sedimentary
Chert (Flint)
Silicon dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, tan, and yellowish; Luster: Waxy to vitreous; Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Brown, tan, and yellowish
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, tan, and yellowish; Luster: Waxy to vitreous; Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed as nodules or layered deposits in limestone or chalk through the accumulation of siliceous remains of organisms like diatoms and radiolarians, which then recrystallize.
Uses & applications
Historically used for stone tools (knapping) and starting fires. Currently used as an abrasive, in construction aggregate, and as a semi-precious gemstone in lapidary work.
Geological facts
Chert has a distinctive conchoidal fracture which allows it to break into sharp edges, making it the primary material for prehistoric arrowheads and spear points. Different colors are caused by trace impurities like iron oxide.
Field identification & locations
Identify by its waxy appearance, extreme hardness (scratches glass), and the way it breaks into smooth, curved 'shell-like' surfaces. Found globally in sedimentary rock basins.
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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