mineral
Lake Superior Agate
Chalcedony (cryptocrystalline Quartz), SiO2
Hardness: 7 on Mohs scale; Color: Red, orange, and tan bands from iron impurities; Luster: Vitreous to waxy; Crystal structure: Hexagonal (trigonal); Cleavage: None/Conchoidal fracture; Specific gravity: 2.58–2.64
- Hardness
- 7 on Mohs scale
- Color
- Red, orange, and tan bands from iron impurities
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Red, orange, and tan bands from iron impurities; Luster: Vitreous to waxy; Crystal structure: Hexagonal (trigonal); Cleavage: None/Conchoidal fracture; Specific gravity: 2.58–2.64
Formation & geological history
Formed in gas vesicles of lava flows approximately 1.1 billion years ago during the Midcontinent Rift. Silica-rich groundwater filled these cavities, depositing layers of chalcedony over time.
Uses & applications
Highly prized by collectors; used in jewelry, lapidary arts, and as the official state gemstone of Minnesota.
Geological facts
The distinctive red color comes from iron leached from local rocks during the silica deposition. They were spread across the Midwest by glaciers during the last Ice Age.
Field identification & locations
Identify by looking for concentric 'banding' or ring patterns and a waxy texture. Commonly found on beaches of Lake Superior, gravel pits, and riverbeds across Minnesota, Wisconsin, and Iowa.
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