sedimentary
Chert (Flint)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: light gray to tan (variable); Luster: dull to waxy; Structure: Cryptocrystalline (microscopic grains); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- light gray to tan (variable)
- Luster
- dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: light gray to tan (variable); Luster: dull to waxy; Structure: Cryptocrystalline (microscopic grains); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the accumulation of silica-rich organic remains such as radiolarians or diatoms, or through chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction as road base, as a component in ceramics, and occasionally for lapidary work.
Geological facts
Chert is so hard it can create sparks when struck against steel, which made its variety, Flint, essential for early firearms and fire-starting throughout human history.
Field identification & locations
Identify by its extreme hardness (scratches glass) and smooth, shell-like (conchoidal) curved fracture surfaces. Commonly found in riverbeds and sedimentary outcrops globally.
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