mineral

Lake Superior Agate

Banded Chalcedony (SiO2)

Hardness: 6.5-7 Mohs; Color: Iron-rich red, orange, and tan with white or translucent bands; Luster: Vitreous to waxy; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.60-2.65

Hardness: 6
Luster: Vitreous to waxy

Identified June 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Keweenawan Rift when gas bubbles in basaltic lava flows filled with silica-rich groundwater. Over time, heavy iron-rich impurities deposited in layers creating the characteristic bands. Glacial activity later dispersed them throughout the Midwest.

Uses & applications

Primarily used for jewelry (cabochons), lapidary art, and as a highly prized collectible specimen. It is the official state gemstone of Minnesota.

Geological facts

Lake Superior Agates are among the oldest agates in the world. They were distributed across Minnesota, Iowa, Wisconsin, and Michigan by the movements of the Superior Lobe glacier during the last ice age.

Field identification & locations

Identify in the field by looking for a waxy 'glassy' texture, translucent skin when wet, and the characteristic 'fortification' banding. Often found in gravel pits, riverbeds, and on the shores of Lake Superior.