sedimentary
Chert (Dark Variety / Flint)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 Mohs; Color: Dark brown, black, or deep grey; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 6
- Color
- Dark brown, black, or deep grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Dark brown, black, or deep grey; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains, such as radiolaria or diatoms, in marine environments, or through chemical precipitation in limestone. It typically dates from the Precambrian to the present.
Uses & applications
Historically used for stone tools and starting fires; currently used in construction as road metal, as a gemstone (Jasper/Flint), and as an architectural decorative stone.
Geological facts
Chert is so hard it can scratch steel. When broken, it creates exceptionally sharp edges, which led to its primary role in the development of early human technology for arrowheads and knives.
Field identification & locations
Identify by its waxy texture and conchoidal (shell-like) fracture patterns. It is commonly found as nodules in limestone or chalk beds. Test by scratching a glass plate or striking with steel to produce sparks.
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