sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Black, brown, gray; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Black, brown, gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Black, brown, gray; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica from groundwater flow through limestone or chalk beds, or from the accumulation of siliceous organisms on the sea floor during the Phanerozoic Eon.
Uses & applications
Historically used for stone tools (knapping) and fire-starting (flint). Currently used in road construction, as an abrasive, and sometimes in jewelry as tumbled stones.
Geological facts
Dark-colored chert is often specifically called 'flint.' It is so hard that it can scratch steel, and its conchoidal fracture creates edges sharper than a surgical scalpel.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found as nodules or layers within carbonate rocks globally.
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