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.

Hardness

6.5-7

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineralcommonQuartz family

Priday Thunderegg

Hardness: 6.5-7
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this priday thunderegg?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try 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.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: red, brown, white, gray, blue.
5
Look at form & habit
Crystal system: trigonal. Typical habit: nodular.

Often confused with

Priday Thunderegg vs. its common look-alikes, and how to tell them apart in the field.

mineral

Agate

Mohs 6.5-7

How to tell apart: Luster reads vitreous on Priday Thunderegg and waxy on Agate.

How to tell apart: Luster reads vitreous on Priday Thunderegg and waxy on Jasper.

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
Colors: Red Brown White Gray Blue
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Trigonal
Crystal habit: Nodular
Cleavage: None
Rarity: Common
Uses: Lapidary, Collector, Decorative
Host rock: rhyolite

Where rockhounds find priday thunderegg

Classic worldwide localities

Jefferson County, Oregon, USA

Field-hunting tip

Where it forms: rhyolite. If you start seeing quartz, calcite, opal in float, you are in the right ground. Typical habit: nodular. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify priday thunderegg?+

Mohs hardness is 6.5-7. Luster: vitreous. The streak is white. Common colors include red, brown, white, gray.

Where is priday thunderegg found?+

Notable localities include Jefferson County, Oregon, USA.

What rocks look like priday thunderegg?+

Priday Thunderegg is most often confused with Agate, Jasper, Common Opal. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with priday thunderegg?+

Priday Thunderegg commonly co-occurs with Quartz, Calcite, Opal. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does priday thunderegg form in?+

Where it forms: rhyolite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is priday thunderegg used for?+

Priday Thunderegg is used in lapidary, collector, decorative.

Find priday thunderegg on the map

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