sedimentary
Chert (often called Flint)
Chert (Microcrystalline or cryptocrystalline Silica, SiO2)
Hardness: 7 (Mohs); Color: Brown, tan, gray-green; Luster: Dull to waxy; Crystal structure: Pathologically small quartz crystals; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs)
- Color
- Brown, tan, gray-green
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs); Color: Brown, tan, gray-green; Luster: Dull to waxy; Crystal structure: Pathologically small quartz crystals; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of limestone by silica-rich fluids within sedimentary rock layers.
Uses & applications
Used historically for stone tools and arrowheads; currently used in construction aggregate and as a semi-precious stone for lapidary work.
Geological facts
Chert is so hard it can create sparks when struck against steel, which made it essential for early firearms (flintlocks) and fire-starting.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Common in limestone and chalk deposits.
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