Stephenson Mountain, located in eastern Oregon near the Oregon–Idaho border, is a lesser-known thunderegg site that has yielded nodules with interesting and varied agate interiors. Thundereggs from this area are typically medium-sized and feature agate in soft grays, whites, and muted tans, occasionally with moss or dendritic inclusions. The rhyolite matrix is often dense and weathered, with some specimens showing a rough, broken surface typical of high desert exposures. While not as prolific or colorful as some of Oregon’s more famous beds, Stephenson Mountain thundereggs are appreciated by collectors for their natural character and occasional surprises when cut.
Rocks of the Cascades – Thundereggs, Tools & Tradition