sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: dark grey to black (Flint variety); Luster: waxy to dull; Structure: microcrystalline; Cleavage: none, exhibits strong conchoidal fracture; SG: 2.65.
- Hardness
- 6
- Color
- dark grey to black (Flint variety)
- Luster
- waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: dark grey to black (Flint variety); Luster: waxy to dull; Structure: microcrystalline; Cleavage: none, exhibits strong conchoidal fracture; SG: 2.65.
Formation & geological history
Formed in marine environments through the accumulation of silica-rich organic remains (diatoms/radiolarians) or chemical precipitation within limestone/chalk beds.
Uses & applications
Used historically for stone tools and fire-starting; currently used as construction aggregate, road surfacing, and sometimes for jewelry as jasper or agate.
Geological facts
Black chert is often called Flint and was the most important material for prehistoric humans to create sharp hunting tools like arrowheads and knives.
Field identification & locations
Identify by its extreme hardness (scratches glass) and smooth, curved (conchoidal) breakage surfaces. Commonly found in riverbeds and limestone outcrops.
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