sedimentary
Chert
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 Mohs scale; Color: Tan, brown, or earthy yellow; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Tan, brown, or earthy yellow
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Tan, brown, or earthy yellow; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often through the accumulation of siliceous biological remains like radiolaria or through chemical replacement of limestone or chalk.
Uses & applications
Historically used for stone tools due to sharp edges; currently used as a road aggregate, in landscaping, and some varieties as semi-precious gemstones.
Geological facts
Chert is essentially the same mineral as flint, though flint is typically used to describe dark varieties. It can have extremely sharp edges when broken, which were once used to make arrowheads and knives.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and its shell-like (conchoidal) fracture pattern. It is common in limestone formations and as river pebbles.
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