sedimentary
Chert (variations include Flint/Jasper)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Brown, reddish-brown, tan, or grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Brown, reddish-brown, tan, or grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Brown, reddish-brown, tan, or grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich microfossils (like radiolarians or diatoms) or via chemical precipitation in limestone/chalk beds. It can span from the Precambrian to recent geological ages.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used in road surfacing, as abrasive material, and sometimes in jewelry (as jasper/agate).
Geological facts
Chert has a conchoidal fracture, allowing it to break into sharp edges, which made it the primary material for prehistoric arrowheads and spear tips. It is extremely chemically stable and resistant to weathering.
Field identification & locations
Identify in the field by its smooth, waxy surface and characteristic shell-like (conchoidal) fracture patterns when broken. It cannot be scratched by a steel knife. Common in riverbeds and limestone outcrops.
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