sedimentary
Chert (often called Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grey, tan, smoky; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grey, tan, smoky
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Grey, tan, smoky; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in construction aggregate, road surfacing, and as a starting material for silicon.
Geological facts
Because of its conchoidal fracture, chert breaks into edges that are sharper than a surgical steel scalpel. Native Americans and early humans worldwide prized it for toolmaking.
Field identification & locations
Identify by its 'waxy' appearance, extreme hardness (it will scratch glass), and its tendency to break in smooth, curved shell-like patterns known as conchoidal fractures. Commonly found in gravel pits or weathered out of limestone cliffs.
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