sedimentary
Lake Superior Agate
Silicon Dioxide (Chalcedony / Quartz, SiO2)
Hardness of 6.5 to 7 on the Mohs scale, waxy to vitreous luster, distinct concentric banding in shades of red, orange, and yellow (caused by iron oxidation), conchoidal fracture, and no true cleavage.
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Physical properties
Hardness of 6.5 to 7 on the Mohs scale, waxy to vitreous luster, distinct concentric banding in shades of red, orange, and yellow (caused by iron oxidation), conchoidal fracture, and no true cleavage.
Formation & geological history
Formed roughly 1.1 billion years ago during the Keweenawan Rift Zone volcanism. Groundwater rich in silica and iron percolated through gas cavities in basalt flows, depositing layers of microcrystalline quartz over millions of years.
Uses & applications
Widely used in jewelry, lapidary arts, and highly prized by rockhounds and mineral collectors.
Geological facts
The Lake Superior agate is the official state gemstone of Minnesota. The vibrant red and orange colors are a hallmark of iron oxide staining unique to the region's geological history.
Field identification & locations
Found primarily in gravel pits, riverbeds, and beaches around the Lake Superior region in North America. Look for waxy luster, translucent banding, and iron-stained exterior pitting when searching in the field.
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