Thundereggs are the official state rock of OregonA thunderegg is a spherical lumpy rock-like object containing agate, jasper, opal, or quartz crystals.
Typically, they range from 2 to 6 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.
Formation of Thundereggs
Thundereggs formed within gas or steam pockets of the rhyolite lava bed that served as molds. These hollow cavities are later filled with a silica-rich solution that gradually solidifies, crystallizes, and forms layers. Silica fluids adhering to 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 is believed to come from Native American legends in Central Oregon, and according to folklore, thunder spirits or Thunderbird beings created these stones during storms.
One story tells of supernatural beings living near Mount Jefferson and Mount Hood who threw these “eggs” during thunderstorms, leaving them scattered across the landscape.
The term “thunderegg” was first recorded in print by J. Lewis Renton of Portland 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. Over time, the tuff has decayed into soft mud and clay, making thundereggs visible.
Value of 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. Over time, the tuff has decayed into soft mud and clay, making thundereggs visible.
Thundereggs occur in several western U.S. states, but the most famous collecting sites are in Oregon, especially in central and eastern regions. Oregon is well known for both public and fee-based collecting locations, which is why it designated the thunderegg as its official state rock.
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Reference
Renton, J. L., 1936, Opal or agate filled "Thunder eggs": The Mineralogist, v. 4, no. 1, p. 12.