sedimentary
Chert (often called Flint)
Chert (Microcrystalline or Cryptocrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: Gray to dark brown with white rind (cortex); Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray to dark brown with white rind (cortex)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray to dark brown with white rind (cortex); Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed by the accumulation of siliceous remains of microscopic organisms on the seafloor or by chemical precipitation. Often occurs as nodules in limestone or chalk beds. Most specimens found today date from the Cretaceous period to the Precambrian.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp edges; used in fire-starting (flint and steel), making ceramics, and as a decorative landscaping stone. High-quality colorful varieties are used in lapidary work.
Geological facts
Chert was one of the most important minerals for early humans because of its predictable conchoidal fracture, allowing it to be chipped into extremely sharp tools. The dark variety found in chalk is specifically referred to as Flint.
Field identification & locations
Identify in the field by its very smooth, waxy texture and shell-like (conchoidal) fractures. It will scratch glass and often has a chalky white outer rind. Commonly found in riverbeds or eroding out of limestone cliffs.
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