mineral

Agate (Thunderegg segment)

Silicon dioxide (SiO2) - Microcrystalline Quartz/Chalcedony

Hardness: 6.5–7 on the Mohs scale; Color: Translucent white to grey-blue agate core with a brown/green rhyolite exterior; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.60–2.65.

Hardness: 6
Luster: Waxy to vitreous

Identified March 11, 2026More mineral →

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Physical properties

Formation & geological history

Formed in volcanic ash layers when gas bubbles or cavities in rhyolitic lava are filled with silica-rich groundwater. Over millions of years (often from the Eocene to Pliocene epochs), the silica crystallizes into agate or chalcedony modules.

Uses & applications

Used primarily for jewelry (cabochons), lapidary arts, decorative bookends, and collection specimens. Some industrial use in precision instruments due to its hardness and resistance to chemicals.

Geological facts

This specimen appears to be a 'Thunderegg', which is the state rock of Oregon. According to Native American legend, these modules were thrown by the Thunder Spirits living in the craters of Mt. Hood and Mt. Jefferson.

Field identification & locations

Identify in the field by looking for lumpy, potato-like nodules within volcanic ash beds. When cut, they reveal a characteristic star-shaped or solid agate core. Found commonly in Oregon (USA), Germany, and Australia.

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