mineral
Lake Superior Agate
Chalcedony (SiO2) with iron inclusions
Hardness: 6.5-7 (Mohs); Color: Iron-red, orange, brown, and cream; Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None; Specific gravity: 2.6
- Hardness
- 6
- Color
- Iron-red, orange, brown, and cream
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Iron-red, orange, brown, and cream; Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None; Specific gravity: 2.6
Formation & geological history
Formed approximately 1.1 billion years ago during the Keweenawan Rift when gas bubbles in basaltic lava flows were filled by silica-rich groundwater. Iron leached from the surrounding rock provided the distinct red and orange banding.
Uses & applications
Used primarily in jewelry (cabochons), lapidary art, and as a popular collector's item due to its unique patterns and history.
Geological facts
The Lake Superior Agate was designated as the official state gem of Minnesota in 1969. It is among the oldest agates in the world, distributed across the Midwest by glaciers during the last Ice Age.
Field identification & locations
Identify by looking for waxy luster, conchoidal fractures, and 'eye' patterns or concentric banding, especially after wetting the stone. Commonly found in gravel pits, riverbeds, and along the shores of Lake Superior.
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