mineral

Banded Agate (specifically Lake Superior Agate type)

Silicon Dioxide (SiO2) - variety Chalcedony

Hardness: 6.5-7 on Mohs scale. Color: alternating bands of red, orange, brown, and translucent white/blue-gray. Luster: waxy to vitreous. Crystal structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.58-2.64.

Hardness: 6
Luster: waxy to vitreous

Identified May 30, 2026More mineral →

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Physical properties

Formation & geological history

Formed approximately 1.1 billion years ago during the Keweenawan Rift. Secondary silica-rich water deposited layers of minerals within gas vesicles (bubbles) in cooling volcanic basalt. The distinct colors are caused by varying levels of iron oxidation during formation.

Uses & applications

Commonly used in jewelry (cabochons, beads), decorative carvings, and as a popular collector's item. In industry, due to its hardness and chemical resistance, chalcedony is sometimes used for mortars and pestles.

Geological facts

Lake Superior Agate is the official state gem of Minnesota. Unlike many other agates, these were distributed across the Midwest by glacial movement during the last Ice Age, meaning they can be found hundreds of miles from their volcanic origin.

Field identification & locations

Identify in the field by looking for 'peeled' or translucent waxy surfaces and concentric banding patterns. They are most commonly found on beaches, in gravel pits, and in glacial till throughout the Lake Superior basin (Minnesota, Wisconsin, Michigan).

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