sedimentary

Lake Superior Agate

Silicon Dioxide (Chalcedony/Quartz), SiO2

Hardness of 6.5 to 7 on the Mohs scale, waxy to vitreous luster, cryptocrystalline crystal structure, conchoidal fracture with no cleavage, specific gravity of approximately 2.60 to 2.65, and typically displays distinctive red, orange, and brown banding caused by iron oxide staining.

Identified July 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Keweenawan Rift Zone event, where hydrothermal fluids deposited silica-rich solutions into gas cavities in basaltic lava flows, later freed by weathering and glacial action.

Uses & applications

Widely used in jewelry, lapidary arts, and as a prized collectible specimen for rockhounds.

Geological facts

Lake Superior agates are the official state gemstone of Minnesota and are famous for their rich, iron-oxide induced red and orange bands, which reflect the ancient volcanic history of the region.

Field identification & locations

Can be identified in the field by its waxy luster, distinct concentric color banding, conchoidal fracture, and resistance to weathering. Commonly found in glacial drift, gravel pits, and along the shores of Lake Superior and surrounding Midwestern states.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sedimentary Sandstone

Arenite (Sedimentary Rock)

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Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

fossil

Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

sedimentary

Shale (or Mudstone/Siltstone)

Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)

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Shale

Shale (clastic sedimentary rock)

sedimentary