sedimentary
Chert (Lake Superior Agate variant)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Brown, tan, and white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
- Hardness
- 6
- Color
- Brown, tan, and white
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Brown, tan, and white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
Formation & geological history
Formed in gas cavities (vesicles) of basaltic lava flows approximately 1.1 billion years ago during the Keweenawan Rift. This piece appears to be a water-worn pebble smoothed by glacial or river action.
Uses & applications
Used primarily by lapidaries for cabochons, jewelry, and as decorative tumbling stones. Traditionally used by indigenous peoples for stone tools and projectile points due to its sharp edges when fractured.
Geological facts
Many small pebbles like this are concentrated around the Lake Superior region due to glacial movement during the Ice Age. The white circular features are likely 'eyes' or small amygdule fillings of chalcedony.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture and hardness—it will scratch steel but cannot be scratched by a knife. Commonly found in gravel pits and along the shores of the Great Lakes.
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