sedimentary
Chert (Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, white, or black; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, white, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, white, or black; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed by the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine or groundwater environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons (arrowheads) due to its sharp edges. Modernly used in road construction, flintlock firearms, and as a low-cost gemstone or lapidary material.
Geological facts
When chert strikes steel, it produces sparks, a property used for centuries to start fires. It is one of the most durable sedimentary rocks and can survive long-distance transport in rivers.
Field identification & locations
Identify in the field by its characteristic shell-like (conchoidal) fracture and its ability to scratch glass. Look for it in limestone outcrops or as rounded pebbles in stream beds.
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