sedimentary
Lake Superior Agate
Cryptocrystalline Quartz (SiO2) with iron impurities
Hardness: 6.5-7.0 (Mohs). Color: Red, orange, yellow, and brown due to iron oxidation. Luster: Waxy to vitreous. Structure: Microcrystalline. Translucent to opaque with banding.
- Hardness
- 6
- Color
- Red, orange, yellow, and brown due to iron oxidation
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7.0 (Mohs). Color: Red, orange, yellow, and brown due to iron oxidation. Luster: Waxy to vitreous. Structure: Microcrystalline. Translucent to opaque with banding.
Formation & geological history
Formed roughly 1.1 billion years ago during the Midcontinent Rift. Gas bubbles in basaltic lava flows were filled by silica-rich groundwater. Over time, distinct iron-stained layers crystallized to form agates.
Uses & applications
Highly prized by collectors and lapidaries. Used for jewelry (cabochons), polished specimens, and state-symbol collecting (state rock of Minnesota).
Geological facts
The Lake Superior Agate was designated the official state rock of Minnesota in 1969. They are unique for their rich iron-red colors compared to other global agates.
Field identification & locations
Found in glacial drift across the Upper Midwest (Minnesota, Wisconsin, Michigan). Identified by its waxy texture, 'peeled' appearance, and characteristic red-and-white banding when wet or backlit.
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