Thunderegg Agate

A thunderegg is a rounded nodule, typically golf-ball to softball size, that forms in rhyolitic volcanic ash and lava. From the outside it looks like a drab, lumpy ball of rock, but when sawn open it reveals a star- or angular-shaped core filled with agate, chalcedony, jasper, opal or quartz.

Thunderegg agate refers to the agate-filled variety, where the inner cavity holds banded or fortification agate. The contrast between the unremarkable rind and the colorful core makes thundereggs a beloved cut-and-surprise stone.

The thunderegg is the official state rock of Oregon, where many famous beds occur, and the name comes from Native American legend.

Thundereggs form within spherical pockets called lithophysae that develop in silica-rich rhyolitic lava and welded ash. As the volcanic glass devitrifies and gases expand, star-shaped cavities form, often outlined by radiating cristobalite or feldspar.

Later, silica-rich groundwater percolates through the host rhyolite and fills these cavities, depositing chalcedony and agate layer by layer. The angular, star-like outline of the original cavity gives the agate core its characteristic shape.

Major thunderegg localities include Oregon (notably the Priday and Richardson beds), other western U.S. states, Mexico, Germany and Argentina, all in rhyolitic volcanic terrains.

Externally, a thunderegg is a roughly spherical, knobby, drab brown-to-gray rhyolite nodule that gives little hint of its interior. The defining feature appears only when cut: a star-shaped or angular agate/chalcedony core, often with banding or a central quartz pocket.

The agate fill has hardness 6.5-7 (scratches glass), waxy to vitreous luster, white streak and translucent edges, while the rhyolite rind is duller and softer-appearing.

Distinguish thundereggs from true geodes, which are typically hollow with inward-pointing crystals; thundereggs are usually solid-filled and have the characteristic star outline within a rhyolite shell.

Thunderegg agate is cut and polished to display the star-shaped core, popular as matched halves, slices, cabochons and display specimens. The element of surprise makes them favorites at rock shops and for cutting demonstrations.

The agate cores are hard and durable enough for cabochons and jewelry, while whole and halved eggs are sold as decorative and educational pieces.

Thundereggs hold cultural significance as the Oregon state rock, named from a Native American legend that thunder spirits hurled the stones; metaphysically agate is regarded as grounding, though these are traditional beliefs.