sedimentary
Chert (Iron-stained or Dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Dark brown, black, or grey with iron staining; Luster: Dull to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Dark brown, black, or grey with iron staining
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark brown, black, or grey with iron staining; Luster: Dull to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation on the ocean floor. Common in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools (arrowheads). Modern uses include road surfacing (as gravel), abrasives, and as a semi-precious gemstone when colorful (Jasper).
Geological facts
Chert is so hard it can strike sparks against steel. It was one of the most vital materials for early humans to create survival tools due to its sharp conchoidal edges.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like (conchoidal) fracture pattern. Often found as nodules in limestone or as bedded deposits.
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