sedimentary

Thunder Egg (Agate/Jasper filled)

Chalcedony variety Agate/Jasper in a Rhyolitic matrix

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs scale); Color: Brown, yellow, ochre, transluscent grey; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None

Hardness: 6
Color: Brown, yellow, ochre, transluscent grey
Luster: Waxy to vitreous

Identified July 20, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Brown, yellow, ochre, transluscent grey; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None

Formation & geological history

Formed within gas bubbles in rhyolitic volcanic ash layers where silica-rich groundwater deposited minerals over millions of years, typically during the Eocene epoch.

Uses & applications

Commonly used for lapidary work, jewelry making, bookends, and as a popular collector's specimen due to its unique internal patterns.

Geological facts

Thunder eggs are the state rock of Oregon. According to Native American legend, they were stones thrown by the spirits of Mount Hood and Mount Jefferson during thunderstorms.

Field identification & locations

Identified by a rough, drab exterior that hides a colorful, star-shaped or solid core of agate or jasper when sliced. Often found in specific beds within volcanic tuff.