sedimentary
Chert (Nodular flint)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Brown, grey, tan, or black; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Brown, grey, tan, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Brown, grey, tan, or black; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed in sedimentary rocks (usually limestone or chalk) through the replacement of calcium carbonate with silica-rich fluids or the accumulation of siliceous organisms like diatoms and radiolaria. Found throughout geological history.
Uses & applications
Historically used for making stone tools (arrowheads, knives) due to its sharp edges. Modernly used as a decorative stone, in construction aggregate, and as a striker for fire-starting.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it creates edges that are sharper than a surgical scalpel. It is one of the most durable rocks and can survive for millions of years after the surrounding limestone has eroded away.
Field identification & locations
Identify in the field by its waxy appearance and smooth, curved fracture surfaces. It cannot be scratched by a steel knife. Common worldwide in limestone regions like the chalk cliffs of Dover or the American Midwest.
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