sedimentary
Chert (often called Flint when dark)
Chert (Microcrystalline or Cryptocrystalline Silica, SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with waxy luster; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or brown with waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with waxy luster; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. It can date back to the Precambrian, though common in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Industrially used as an abrasive, in road construction (aggregate), and occasionally polished as semi-precious gemstones or decorative landscape stone.
Geological facts
Chert was the high-tech material of the Stone Age due to its ability to be chipped into razor-sharp edges. It is so tough that it can spark when struck against steel, a property that led to the development of flintlock firearms.
Field identification & locations
Identify in the field by its smooth, waxy surface and curved, shell-like (conchoidal) fractures. It will easily scratch glass. Common in riverbeds, limestone outcrops, and coastal cliffs globally.
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