sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Light gray to white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Light gray to white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Light gray to white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed in marine environments as biological silica from radiolarians or diatoms accumulates and recrystallizes into a dense, cryptocrystalline rock within limestone or chalk beds. Commonly formed during the Cretaceous period.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges. Modernly used in construction aggregate, road metal, and as a starting material for fire (flint and steel).
Geological facts
Chert has been used by humans for over 2 million years. It is so hard that it can scratch steel, and when struck against steel, it produces sparks.
Field identification & locations
Identify by its distinct conchoidal (shell-like) fracture patterns and sharp edges. It is commonly found as nodules or layers within sedimentary rocks like limestone.
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