mineral

Banded Agate (specifically Lake Superior Agate type)

Silicon Dioxide (SiO2) - Microcrystalline Chalcedony

Hardness: 6.5–7 on Mohs scale; Color: Translucent white to tan with rust-colored iron bands; Luster: Waxy to vitreous; Crystal structure: Hexagonal (trigonal) - cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.58–2.64.

Hardness: 6
Color: Translucent white to tan with rust-colored iron bands
Luster: Waxy to vitreous

Identified March 28, 2026More mineral →

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Physical properties

Formation & geological history

Formed roughly 1.1 billion years ago during the Midcontinent Rift. Silica-rich groundwater percolated through gas vesicles (bubbles) in cooling basaltic lava flows, precipitating mineral layers over time. This specimen shows water-worn rounding from glacial transport.

Uses & applications

Primary use is for lapidary work, jewelry (cabochons), and as a popular collectible gemstone. Larger or more colorful specimens are used for bookends or decorative carvings.

Geological facts

The Lake Superior Agate is the official state gemstone of Minnesota. The distinctive red and brown bands come from iron impurities (limonite and hematite) that were present during the deposition process.

Field identification & locations

Identify in the field by looking for a waxy translucency, 'peeled' looking surfaces, and colorful banding often visible when wet. Commonly found in gravel pits, riverbeds, and official lake beaches across the Upper Midwest USA (Minnesota, Wisconsin, Michigan).

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