mineral
Lake Superior Agate
Silicon Dioxide (SiO2) - Banded Chalcedony
Hardness: 6.5 - 7 on the Mohs scale; Color: Red, orange, white, and tan concentric banding; Luster: Waxy to vitreous; Crystal system: Trigonal/Microcrystalline; Cleavage: None; Conchoidal fracture.
- Hardness
- 6
- Color
- Red, orange, white, and tan concentric banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5 - 7 on the Mohs scale; Color: Red, orange, white, and tan concentric banding; Luster: Waxy to vitreous; Crystal system: Trigonal/Microcrystalline; Cleavage: None; Conchoidal fracture.
Formation & geological history
Formed during volcanic activity about 1.2 billion years ago in the Midcontinent Rift System. Iron-rich silica-bearing groundwater filled gas cavities (vesicles) in ancient basalt lava flows, slowly precipitating chalcedony layers.
Uses & applications
Popular for lapidary work, jewelry (cabochons), tumbling, and mineral specimen collecting. Minnesota's official state gemstone.
Geological facts
Glaciers dispersed Lake Superior agates across Minnesota, Iowa, Wisconsin, and Michigan during the Ice Age, so they are frequently found far from their original source basalt beds.
Field identification & locations
Look for a pitted, iron-stained husk with concentric red, orange, and white bands exposed on worn surfaces or translucent edges under light. Commonly found in glacial till, riverbeds, and gravel pits in the Upper Midwest USA.
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