sedimentary
Chert / Lake Superior Agate
Cryptocrystalline Silicon Dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: tan, gray, and brown with faint banding; Luster: waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- tan, gray, and brown with faint banding
- Luster
- waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: tan, gray, and brown with faint banding; Luster: waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the deposition of silica-rich fluids in volcanic cavities (vesicles) or within sedimentary layers. These specific specimens are often associated with the Proterozoic Midcontinent Rift System, approximately 1.1 billion years old, and redistributed by glacial activity.
Uses & applications
Primarily used for lapidary work, jewelry making, and as collector specimens. Historically used by indigenous peoples for stone tools due to its sharp conchoidal fracture.
Geological facts
This specimen appears to be a small water-worn chert nodule or a low-grade 'Laker' (Lake Superior Agate). It shows the characteristic 'pit' marks or 'husks' often found on stones tumbled by glacial movement and river currents.
Field identification & locations
Can be identified in the field by its waxy luster, conchoidal fracture (chip marks that look like shell curves), and extreme hardness (will scratch glass). Commonly found in glacial till and gravel pits throughout the Upper Midwest United States.
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