Thunderegg

A knobbly rhyolite nodule that, cut open, reveals a star-shaped core of agate or jasper.

Thunderegg specimen — Photo: David Rix Eibonvale, CC BY 3.0, via Wikimedia Commons

How to identify thunderegg

Baseball-sized, bumpy brown nodule.
Star-shaped agate center when sawed.
Heavy and solid (unlike hollow geodes).

Often confused with

Geode
Geodes are hollow with crystal-lined insides.
Compare side by side
Agate
Agate is the filling; the thunderegg is the whole nodule.
Compare side by side

Where it's found

Central Oregon
Queensland, Australia
Chihuahua, Mexico
Germany

Uses

Cut and polished as display pieces.

Rock tumbling

Not suitable for tumbling. Usually sawn and polished flat instead.

Did you know?

It's Oregon's state rock; local legend says thunder spirits threw them at each other.

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