Priday Thundereggs are spherical, nodular volcanic rocks found in rhyolite beds that contain intricate inner fillings of agate, jasper, or quartz crystals. When sliced and polished, they reveal vibrant, often plume-like or mossy patterns that are highly prized by lapidary artists. They are iconic geological specimens specifically associated with the Priday Ranch locality in Oregon.
Is this priday thunderegg?
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 priday thunderegg with a known reference. Priday Thunderegg 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. Priday Thunderegg leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Priday Thunderegg typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: red, brown, white, gray, blue.
- 5Look at form & habitCrystal system: trigonal. Typical habit: nodular.
Often confused with
Priday Thunderegg vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside priday thunderegg
Minerals reported to co-occur with priday thunderegg. 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
- 2.6-2.7 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 specimen
Where rockhounds find priday thunderegg
Classic worldwide localities
- Jefferson County, Oregon, USA
Field-hunting tip
Look in rhyolite country — that is the host setting where priday thunderegg typically forms. If you start seeing quartz, calcite, 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.
Common questions
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Find priday thunderegg on the map
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