Rock Identifier
Banded Agate (specifically Lake Superior Agate type) (Silicon Dioxide (SiO2) - variety Chalcedony) — mineral
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
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Physical properties

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.

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).

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