sedimentary
Chert (Chalk-associated Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or white with a chalky outer cortex; Luster: Waxy to vitreous (glassy) on fresh breaks; Structure: Microcrystalline, showing conchoidal fracture (curved, shell-like breaks); Cleavage: None.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or white with a chalky outer cortex
- Luster
- Waxy to vitreous (glassy) on fresh breaks
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or white with a chalky outer cortex; Luster: Waxy to vitreous (glassy) on fresh breaks; Structure: Microcrystalline, showing conchoidal fracture (curved, shell-like breaks); Cleavage: None.
Formation & geological history
Formed by the precipitation of silica within sedimentary rock layers, often limestone or chalk. It frequently occurs as nodules or beds originating from the skeletons of marine organisms like sponges or diatoms. Most common in Mesozoic and Cenozoic aged strata.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges; modernly used as a road aggregate, in glass manufacturing, and occasionally as an ornamental stone or for starting fires (flint and steel).
Geological facts
Flint is a variety of chert that occurs specifically in chalk or marly limestone. It was the primary material for human technology for over two million years, used to make arrowheads, axe heads, and scrapers.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its 'conchoidal' fracture patterns. Look for a smooth, glass-like interior covered by a rough, white weathering rind. It is commonly found in the white cliffs of Dover, the Jurassic coast of the UK, and throughout the American Midwest.
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