sedimentary
Chert (Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically grey, brown, or tan; Luster: waxy to dull; Crystal structure: hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- typically grey, brown, or tan
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically grey, brown, or tan; 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 hydrothermal fluids in limestone or chalk beds. Most specimens date from the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools and weapons (arrowheads); currently used as road aggregate, in glass making, and as strikers for flintlock firearms or fire-starting kits.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it produces edges that can be sharper than a surgical steel scalpel. Darker varieties are typically referred to as 'Flint'.
Field identification & locations
Identify in the field by its characteristic smooth, curved fracture surfaces and its ability to scratch glass. It often occurs as nodules within limestone. Found globally, notably in the Dover cliffs of the UK and across the midwestern USA.
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