sedimentary

Chert / Lake Superior Agate

Cryptocrystalline Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

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

Identified July 20, 2026More sedimentary →

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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.