Thunderegg Agate Identification Guide
Identify thundereggs by their rhyolite rind and agate-filled core, and distinguish them from geodes, nodules, and plain agate.
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What Thunderegg Agate Looks Like
A thunderegg is a nodule that forms in rhyolitic volcanic ash/lava with a knobby rhyolite (or tuff) outer rind and an interior cavity filled with agate, chalcedony, jasper, quartz, or opal. Unsliced, it looks like a drab brown ball; sliced, it reveals a star-shaped or angular agate core.
- Color: dull brown, tan, or gray exterior; interior shows banded agate, chalcedony, and quartz in many colors.
- Luster: rough/earthy outside; waxy to vitreous on the cut interior.
- Transparency: opaque rind; translucent agate core.
- Form: roughly spherical to lumpy nodules, typically golf-ball to softball size; interior often shows a characteristic 4- or 5-pointed angular pattern.
Step-by-Step Field ID Checklist
- Recognize the rind. A knobby, warty rhyolite/tuff exterior on a roundish nodule is the key clue.
- Heft and ring. Solid (agate-filled) thundereggs feel dense; hollow ones (geode-like) may be lighter or rattle.
- Cut or look at a sawn face. A solid agate/chalcedony fill, often with an angular star pattern, confirms a thunderegg (versus a hollow crystal-lined geode).
- Hardness. The agate interior is Mohs 7—scratches glass; the rhyolite rind is softer and granular.
- Streak. White (agate).
- Setting. Weathering out of rhyolite/welded tuff beds.
Key Diagnostic Tests
- Hardness: interior ~7 (chalcedony/quartz); rind softer.
- Streak: white (core).
- Fracture: conchoidal in the agate core.
- Acid: core does not fizz; any calcite fill or carbonate matrix may react.
- Density: moderate to high if solidly agate-filled.
Common Look-Alikes
- Geode: also a round nodule, but a geode is typically hollow with a crystal-lined cavity (often quartz/calcite), whereas a thunderegg is usually solid-filled with agate and has a rhyolite rind. Hollowness and a crystal-lined interior point to a geode.
- Agate nodule: lacks the distinct knobby rhyolite rind; a thunderegg has a clear ash/rhyolite shell.
- Concretion: sedimentary, lacks the agate/chalcedony fill and rhyolite host.
- Plain river agate: translucent throughout with no rough volcanic rind.
Where It Is Found
Thundereggs form in silicic volcanic rocks (rhyolite tuffs). Famous localities include Oregon (the state rock of Oregon), especially the Ochoco Mountains and Richardson's Rock Ranch, plus Idaho, Arizona, Utah, Mexico (Chihuahua), Germany, and Argentina.
Frequently asked questions
How can you tell if a rock is a thunderegg?
Look for a roughly spherical nodule with a knobby rhyolite or tuff rind weathering out of volcanic ash beds. When cut, a thunderegg reveals a solid agate or chalcedony core, often with an angular star-shaped pattern.
What is the difference between a thunderegg and a geode?
A geode is usually hollow with a crystal-lined cavity, while a thunderegg is typically solid, filled with agate or chalcedony, and has a distinctive rhyolite rind. Some thundereggs do have small cavities, but the agate fill and volcanic rind are the giveaway.
What does a thunderegg look like inside?
Cut thundereggs reveal banded agate, chalcedony, jasper, or quartz, frequently in a four- or five-pointed angular star pattern, in many colors.
Where are thundereggs found?
They form in rhyolitic volcanic rocks. Oregon is the most famous source (thundereggs are Oregon's state rock), along with Idaho, Arizona, Mexico, Germany, and Argentina.
Thunderegg Agate identified by the community
Recent Thunderegg Agate specimens identified with Rock Identifier.
