. They start as gas bubbles or steam pockets in the lava, which act as molds. Over time, these are filled with silica-rich fluids, mainly chalcedony, agate, jasper, or opal, creating intricate patterns and colors inside.

Appearance: On the outside, thundereggs look like ordinary, rough-textured rocks, often with a white-gray to reddish-brown surface. However, when cut and polished, they reveal a variety of internal structures, from solid agate or chalcedony cores to sometimes hollow centers with crystals.

Size: They can range from small, about the size of a baseball, to larger specimens over a meter across, though most commonly they are between two to six inches in diameter.

Locations: While Oregon, USA, is particularly famous for thundereggs, especially in counties like Crook, Jefferson, Malheur, Wasco, and Wheeler, they are found globally wherever the geological conditions are right. Other notable locations include Germany, Australia, Brazil, Canada, Mexico, Poland, Romania, and Turkey.

Cultural Significance: In Oregon, the thunderegg was designated as the state rock in 1965, reflecting its importance in local geology and culture. Native American legends attribute their formation to thunder spirits or gods throwing these rocks during thunderstorms.

Collection and Use: Thundereggs are popular among rock collectors for their unique beauty once cut open. They're often used in jewelry making or as display pieces due to their aesthetic appeal after being sliced and polished.

Variations: Each thunderegg is unique due to the minerals present in the area where it forms, leading to a wide variety in color, pattern, and structure even from the same bed.