mineral
Lake Superior Agate
Banded Chalcedony (Silicon Dioxide, SiO2)
Hardness: 7 Mohs. Color: Iron-stained red, orange, and tan with white banding. Luster: Waxy to vitreous. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.60–2.64.
- Hardness
- 7 Mohs
- Color
- Iron-stained red, orange, and tan with white banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs. Color: Iron-stained red, orange, and tan with white banding. Luster: Waxy to vitreous. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.60–2.64.
Formation & geological history
Formed approximately 1.1 billion years ago during the Keweenawan Rift. Silica-rich groundwater deposited layers of quartz inside gas vesicles (bubbles) of cooling basaltic lava flows.
Uses & applications
Highly prized by collectors as a state gemstone, used in lapidary work for cabochons, jewelry, and decorative polished display pieces.
Geological facts
It is the official state gemstone of Minnesota. The distinctive red color is caused by the oxidation of iron that was present during the mineral's formation within volcanic rock.
Field identification & locations
Identify by looking for 'peeling' layers, translucent banding, and a waxy texture. Commonly found on beaches of Lake Superior, in gravel pits, and glacial till across Minnesota, Wisconsin, and Iowa.
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