igneous

Thunderegg (Agetized Geode nodule)

Rhyolitic Agate Nodule (SiO2 filling volcanic cavities)

Hardness: 6.5-7 (Mohs scale); Color: Brownish-red rhyolite matrix with white/blue/translucent agate or chalcedony core; Luster: Vitreous to waxy in center, dull on exterior; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.55-2.70

Hardness: 6
Luster: Vitreous to waxy in center, dull on exterior

Identified March 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed as gas bubbles or vesicles within rhyolitic lava flows. Over millions of years (typically Cenozoic or older), silica-rich hydrothermal fluids seeped into the cavities, crystallizing into agate or jasper. Found in varying ages, most famous from the Oligocene-Miocene epochs (approx. 20-30 million years ago).

Uses & applications

Primarily used as lapidary material for jewelry, paperweights, and decorative display pieces. They are highly popular among rockhounds and mineral collectors.

Geological facts

Thundereggs are the official state rock of Oregon. According to Native American legend, the spirits of Mt. Hood and Mt. Jefferson would hurl these stones at each other during 'thundery' displays of volcanic anger.

Field identification & locations

Identify in the field by looking for spherical, 'lumpy' nodules with a rough, star-shaped or chaotic internal pattern when cut. Commonly found in Oregon, New Mexico, and Germany. Collectors should look for heavy, round rocks that feel denser than standard basalt or rhyolite chunks.

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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