sedimentary
Agate (Lake Superior Agate variant)
Chalcedony / Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs). Color: Translucent grey-blue mixed with iron-rich reds and oranges. Luster: Waxy to vitreous. Crystal Structure: Hexagonal (microcrystalline). Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Translucent grey-blue mixed with iron-rich reds and oranges
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs). Color: Translucent grey-blue mixed with iron-rich reds and oranges. Luster: Waxy to vitreous. Crystal Structure: Hexagonal (microcrystalline). Cleavage: None (conchoidal fracture).
Formation & geological history
Formed approximately 1.1 billion years ago during the Midcontinent Rift. Volcanic activity created gas bubbles (amygdules) in basalt, which were later filled by silica-rich groundwater. Iron oxides provide the red staining.
Uses & applications
Highly prized by lapidaries for polishing, jewelry making (cabochons), and as a popular collectible gemstone. Used historically for tool making (sharp flakes).
Geological facts
The Lake Superior Agate is the state gemstone of Minnesota. Its unique banded and translucent properties are created by the slow deposition of quartz layers over millions of years.
Field identification & locations
Identify in the field by their translucent 'glow' when held to light, waxy texture, and presence of iron-oxide staining. Common in gravel pits and beaches across the Great Lakes region.
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