igneous
Thunderegg (Agate Geode)
Agate/Chalcedony filled Rhyolite nodule (SiO2)
Hardness: 6.5-7 (Mohs), Color: Brownish-red exterior with possible white or translucent interiors, Luster: Dull/Waxy, Crystal Structure: Cryptocrystalline, Specific Gravity: 2.6
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs), Color: Brownish-red exterior with possible white or translucent interiors, Luster: Dull/Waxy, Crystal Structure: Cryptocrystalline, Specific Gravity: 2.6
Formation & geological history
Formed within rhyolitic volcanic ash layers during the Cenozoic era. Gas bubbles or voids in cooling lava were later filled with silica-rich groundwater that crystallized over millions of years.
Uses & applications
Used primarily for lapidary work, decorative cabochons for jewelry, and as a popular collectible for rockhounds. Often sliced and polished to reveal internal patterns.
Geological facts
Thundereggs are the state rock of Oregon. According to Native American legend, they were weapons thrown by the thunder spirits living on Mt. Hood and Mt. Jefferson.
Field identification & locations
Identify by a rough, lumpy, star-shaped exterior often with knobby surfaces. Commonly found in Oregon, Nevada, and New Mexico. They look like ordinary rocks until cut open.
