sedimentary
Lake Superior Agate
Chalcedony (Microcrystalline Quartz, SiO2) with iron impurities
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
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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.
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