mineral

Lake Superior Agate

Silicon Dioxide (SiO2) with iron impurities

Hardness: 7 (Mohs scale), Color: Rich red, orange, and brown (iron-stained), Luster: Vitreous to waxy when wet/polished, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.60-2.65

Identified June 18, 2026More mineral →

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Physical properties

Formation & geological history

Formed in gas vesicles (bubbles) of basaltic lava flows approximately 1.1 billion years ago during the Midcontinent Rift. Silica-rich groundwater filled these cavities, depositing layers of chalcedony.

Uses & applications

Highly prized by collectors, used in lapidary work for jewelry making (cabochons), and as a state gemstone for Minnesota.

Geological facts

Lake Superior agates are distinguished by their rich red and orange colors, which are caused by iron oxidation. They were spread across the Midwest by glaciers during the last ice age.

Field identification & locations

Identify by looking for waxy luster, 'peel' marks, or concentric banding on a translucent surface. Frequently found on gravel beaches of Lake Superior and in glacial till throughout Minnesota and Wisconsin.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral