sedimentary
Chert (Flint)
Chert (Cryptocrystalline silicon dioxide, SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed from the accumulation of siliceous remains of microscopic organisms (diatoms, radiolarians) or through chemical precipitation in marine or freshwater environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges; used in modern times for industrial grinding media and as flint for fire-starting. Occasionally used in jewelry as decorative stones.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create arrowheads and scrapers. When struck against steel, it can produce sparks, which led to its extensive use in flintlock firearms.
Field identification & locations
Identify in the field by its very smooth ‘waxy’ texture and sharp, shell-like (conchoidal) fractures where broken. It is harder than steel and will scratch a glass plate. Common in riverbeds and limestone outcrops.
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