sedimentary
Lake Superior Agate
Silicon Dioxide (SiO2) - variety: banded chalcedony quartzite
Hardness: 7 Mohs; Color: Iron-rich red, orange, and white; Luster: Vitreous to waxy; Crystal structure: Trigonal/Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 2.6-2.64
- Hardness
- 7 Mohs
- Color
- Iron-rich red, orange, and white
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 Mohs; Color: Iron-rich red, orange, and white; Luster: Vitreous to waxy; Crystal structure: Trigonal/Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 2.6-2.64
Formation & geological history
Formed roughly 1.1 billion years ago during the Midcontinent Rift System. Siliceous fluids filled gas vesicles (bubbles) in cooling basaltic lava. Iron-rich groundwaters later precipitated minerals in concentric bands.
Uses & applications
Highly sought after for lapidary work, jewelry, and gemstone collection. Frequently used in polished cabochons and as decorative display specimens.
Geological facts
It is the official state gemstone of Minnesota. The distinctive red color is caused by iron oxidation, reflecting the iron-rich ores of the Lake Superior region.
Field identification & locations
Identify by looking for translucent waxy surfaces and 'onion-skin' concentric banding. Found in glacial drift across Minnesota, Wisconsin, Iowa, and Michigan, particularly on beaches and in gravel pits.
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