sedimentary
Chert (specifically Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Tan, brown, and grey with a waxy luster; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 6
- Color
- Tan, brown, and grey with a waxy luster
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan, brown, and grey with a waxy luster; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed as nodules or beds in sedimentary rocks (usually limestone or chalk) via the replacement of carbonates by silica-rich fluids. Often contains microfossils from many geological eras, including the Cretaceous.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and fire-starting. Currently used as road aggregate, in jewelry as semi-precious gemstones, and in glassmaking.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's scalpel. It was the premier technology for toolmaking during the Stone Age.
Field identification & locations
Identify in the field by its smooth, waxy surface and the sharp, curved (shell-like) edges where it has broken. Commonly found in riverbeds or within limestone outcrops. Key locations include the UK (Dover), Central USA, and France.
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