sedimentary
Chert (often called Flint or Jasper)
Microcrystalline silica (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, white, and reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, white, and reddish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, white, and reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by chemical precipitation from hydrothermal fluids in limestone or chalk beds. Most specimens date from the Phanerozoic eon.
Uses & applications
Used historically for stone tools and weapons (arrowheads). Modern uses include road surfacing, decorative garden stones, and occasionally as semi-precious gemstones in jewelry (Jasper).
Geological facts
Chert was one of the most important technological materials for early humans because its conchoidal fracture creates extremely sharp edges; it is also capable of creating sparks when struck against steel.
Field identification & locations
Identify by its waxy texture, extreme hardness (it will scratch glass), and smooth, shell-like (conchoidal) fractures. Common in riverbeds and limestone outcrops globally.
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