sedimentary
Chert (often called Flint or Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Variable (grey, brown, yellow-ochre); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Variable (grey, brown, yellow-ochre)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Variable (grey, brown, yellow-ochre); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids. This example shows a weathered exterior with a distinct yellow/brown inner core.
Uses & applications
Historical use for stone tools/weaponry due to conchoidal fracture; modern use in construction aggregate and as lapidary material for jewelry (Jasper/Agate varieties).
Geological facts
Chert was one of the most important materials for early humans because its conchoidal fracture creates extremely sharp edges for knives and arrowheads. It is often found as nodules in limestone or chalk.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its smooth waxy feel on breaks, and its characteristic curved (conchoidal) fracture surfaces that look like a shell. Commonly found in riverbeds or within limestone outcrops.
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