sedimentary
Banded Agate (Lake Superior Agate)
Silicon Dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 6.5-7 Mohs; Color: Brown, grey, white, tan with concentric banding; Luster: Vitreous to waxy; Structure: Microcrystalline (trigonal); Conchoidal fracture.
- Hardness
- 6
- Color
- Brown, grey, white, tan with concentric banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, grey, white, tan with concentric banding; Luster: Vitreous to waxy; Structure: Microcrystalline (trigonal); Conchoidal fracture.
Formation & geological history
Formed roughly 1.1 billion years ago during the Midcontinent Rift. Silica-rich groundwater deposited layers of iron-stained quartz inside gas vesicles of volcanic basalt.
Uses & applications
Primarily used for lapidary work, jewelry making (cabochons), and as highly sought-after collector items for rockhounds.
Geological facts
The Lake Superior Agate is the official state rock of Minnesota. Its distinct red and orange colors come from iron oxidation, while grey and white bands represent pure silica layers.
Field identification & locations
Identified by fine, concentric 'fortification' bands and a waxy texture. Commonly found on beaches of Lake Superior or in glacial gravel pits across the Upper Midwest USA.
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