Oregon designated the thunder-egg (geode) as the official state rock in 1965 after the rockhounds of Oregon voted it as their first choice. Found chiefly in Crook, Jefferson, Malheur, Wasco and Wheeler counties of Oregon, each geode is unique in pattern, design and color. Cut and polished, thunder-eggs reveal exquisite designs. They are a wonder of nature and are sought after and highly prized around the globe.

According to legend, thunder-eggs were named by the Native Americans of central Oregon. The Indians of this region are said to have believed these strage, 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 geodes 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.

Source:

State Symbols: Oregon Blue Book
Thunder-Egg Photos & Facts: Richardson's Rock Ranch, OR

Links:

Geological Symbols - 50 States List