mineral
Lake Superior Agate
Silicon Dioxide (SiO2)
AI-generated identification · may be incorrectHardness: 7 on Mohs scale; Color: Red, orange, white, and translucent gray; Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline chalcedony); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64
- Hardness
- 7 on Mohs scale
- Color
- Red, orange, white, and translucent gray
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Red, orange, white, and translucent gray; Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline chalcedony); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64
Formation & geological history
Formed roughly 1.1 billion years ago during the Midcontinent Rift. Silica-rich groundwater deposited layers of chalcedony within gas vesicles (bubbles) in Precambrian basaltic lava flows. Glacial activity later eroded these from the basalt and distributed them across the Midwest.
Uses & applications
Primarily used for lapidary work, jewelry (cabochons), and as a highly sought-after collectible specimen. It is the official state gem of Minnesota.
Geological facts
Lake Superior Agates are known for their rich iron-oxide staining which creates vibrant red and orange colors. Some specimens exhibit 'eye' patterns or 'fortification' banding that mimics the shape of a fortress.
Field identification & locations
Identify in the field by looking for a waxy translucence, banded patterns, and a 'pitted' husk or peel. Commonly found on beaches of Lake Superior, in gravel pits, and in glacial till across Minnesota, Wisconsin, and Michigan.
