sedimentary
Chert (Black Chert/Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (diatoms, radiolarians) or chemical precipitation in deep marine or limestone environments. Most specimens are from the Phanerozoic eon.
Uses & applications
Historically used for stone tools and starting fires (flint). Currently used as a gemstone (Jasper varieties), in jewelry, as road surfacing material, and as collector specimens.
Geological facts
Black chert is often called flint when it occurs in chalk or marly limestone. It was the primary material for spearheads and arrowheads due to its razor-sharp edges when fractured.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture and its ability to scratch glass. Commonly found in limestone beds or as rounded pebbles in stream beds.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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