sedimentary
Chert (often called Flint)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, or brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed as nodules or layers in limestone through the accumulation of silica-rich organic remains (like radiolarians or sponge spicules) or by secondary chemical replacement of host carbonates during diagenesis.
Uses & applications
Historically used for stone tools (knapping) and starting fires; currently used as road construction aggregate, gemstone cabochons, and as a refractory material.
Geological facts
Flint is a variety of chert that occurs specifically in chalk or marl. It was one of the first materials traded over long distances by prehistoric humans due to its ability to hold a sharp edge.
Field identification & locations
Identify in the field by its characteristic shell-like (conchoidal) fracture pattern and sharp edges; it will scratch glass easily. Commonly found in river beds, chalk cliffs, and limestone quarries.
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