sedimentary

Chert (with Limestone and Iron Oxide staining)

Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified March 28, 2026More sedimentary →

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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.