Thundereggs - Rough Rock with Unexpected Surprise Inside

Thunderegg is a spherical lumpy rock-like object containing agate, jasper, opal, or quartz crystals. Typically, thundereggs range from two to six inches in diameter, although some can be as large as one to three feet and weigh over a ton. Their exterior surface varies from white-gray to reddish-brown and has a clay appearance with ridges.

Thundereggs Formation

Thundereggs formed within gas or steam pockets of rhyolite lava bed that served as molds. These hollow cavities were filled with a silica-rich solution that later solidified, crystallized, and layered. Silica fluids adhering with tuff built a shell around the chalcedony core.

Hollow thunder eggs are known as geodes but not all geodes are thundereggs.

Why are They Called Thundereggs?

The name of those unique rocks may come from the thunderbirds of Native American folklore. These supernatural spirits have a significant presence in the stories, history, and culture of the First People. Native Americans of Central Oregon believed that thundereggs were the eggs of thunderbirds, giving the mineral its unique name.

According to legend, spirits living on Mount Jefferson and Mount Hood waged war against each other, launching thundereggs in rage during thunderstorms. As a result, these "supernatural projectiles" can now be found and collected.

J. Lewis Renton from Portland first used this term in a publication in 1936.

How to Spot Thundereggs?

Thundereggs can be easily distinguished from other rocks by their round and lumpy exteriors. Generally, thundereggs lie relatively close to the ground, embedded in the tuff clay. Throughout the time, the tuff has been decayed to soft mud and clay, making thundereggs visible.

What is the value of thundereggs?

The price of polished thundereggs can range from a few to hundreds of dollars. The highest prices, up to $200, are for specimens with moss or plumes that resemble plants or flowers.