sedimentary
Chert (Chalcedony/Flint)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan, brown, grey, translucent; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.6-2.7
- Hardness
- 6
- Color
- Tan, brown, grey, translucent
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan, brown, grey, translucent; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary basins. Often found as nodules or layers in limestone or chalk beds.
Uses & applications
Historically used for stone tools and fire-starting (flint). Currently used as an abrasive, in construction aggregate, and occasionally for lapidary work or semi-precious jewelry.
Geological facts
Chert is a cryptocrystalline variety of quartz. It was the primary material used by prehistoric humans to create sharp-edged tools like arrowheads and knives due to its predictable conchoidal fracturing.
Field identification & locations
Identify in the field by its sharp, glass-like edges when broken and its ability to scratch steel and glass. Look for waxy textures in sedimentary rock layers or river beds.
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