Agate (specifically a Thunderegg/Agate Nodule)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Variegated brown, grey, and translucent white banding; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58–2.64

Formation & Geological History

Formed within cavities in volcanic rocks (rhyolite or basalt) from silica-rich groundwater. Over millions of years, the silica precipitates out in layers, creating the characteristic banding. Most specimens date back to the Cenozoic era or earlier.

Uses & Applications

Used primarily for jewelry (cabochons), lapidary art, collection, and industrial abrasives. High-quality nodules are often polished or sliced for display.

Geological Facts

Thundereggs are the 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 battles. The 'eye' or center of this specimen contains a small cavity or druzy quartz center.

Field Identification & Locations

Identify in the field by their rough, cauliflower-like exterior contrasted with a glassy, banded interior when broken. Commonly found in areas with past volcanic activity like Oregon, Idaho, and Arizona.

Identified on: 4/19/2026

Mode: Standard