sedimentary
Chert (specifically Flint)
Silicon dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Dark grey to brown with a light tan/white outer cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7
- Hardness
- 7 (Mohs scale)
- Color
- Dark grey to brown with a light tan/white outer cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark grey to brown with a light tan/white outer cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7
Formation & geological history
Formed as nodules or masses in sedimentary rocks like limestone or chalk. It is created through biochemical precipitation of silica from the remains of diatoms or sponges, often dating from the Cretaceous period to the Paleozoic era.
Uses & applications
Historically used for making stone tools (arrowheads, knives) and for starting fires (striking steel). Industrially used in road construction, ceramics, and occasionally as a gemstone in tumbled jewelry.
Geological facts
Flint was the primary material for the 'Stone Age' due to its ability to flake into razor-sharp edges. It is also the specific variety of chert used commonly in flintlock firearms to ignite gunpowder.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It often features a white, chalky outer 'skin' or cortex. Common in the UK, France, and the Midwestern United States.
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