mineral

Lake Superior Agate

Cryptocrystalline Quartz (SiO2) with Iron impurities

Hardness: 6.5-7 Mohs; Color: Red, orange, and brown banding due to iron oxidation; Luster: Vitreous to waxy; Crystal structure: Hexagonal/Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

Hardness: 6
Color: Red, orange, and brown banding due to iron oxidation
Luster: Vitreous to waxy

Identified June 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed approximately 1.1 billion years ago during the Midcontinent Rift system. Silica-rich groundwater filled gas vesicles in basaltic lava flows, precipitating layers of quartz and iron over time. Glacial movements later distributed these from the Lake Superior basin throughout the Midwest.

Uses & applications

Highly prized for lapidary work, jewelry (cabochons), and as a state gemstone. It is primarily collected as a specimen or used in decorative arts.

Geological facts

The Lake Superior Agate was designated as the official state gemstone of Minnesota in 1969. Unlike many other agates, its rich red color comes from high concentrations of iron leached from the local volcanic rocks during its formation.

Field identification & locations

Identify in the field by looking for translucent banding, a pitted 'peeled' potato-skin exterior, and rich rusty-red hues. Common in gravel pits, beach shorelines, and glacial till across Minnesota, Wisconsin, and Iowa.

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