sedimentary
Chert
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 Mohs scale; Color: Beige, tan, or light gray; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); High specific gravity around 2.6
- Hardness
- 7 Mohs scale
- Color
- Beige, tan, or light gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 Mohs scale; Color: Beige, tan, or light gray; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); High specific gravity around 2.6
Formation & geological history
Formed via the accumulation of siliceous oozes or through the replacement of calcium carbonate by silica in sedimentary basins. Most commonly found in limestone or chalk beds.
Uses & applications
Historically used for stone tools and arrowheads; currently used as road base, decorative landscaping stone, and as an industrial abrasive.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create sharp-edged tools due to its conchoidal fracture pattern. Flint is a well-known dark variety of chert.
Field identification & locations
Identifiable by its extreme hardness (scratches glass), waxy feel, and smooth curved (conchoidal) breaks. Often found as nodules or layers within sedimentary rock outcrops.
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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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Shale or Slaty Mudstone
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Arenite (primarily SiO2)
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