sedimentary
Chert (Flint)
Chert (SiO2)
Hardness: 6.5-7 (Mohs); Color: Grey, black, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Grey, black, or tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Grey, black, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed as nodules or layered deposits in limestone or chalk through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater.
Uses & applications
Historically used for stone tools (arrowheads, knives) and strike-a-lights. Currently used in construction aggregate and as a collector's specimen.
Geological facts
Chert is so hard it can scratch steel and was the primary material for the 'Stone Age' technology. When struck against steel, it produces sparks used for starting fires.
Field identification & locations
Identify by its extremely smooth, shell-like (conchoidal) fracture and waxy texture. Commonly found in riverbeds, glacial till, or embedded within limestone outcrops.
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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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