sedimentary
Chert (specifically Flint or Common Chert)
Silicon dioxide (SiO2) - Microcrystalline quartz
Hardness: 7 on Mohs scale; Color: White, tan, or gray with iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- White, tan, or gray with iron staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: White, tan, or gray with iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or sponge spicules) within limestone or chalk beds. Often found in nodules.
Uses & applications
Historically used for making stone tools and fire-starting strikers. Today used as road aggregate, architectural stone, and in some gemstone applications like tumbling.
Geological facts
Chert has been used by humans for over two million years to create sharp-edged tools like arrowheads. When struck against steel, it produces sparks, making it essential for flintlock firearms.
Field identification & locations
Identify in the field by its sharp, glass-like 'shell-shaped' (conchoidal) fractures and its ability to scratch glass. Commonly found in chalk or limestone outcrops and riverbeds.
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