sedimentary
Chert / Flint
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Gray, brown, or black (often dull but can be waxy); Luster: Vitreous to waxy; Crystal Structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Gray, brown, or black (often dull but can be waxy)
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on the Mohs scale; Color: Gray, brown, or black (often dull but can be waxy); Luster: Vitreous 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 diatoms or radiolarians) on the ocean floor or through the chemical replacement of limestone/chalk by silica-rich fluids. It can range from Precambrian to modern in age.
Uses & applications
Historically used for making stone tools (arrowheads, knives) due to its sharp edges; used in modern times as a high-quality road aggregate, in glass manufacturing, and as 'flint' for fire-starting tools.
Geological facts
Chert was one of the most important minerals for early humans. Its ability to break with a conchoidal fracture creates edges sharper than a surgical scalpel. It is the official state rock of Ohio (as Flint Ridge flint).
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its smooth, curved 'shell-like' fracture surfaces, and a dull, stony look. It is commonly found as nodules in limestone or chalk beds globally.
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