mineral
Lake Superior Agate
Chalcedony (SiO2 with iron impurities)
Hardness: 7 Mohs; Color: Red, orange, tan, and brown banding; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64
- Hardness
- 7 Mohs
- Color
- Red, orange, tan, and brown banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Red, orange, tan, and brown banding; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64
Formation & geological history
Formed during the Midcontinent Rift system approximately 1.1 billion years ago. Silica-rich groundwater filled gas vesicles in basaltic lava flows, slowly precipitating layers of quartz and iron minerals.
Uses & applications
Widely used for lapidary work, jewelry (cabochons), and highly prized by mineral collectors. Small tumbled stones are popular souvenirs.
Geological facts
The Lake Superior Agate was designated as the official state rock of Minnesota in 1969. The red color characteristic of these agates comes from iron leached from the surrounding volcanic rocks.
Field identification & locations
Identified by concentric banding (eyes), waxy texture when wet, and a distinctive 'pitted' husk. Most commonly found along the shores of Lake Superior, in glacial till, and in gravel pits across the Upper Midwest.
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metamorphic
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Mineral
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Banded Gneiss (River Rock/Pebble)
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