igneous

Thunderegg (specifically a Jasper/Agate-filled Geode nodule)

Silicon Dioxide (SiO2) within Rhyolitic Matrix

Hardness: 6.5-7 Mohs; Color: Tan/gray matrix with tan, mahogany, and black interior jasper; Luster: Waxy to vitreous (interior), Dull (exterior); Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 6
Luster: Waxy to vitreous (interior), Dull (exterior)

Identified March 11, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed in gas pockets (vesicles) within rhyolitic volcanic ash or lava flows. Over time, silica-rich groundwater percolates through the rock, depositing layers of chalcedony, jasper, or opal. These typically date to the Cenozoic era.

Uses & applications

Used primarily as lapidary material for jewelry (cabochons), paperweights, and as popular display specimens for mineral collectors.

Geological facts

Thundereggs are the state rock of Oregon, USA. Legend has it they were thrown by thunder spirits (Thunderbirds) during storms. Unlike regular geodes, they are usually solid rather than hollow.

Field identification & locations

Identify by their rough, cauliflower-like brownish-gray exterior and star-shaped or irregular interior patterns when cut. Commonly found in Oregon, Idaho, and Germany. Collectors look for high-contrast colors and unique interior geometry.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral