sedimentary
Chert (with Quartz veins)
Chert (Microcrystalline Silica, SiO2)
Hardness: 7 on Mohs scale; Color: Gray to black with white veins; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray to black with white veins
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray to black with white veins; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) in deep-sea environments or by chemical precipitation. The white veins are later quartz infillings of fractures.
Uses & applications
Used historically for stone tools due to its sharp edges; currently used in construction aggregate, road surfacing, and as decorative river stones.
Geological facts
Chert is so hard it can scratch steel and was often used alongside steel as a 'firestone'. It is chemically identical to flint, but the name 'flint' is usually reserved for specimens found in chalk or marly limestone.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a knife) and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds and coastal areas as rounded pebbles.
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