sedimentary
Chert (varieties include Flint and Mudstone)
Microcrystalline Silica (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, brown, or dull green; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, brown, or dull green
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, or dull green; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. It can date from the Precambrian to the present.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in road construction, as abrasives, or as a decorative landscaping stone.
Geological facts
Chert was one of the first materials used by early humans for tool-making. It produces sparks when struck against steel, which made it essential for flintlock firearms and early fire-starting.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture pattern and its ability to scratch glass. It is commonly found in sedimentary basins and as nodules in limestone outcrops.
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