sedimentary
Lake Superior Agate
Silicon Dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 7 Mohs; Color: Red, orange, yellow with iron-oxide staining; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
- Hardness
- 7 Mohs
- Color
- Red, orange, yellow with iron-oxide staining
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Red, orange, yellow with iron-oxide staining; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
Formation & geological history
Formed roughly 1.1 billion years ago in gas vesicles of basaltic lava flows during the Midcontinent Rift. Iron-rich silica-bearing groundwater filled these cavities over millions of years.
Uses & applications
Used primarily for jewelry (cabochons), lapidary art, and is highly sought after by mineral collectors. It is the official state gemstone of Minnesota.
Geological facts
These agates were dispersed across the Midwest by glaciers during the last Ice Age. Famous for their rich 'candy-like' red and orange concentric banding caused by iron impurities.
Field identification & locations
Identify in the field by looking for a waxy translucence, pitted 'potato skin' exterior, and bright red/orange banding. Found in gravel pits, lake shores, and riverbeds across MN, WI, MI, and IA.
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