(pendant only - chain sold separately)
According to legend, thundereggs were named by the Native Americans of central Oregon. The Indians of this region are said to have believed 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.
Reference - http://www.statesymbolsusa.org/symbol-official-item/oregon/state-mineral-rock/thunderegg