sedimentary
Chert (Flint)
Chert (SiO2)
Hardness: 7 (Mohs), Color: Tan/Cream/Beige with waxy luster, Structure: Microcrystalline/cryptocrystalline quartz, Breakage: Distinct conchoidal (shell-like) fracture with sharp edges, Specific gravity: ~2.65
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Physical properties
Hardness: 7 (Mohs), Color: Tan/Cream/Beige with waxy luster, Structure: Microcrystalline/cryptocrystalline quartz, Breakage: Distinct conchoidal (shell-like) fracture with sharp edges, Specific gravity: ~2.65
Formation & geological history
Formed as nodules or bedded layers in carbonate rocks (limestone/chalk) through the replacement of calcium carbonate by silica-rich fluids. Often contains microscopic remains of silica-shelled organisms.
Uses & applications
Used historically for stone tools (knapping) and fire-starting. Currently used as road aggregate, in landscaping, and as a raw material in some ceramics.
Geological facts
Chert is so hard it will scratch steel and glass. Because it breaks with incredibly sharp edges, it was the primary material for prehistoric arrowheads, scrapers, and blades.
Field identification & locations
Identify by its smooth, waxy surface and the presence of curved, shell-shaped impact scars (conchoidal fracture). Often found in riverbeds or protruding from limestone outcrops.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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