mineral
Lake Superior Agate
Silicon dioxide (SiO2) - variety of Chalcedony
Hardness: 7 Mohs scale; Color: Red, orange, tan, and brown banding caused by iron; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.60-2.65
- Hardness
- 7 Mohs scale
- Color
- Red, orange, tan, and brown banding caused by iron
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs scale; Color: Red, orange, tan, and brown banding caused by iron; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.60-2.65
Formation & geological history
Formed roughly 1.1 billion years ago during the Midcontinent Rift System. Gas bubbles in basaltic lava flows were filled by mineral-rich groundwater, depositing layers of silica and iron over time.
Uses & applications
Primarily used for jewelry, lapidary arts, and specimen collecting. It is the official state gemstone of Minnesota.
Geological facts
The distinctive red and orange colors are due to the presence of iron oxidation. They are often found in glacial drift throughout the Upper Midwest of the United States.
Field identification & locations
Identify in the field by looking for translucent, banded patterns often exposed by a waxy, pitted outer 'skin' or husk. Commonly found on beaches of Lake Superior, in gravel pits, and on dirt roads in Minnesota and Wisconsin.
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