sedimentary
Chert (specifically Flint or Common Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, or dull green in this specimen; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, or dull green in this specimen
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, or dull green in this specimen; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or sponge spicules) or by the chemical precipitation of silica in limestone or chalk beds. It can be found in strata dating from the Precambrian to recent times.
Uses & applications
Historically used for stone tools (knapping) and sparking fire. In modern industry, it is used as a decorative stone, in road construction, and as a raw material for ceramics and silicon.
Geological facts
Because it fractures conchoidally to create extremely sharp edges, it was the primary material for prehistoric arrowheads, knives, and scrapers. It is also the 'flint' used in flintlock firearms to create sparks.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), waxy feel, and characteristic curved 'clamshell' (conchoidal) fractures. Often found as nodules or layers within sedimentary limestone or chalk.
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