sedimentary
Chert (Black Chert / Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Black to dark grey; Luster: Waxy to sub-vitreous; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Black to dark grey
- Luster
- Waxy to sub-vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Black to dark grey; Luster: Waxy to sub-vitreous; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical precipitation of silica from groundwater within limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Modernly used in road construction, as a gemstone (when colorful), and in fire-starting kits (flint).
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it produces sparks, which is why it was essential for flintlock firearms and early survival tools.
Field identification & locations
Identify in the field by its waxy appearance and smooth, shell-like (conchoidal) curved breaks. It often has a distinct 'clink' sound when struck. Common in sedimentary basins worldwide.
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