sedimentary
Chert (often called Flint or Jasper when red)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale); Color: Brown, reddish-tan, or earth tones; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 7 (Mohs scale)
- Color
- Brown, reddish-tan, or earth tones
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, reddish-tan, or earth tones; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine or groundwater environments. Can occur as nodules in limestone or as bedded deposits.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp edges. Currently used in road construction as aggregate, in jewelry (as jasper), and occasionally for starting fires with steel.
Geological facts
Chert was one of the most vital resources for prehistoric humans. Because it breaks with a conchoidal fracture, it creates edges sharper than a surgical scalpel. Some beds of chert are hundreds of feet thick.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its curved, shell-like fracture patterns. Look for it in limestone outcrops or as smooth river pebbles. Collectors look for unique color patterns or evidence of 'knapping' (shaping by humans).
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