sedimentary
Chert (Jasper)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Brown, tan, reddish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from hydrothermal fluids in sedimentary basins. It can occur in nodules or bedded deposits.
Uses & applications
Historically used for stone tools (arrowheads). Currently used as decorative stones, in landscaping, as a source for silica in glassmaking, and occasionally for lapidary work in jewelry.
Geological facts
Chert has a conchoidal fracture which means it breaks like glass, producing very sharp edges. This made it the premier material for prehistoric toolmaking alongside flint.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its smooth waxy feel, and the way it breaks into curved, shell-like flakes. Commonly found in limestone or chalk beds and along riverbeds as rounded pebbles.
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