mineral
Lake Superior Agate
Chalcedony (SiO2 with Iron inclusions)
Hardness: 6.5-7 (Mohs); Color: Red, orange, brown with white bands; Luster: Vitreous to waxy; Crystal structure: Hexagonal/Trigonal (Cryptocrystalline); Conchoidal fracture.
- Hardness
- 6
- Color
- Red, orange, brown with white bands
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Red, orange, brown with white bands; Luster: Vitreous to waxy; Crystal structure: Hexagonal/Trigonal (Cryptocrystalline); Conchoidal fracture.
Formation & geological history
Formed in gas vesicles (voids) of basaltic lava flows about 1.1 billion years ago during the Midcontinent Rift. Glacial activity later dispersed these nodules throughout the Midwest region.
Uses & applications
Highly prized by collectors; used for lapidary work, jewelry making (cabochons), and as decorative display specimens.
Geological facts
The Lake Superior Agate was designated as the state rock of Minnesota in 1969. Some are found deep in glacial till, while others are found along the shores of the Great Lakes.
Field identification & locations
Identified by concentric banding, a glassy luster when wet or polished, and a characteristic waxy or pitted 'peel' on the exterior. Common in gravel pits and beaches in Minnesota, Wisconsin, and Michigan.
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Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Schist
Schist
Metamorphic
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Gypsum (variety Selenite or Alabaster)
Calcium sulfate dihydrate (CaSO4·2H2O)
Mineral
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Nephrite
Mineral/Rock
Banded Gneiss (River Rock/Pebble)
Gneiss (specifically a banded variety often found as a river rock)
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