sedimentary

Lake Superior Agate

Banded Chalcedony (Silicon Dioxide, SiO2)

Hardness: 7 (Mohs scale); Color: Iron-red, orange, tan, and brown bands; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.60-2.65

Hardness: 7 (Mohs scale)
Color: Iron-red, orange, tan, and brown bands
Luster: Waxy to vitreous

Identified July 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Midcontinent Rift System. Silicia-rich groundwater deposited layers of quartz inside gas vesicles (bubbles) of volcanic basalt. Later released by erosion and transported by glaciers.

Uses & applications

Highly sought after by collectors; used in jewelry (cabochons), lapidary art, and as the official state gemstone of Minnesota.

Geological facts

Lake Superior Agates are over a billion years old, making them significantly older than most other agate varieties. They are famous for their rich red and orange colors, which are caused by iron oxidation.

Field identification & locations

Identify by looking for 'peeling' layers, waxy texture, and distinct concentric banding. Commonly found on beaches of Lake Superior and in glacial till throughout Minnesota, Wisconsin, Iowa, and Michigan.

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Cone-in-cone structure (Secondary Sedimentary Structure)

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Brecciated Chert (Microcrystalline Silica)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

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Sandstone

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