Thundereggs are spherical, nodular structures found within rhyolitic volcanic ash flows that contain centers of agate or chalcedony. They appear as plain, weathered rocks on the outside but reveal colorful, patterned, or banded agate interiors when cut and polished. They are highly prized by collectors for their intricate internal formations and unique locality-specific color patterns.
Is this thundereggs?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch thundereggs with a known reference. Thundereggs sits at Mohs 6.5-7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Thundereggs leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Thundereggs typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: multicolored, brown, gray, blue, red.
- 5Look at form & habitCrystal system: trigonal. Typical habit: nodular.
Often confused with
Thundereggs vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside thundereggs
Minerals reported to co-occur with thundereggs. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SiO₂
- Mohs hardness
- 6.5-7
- Density
- 2.65 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Nodular
- Cleavage
- None
- Rarity
- Common
- Uses
- Lapidary, Collector, Decorative
- Host rock
- Rhyolite
- Typical price
- $10-100 per nodule depending on size and quality
Where rockhounds find thundereggs
15 mapped spotsClassic worldwide localities
- Oregon, USA
- Germany
- New Mexico, USA
- Ethiopia
- Mexico
U.S. states with thundereggs
Each link opens a state-specific list of mapped rockhounding spots that produce thundereggs.
Field-hunting tip
Look in rhyolite country — that is the host setting where thundereggs typically forms. If you start seeing quartz, chalcedony, opal in float, you are in the right ground. Field specimens usually show a nodular habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Oregon, Arizona, California — start trip planning there.
Common questions
How do you identify thundereggs?+
Where is thundereggs found?+
Can I find thundereggs in the United States?+
How much is thundereggs worth?+
What rocks look like thundereggs?+
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Find thundereggs on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
