mineral
Lake Superior Agate
Chalcedony (Silicon Dioxide, SiO2) with Iron impurities
Hardness: 7 on Mohs scale; Color: Red, orange, and tan bands from iron; Luster: Vitreous to waxy; Crystal structure: Microcrystalline (trigonal); Cleavage: None; Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Red, orange, and tan bands from iron
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Red, orange, and tan bands from iron; Luster: Vitreous to waxy; Crystal structure: Microcrystalline (trigonal); Cleavage: None; Specific Gravity: 2.6-2.7
Formation & geological history
Formed approximately 1.1 billion years ago during the Midcontinent Rift system. Silica-rich groundwater filled gas cavities in basaltic lava flows, precipitating layers of chalcedony. Glaciers later dispersed them across the Upper Midwest.
Uses & applications
Highly popular for lapidary work, jewelry (cabochons), and as a state gemstone for Minnesota. Many are kept as raw specimens by collectors.
Geological facts
The Lake Superior Agate is the official state rock of Minnesota. Its unique red and white banding is caused by the oxidation of iron that occurred during the rock's formation over a billion years ago.
Field identification & locations
Identified by concentric 'fortification' banding and a translucent waxy appearance when wet. Commonly found on the shores of Lake Superior, in gravel pits, and glacial till across Minnesota, Wisconsin, and Iowa.
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