sedimentary
Chert (Jasper/Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown to tan with banding, Luster: Waxy to dull, Crystal Structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown to tan with banding, Luster: Waxy to dull, Crystal Structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed via the accumulation of silica-rich organic remains or chemical precipitation within sedimentary rock layers; widely distributed in glacial drift in the Minnesota region.
Uses & applications
Historically used for stone tools and arrowheads; currently used for decorative stones, tumbling, and as a minor gemstone.
Geological facts
Chert is so hard it can scratch steel; when broken, it creates razor-sharp edges which is why it was the primary material for early human tools and weapons.
Field identification & locations
Commonly found in glacial till and gravel pits in the Midwest (including Minnesota). Identify by its waxy texture, smooth conchoidal (shell-like) fractures, and its ability to scratch glass.
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