Thunderegg
Thunderegg is igneous (rhyolitic lithophysa with agate core), Rhyolite shell + SiO2 (chalcedony/opal) core. In the hand, it rates 6.5 to 7 core; 6 shell on the Mohs scale, and it runs drab brown knobby rind and blue-gray. Affordable; exceptional blue-cored or plume-cored Oregon eggs reach collector prices.
The geology.
Thunderegg carries the formula Rhyolite shell + SiO2 (chalcedony/opal) core. It belongs to the not applicable (rock with chalcedony core) system. Catalogues list it as igneous (rhyolitic lithophysa with agate core).
It sits at 6.5 to 7 core; 6 shell on the Mohs scale. The surface reads dull rind; waxy polished core. Light behaviour is opaque rind, translucent core. Specific gravity runs 2.5 to 2.7. The streak is white (core). It breaks with uneven; core breaks conchoidally.
Knobby, ribbed ball of drab rhyolite that saws open to reveal a star or square shaped agate, opal, or chalcedony center, often with banding or moss.
It reaches buyers as cut and polished halves, whole unopened nodules or bookends.
The origins.
Rhyolite ash-flow tuffs; Oregon (USA) is the world capital, also New Mexico, Germany, Poland, and Mexico.
On value, the database notes: affordable; exceptional blue-cored or plume-cored Oregon eggs reach collector prices.
Traditional associations.
In energy work traditions, associated with hidden inner gifts and finding beauty beneath a rough exterior.
We keep this section because people ask. It sits below the geology on purpose, and none of it describes something the stone does.
Spotting the real thing.
Knobby, ribbed ball of drab rhyolite that saws open to reveal a star or square shaped agate, opal, or chalcedony center, often with banding or moss.
Unopened 'guaranteed crystal' eggs are a gamble by nature; some sellers glue polished agate slices onto plain halves, check the seam.
| Looks like | How to tell them apart |
|---|---|
| Plain agate nodule | Agate nodules have smooth chalcedony rinds; thundereggs wear a thick knobby rhyolite jacket with a star-shaped core outline |
| Geode | Geodes are hollow with crystal-lined cavities; most thundereggs are solid-centered |
| Concretion | Concretions are sedimentary and often fizz or shed sandy layers; thunderegg shells are hard volcanic rhyolite |
At-home tests.
Before anyone talks you into a lab report, these are the checks worth doing yourself on thunderegg.
| Test | What you should see |
|---|---|
| Scratch check | Core scratches glass (Mohs 6.5 to 7) |
| By eye | Cut face shows characteristic star or angular core outline |
| By eye | Rind is ribbed and knobby, unlike smooth river cobbles |
If these checks leave you unsure, that is what appraisers are for. Do not file, heat or soak a piece to find out.
Care & handling.
At 6.5 to 7 core; 6 shell on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with uneven; core breaks conchoidally, which is worth knowing before you set it or drop it.
Most damage happens during cleaning, not wearing. Avoid steamers, ultrasonic tanks and anything from under the sink.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is Rhyolite shell + SiO2 (chalcedony/opal) core. Crystal system: Not applicable (rock with chalcedony core). Specific gravity: 2.5 to 2.7. Streak: White (core). Cleavage and fracture: Uneven; core breaks conchoidally.
How it reaches the market
Rhyolite ash-flow tuffs; Oregon (USA) is the world capital, also New Mexico, Germany, Poland, and Mexico.
What it gets confused with
Plain agate nodule: Agate nodules have smooth chalcedony rinds; thundereggs wear a thick knobby rhyolite jacket with a star-shaped core outline. Geode: Geodes are hollow with crystal-lined cavities; most thundereggs are solid-centered. Concretion: Concretions are sedimentary and often fizz or shed sandy layers; thunderegg shells are hard volcanic rhyolite.
What it is worth
Affordable; exceptional blue-cored or plume-cored Oregon eggs reach collector prices.
Where buyers get caught
Unopened 'guaranteed crystal' eggs are a gamble by nature; some sellers glue polished agate slices onto plain halves, check the seam.
One more thing
Oregon made the thunderegg its state rock in 1965; Warm Springs legend says thunder spirits on Mount Hood and Mount Jefferson hurled the eggs at each other during storms.
An education-first guide.
There is nothing to buy on this page. Thunderegg is not part of our catalogue, and the guide is here anyway because somebody searching the name deserves a straight answer.
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