sedimentary

Lake Superior Agate

Silicon Dioxide (Chalcedony / Quartz), SiO2

Hardness of 6.5-7 on the Mohs scale; displays concentric banding in various colors including red, orange, yellow, and white due to iron oxide staining; waxy to vitreous luster; cryptocrystalline structure; conchoidal fracture with no true cleavage; specific gravity of approximately 2.60-2.65.

Identified August 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Keweenawan Rift Zone volcanic activity. Gas cavities in basalt lava flows were filled by hydrothermal groundwater rich in dissolved silica, precipitating layer by layer over millions of years.

Uses & applications

Popular among rockhounds and collectors for lapidary work, tumbling, cabochons, and jewelry making.

Geological facts

Lake Superior agates are the official state gemstone of Minnesota. The vibrant red and orange colors characteristic of these agates are a result of oxidized iron leached from surrounding rocks.

Field identification & locations

Found predominantly in the Upper Midwest of the United States (Minnesota, Wisconsin, Michigan) in glacial drift, gravel pits, and along river beds. Identify in the field by their characteristic waxy luster, conchoidal chipping, and iron-stained parallel color banding.

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Sandstone or Siltstone

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Quartz pebble

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Fossiliferous Concretion

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Sedimentary Sandstone

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Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

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