sedimentary

Lake Superior Agate

Silicon Dioxide (Chalcedony variety)

AI-generated identification · may be incorrect

Hardness of 6.5 to 7 on the Mohs scale, exhibits distinct concentric banding patterns in red, brown, and yellow tones, vitreous to waxy luster, microcrystalline (cryptocrystalline) crystal structure, conchoidal fracture with no true cleavage, and a specific gravity around 2.6

Identified July 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Keweenawan rifting event in the Midcontinent Rift System, where mineral-rich hydrothermal fluids filled gas cavities in basaltic lava flows

Uses & applications

Popularly used as a gemstone in jewelry, lapidary art, and highly sought after by rockhounds and mineral collectors

Geological facts

Lake Superior agates are the official gemstone of the US state of Minnesota, and their characteristic red and orange banding is caused by oxidized iron within the ancient volcanic environment

Field identification & locations

Identified primarily by its bold, concentric fortification bands and rich iron-oxide coloration; commonly found in glacial drift, gravel pits, and along the shores of Lake Superior and surrounding Midwestern US states