sedimentary
Chert (variously Flints)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: dark grey to black; Luster: waxy to dull; Structure: microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 6
- Color
- dark grey to black
- Luster
- waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: dark grey to black; Luster: waxy to dull; Structure: microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from sea water or by the replacement of calcium carbonate by silica in sedimentary rock layers. Often found in nodules in limestone or chalk.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Modernly used as road aggregate, in glass making, and for sparks in flintlock mechanisms.
Geological facts
Chert was one of the most important raw materials for early humans because its conchoidal fracture produces edges that are sharper than a surgical scalpel. It is highly resistant to chemical weathering.
Field identification & locations
Can be identified by its extreme hardness (will scratch steel) and its shell-like curved fracture pattern. Found globally in sedimentary basins and as river pebbles.
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