mineral

Lake Superior Agate

Silicon Dioxide (SiO2) - Banded Chalcedony

Hardness: 6.5-7 on Mohs scale; Color: Red, orange, yellow, and white banding due to iron impurities; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.6-2.65.

Hardness: 6
Luster: Waxy to vitreous

Identified July 14, 2026More mineral →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Midcontinent Rift System. Silica-rich groundwater deposited layers of chalcedony within gas vesicles (bubbles) in basaltic lava flows.

Uses & applications

Primarily used for lapidary work, jewelry (cabochons), and as a highly sought-after collectible specimen. It is the official state gem of Minnesota.

Geological facts

The distinctive 'fortification' banding resembles the aerial view of a medieval fortress. They were spread across the Midwest by glaciers during the last Ice Age, meaning they can be found hundreds of miles from their volcanic source.

Field identification & locations

Identify by looking for waxy luster, translucent bands, and a pitted basalt 'wrapper' or husk. Commonly found on beaches, in gravel pits, and along riverbeds in Minnesota, Wisconsin, and Michigan.

Other mineral specimens

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Sandstone with Mineral Veining or Crust

Sedimentary Sandstone (mostly SiO2 with Fe2O3 tinting)

sedimentary

Schist

Metamorphic

Potassium Feldspar (Orthoclase)

Orthoclase (KAlSi3O8)

mineral

Gypsum (variety Selenite or Alabaster)

Calcium sulfate dihydrate (CaSO4·2H2O)

Mineral

Epidote

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

metamorphic