Agate (Chert/Chalcedony)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, tan, and cream with translucent areas; Luster: Waxy to vitreous; Crystal structure: Microcrystalline/Cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65.

Formation & Geological History

Formed in cavities of volcanic or sedimentary rocks where silica-rich groundwater precipitates over long periods. Often found in riverbeds or glacial tills as weathered nodules.

Uses & Applications

Used primarily in lapidary arts, jewelry making, ornamental carvings, and historical tool making (knapping). Industrial uses include precision mortars and pestles.

Geological Facts

Agates are essentially banded chalcedony. The specimen shown exhibits a pitted surface typical of Lake Superior Agates or similar glacial drift agates where softer matrix has eroded away.

Field Identification & Locations

Identify in the field by looking for a waxy luster, translucency when held to light, and characteristic surface pitting or 'gas bubbles'. Commonly found in gravel pits and shorelines across the Upper Midwest USA.

Identified on: 4/30/2026

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