sedimentary

Chert (Iron-stained)

Microcrystalline Quartz (SiO2)

Hardness (Mohs): 7. Color: Dark gray to black with brownish-yellow iron staining. Luster: Dull to waxy. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Color: Dark gray to black with brownish-yellow iron staining
Luster: Dull to waxy

Identified May 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules or layers in limestone.

Uses & applications

Historically used for stone tools (arrowheads). Modernly used in construction aggregate, road base, and as decorative landscaping stones. Highly silica-rich cherts are used in some industrial abrasives.

Geological facts

Chert is so hard it will spark when struck against steel. Flint is a variety of chert. The distinct conchoidal (shell-like) fracture allows it to break into extremely sharp edges, which is why it was the primary material for prehistoric tools.

Field identification & locations

Identify in the field by its hardness (it should scratch glass) and its curved, glass-like fracture surfaces. Often weathered with a white or brownish rind. Common in sedimentary basins worldwide.

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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