sedimentary

Lake Superior Agate

Chalcedony (Microcrystalline Quartz, SiO2) with iron impurities

AI-generated identification · may be incorrect

Hardness: 7 Mohs; Color: Red, orange, yellow, and brown with concentric banding; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture).

Hardness: 7 Mohs
Color: Red, orange, yellow, and brown with concentric banding
Luster: Waxy to vitreous

Identified July 18, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Red, orange, yellow, and brown with concentric banding; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture).

Formation & geological history

Formed approximately 1.1 billion years ago during the Midcontinent Rift. Gas bubbles in basaltic lava flows were filled by silica-rich groundwater, depositing layers of chalcedony over time.

Uses & applications

Highly prized by collectors, used in lapidary for polishing, tumbling, and making jewelry such as pendants and rings.

Geological facts

The Lake Superior Agate was designated as Minnesota's state gemstone in 1969. The red/orange coloring is caused by iron oxidation (limonite and hematite) present during formation.

Field identification & locations

Identified by concentric banding and a translucent waxy appearance when wet. Commonly found in glacial till and on beaches surrounding Lake Superior, especially in Minnesota and Wisconsin.