sedimentary
Chert (varieties include Flint and Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, brown, or white; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, brown, or white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, brown, or white; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or inorganic precipitation from groundwater. It typically forms as nodules or layered beds within limestone or chalk deposits.
Uses & applications
Historically used for stone tools and arrowheads due to sharp edges; currently used for road construction, jewelry (when colored/banded), and as a silica source in glassmaking.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture produces razor-sharp edges. When struck against steel, it can produce sparks, though 'flint' is the specific variety famous for this.
Field identification & locations
Identify by its waxy luster, extreme hardness (it will scratch glass), and its shell-like (conchoidal) breaking pattern. Frequently found in stream beds or eroded out of limestone outcrops.
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