Thunderegg; from Succor Creek, Oregon Photo by RhyoR/Flickr (Noncommercial Use Permitted with Attribution).
Official State Rock of Oregon
Oregon designated the thunderegg as the official state rock in 1965 after the rockhounds of Oregon voted it as their first choice. All State Rocks
Found chiefly in Crook, Jefferson, Malheur, Wasco and Wheeler counties of Oregon, each thunderegg is unique in pattern, design and color. Cut and polished, thundereggs reveal exquisite designs. They are a wonder of nature and are sought after and highly prized around the globe.
According to legend, thundereggs were named by the Native Americans of central Oregon, who are said to have believed that these strange, agate-filled stones were thrown by fighting "thunder spirits" who dwelt on Mount Jefferson and Mount Hood (two of several snow-capped peaks high in the Cascade Range). The Native Americans believed these rival, jealous gods hurled large numbers of the round-shaped rocks at each other in fury during thunderstorms.
Thundereggs are found in the rhyolite lava flows which spread over Oregon an estimated 60 million years ago during the Eocene Age. Geologists believe the round-shaped thundereggs were formed in gas pockets (bubbles) within the lava, which served as molds.
The cooled bubbles were gradually filled by water percolating through the porous rock carrying rich quantities of silica (quartz). The deposits lined and in many cases filled the cavity, first with the darker matrix material, then the inner core of agate or chalcedony. The beautiful colors were derived from minerals present in the soil.