Lake Superior Agate

Physical Properties

Hardness: 7 (Mohs scale); Color: Red, orange, yellow with translucent banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.60–2.64.

Formation & Geological History

Formed approximately 1.1 billion years ago during the Keweenawan Rift. Gas bubbles trapped in cooling basaltic lava flows were later filled by silica-rich groundwater, depositing minerals in layers.

Uses & Applications

Highly valued by lapidaries for jewelry (cabs), gemstone collecting, and as a popular state rock for Minnesota.

Geological Facts

Lake Superior Agates are renowned for their rich red and orange colors, which are caused by iron oxidation. They were spread throughout the Midwest by the movement of glaciers during the Ice Age.

Field Identification & Locations

Identify by looking for 'peeled' surfaces that reveal concentric banding or a waxy, translucent appearance when wet. Commonly found on beaches, gravel pits, and riverbeds in the Lake Superior region and glaciated areas of the US Midwest.

Identified on: 4/13/2026

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