sedimentary
Chert (often referred to as Flint)
Chert (Microcrystalline or cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: typically gray, tan, or brown with possible iron staining; Luster: waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- typically gray, tan, or brown with possible iron staining
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically gray, tan, or brown with possible iron staining; Luster: waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the seafloor, or through the chemical replacement of limestone/dolomite by silica. Primarily found in Phanerozoic sedimentary sequences.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used in modern times as road aggregate, in glassmaking (if pure), and as a traditional fire-starting flint.
Geological facts
Chert is so hard it can scratch steel. When broken, it exhibits 'conchoidal fracture,' producing curved, glass-like surfaces and extremely sharp edges that were vital to prehistoric human technology.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a knife), waxy appearance, and shell-like curved fracture surfaces. Commonly found as nodules or layers within limestone beds or as rounded pebbles in riverbeds.
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