sedimentary
Chert (variously Flint or Jasper gravel)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, or grey; Luster: Dull to waxy; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, or grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, or grey; Luster: Dull to waxy; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed as nodules or layers in sedative rocks (mostly limestone or chalk) through the replacement of calcium carbonate by silica-rich fluids. Often millions of years old, typically dating from the Cretaceous to the Precambrian periods.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to conchoidal fracturing. Modernly used as construction aggregate, road base, and occasionally as semi-precious gemstones when colorful (Jasper).
Geological facts
Chert is so hard it can scratch glass and steel. When struck against steel, it produces sparks, which led to its historic use in 'flintlock' firearms. It is incredibly durable and resistant to chemical weathering.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch a knife blade. Commonly found in riverbeds, glacial till, and embedded in limestone outcrops. Collectors look for unique banding or translucent edges.
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