mineral

Agate Geode / Thunderegg

Chalcedony (cryptocrystalline variety of Quartz), SiO2

Hardness: 6.5-7 (Mohs scale); Color: Gray, white, and translucent blue-gray bands with clear to smoky quartz interior; Luster: Vitreous to waxy; Crystal structure: Hexagonal/Trigonal; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.60-2.65.

Hardness: 6
Luster: Vitreous to waxy

Identified March 27, 2026More mineral →

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Physical properties

Formation & geological history

Formed in volcanic host rocks (rhyolite or basalt) where gas bubbles or cavities were filled over millions of years by silica-rich groundwater. As layers of silica deposited, they formed the characteristic banding of agate, with the center occasionally hosting macro-crystalline quartz.

Uses & applications

Commonly used in jewelry (cabochons), lapidary art, paperweights, and as collector specimens. Historically used for making precision bearings and mortars/pestles due to its hardness.

Geological facts

This specimen is often referred to as a 'Thunderegg' if it originated from certain rhyolitic lava flows. Legend among Native American cultures (Warm Springs) says these were 'eggs' thrown by thunder spirits during storms.

Field identification & locations

Identify in the field by looking for rough, spherical, or nodular stones with a 'cauliflower' or warty exterior texture that feels heavier than a normal rock. Common locations include Brazil, Uruguay, and the western United States (Oregon/Arizona). collectors look for intact crystalline centers and high-contrast banding after cutting and polishing.

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