sedimentary
Chert (and Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically brown, tan, or gray; Luster: waxy to dull; Crystal structure: hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- typically brown, tan, or gray
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically brown, tan, or gray; Luster: waxy to dull; Crystal structure: hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Most common in Paleozoic to Mesozoic strata.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, jewelry (when colorful like jasper), and fire-starting kits.
Geological facts
Chert was one of the most vital materials for prehistoric humans to create arrowheads and scrapers. Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's scalpel.
Field identification & locations
Identify in the field by its waxy texture, smooth curved (conchoidal) fractures, and its ability to scratch glass. Found globally in sedimentary basins, often protruding as nodules from weathered limestone cliffs.
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