sedimentary
Chert (Flint)
Chert (SiO2)
Hardness: 7 (Mohs); Color: Grey, brown, tan, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs)
- Color
- Grey, brown, tan, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs); Color: Grey, brown, tan, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or via chemical precipitation from silica-rich fluids in sedimentary basins. Often found as nodules in limestone or chalk.
Uses & applications
Historical use for stone tools and weapons (arrowheads); industrial use in road surfacing and as a gemstone component; historically used to start fires.
Geological facts
Chert has been used by humans for over two million years to create sharp-edged tools. It is so tough that it was the primary material for the 'Stone Age'.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture, wax-like luster, and its ability to scratch glass. Commonly found in riverbeds or eroding out of limestone cliffs.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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