sedimentary
Chert (varieties including Flint or Jasper)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Brown, tan, red, and grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Brown, tan, red, and grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Brown, tan, red, and grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Forms in sedimentary rocks through the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or via chemical precipitation from silica-rich groundwater replacing carbonate or wood.
Uses & applications
Historically used for stone tools and fire-starting; currently used for road surfacing, gravel, jewelry (as jasper), and as a collector's specimen.
Geological facts
Because it breaks with a conchoidal fracture creating sharp edges, it was the primary material for prehistoric arrowheads and knives. It is chemically very stable and resistant to weathering.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy appearance, and shell-like (conchoidal) fracture patterns. It is frequently found as nodules in limestone or as bedded deposits.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Brecciated Chert (Microcrystalline Silica)
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