sedimentary
Chert (with Limestone and Iron Oxide staining)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark gray to tan with reddish iron streaks, Luster: Waxy to dull, Structure: Microcrystalline with conchoidal (shell-like) fracture, Cleavage: None
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark gray to tan with reddish iron streaks, Luster: Waxy to dull, Structure: Microcrystalline with conchoidal (shell-like) fracture, Cleavage: None
Formation & geological history
Formed through the precipitation of silica from seawater or the replacement of calcium carbonate in limestone. The bands suggest changing chemical environments or oxidation of iron minerals during lithification.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction aggregate, road base, and as decorative landscaping stone.
Geological facts
Chert is so hard it can create sparks when struck against steel. The dark gray color often comes from organic matter trapped within the silica matrix.
Field identification & locations
Identify in the field by its conchoidal fracture (curved, glass-like breaks) and its ability to scratch glass. Widely found in the Mid-western US and flint-rich formations globally.
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