sedimentary
Chert (varietal Jasper or Mudstone Flint)
Silicon dioxide (SiO2) with impurities
Hardness: 6.5-7 Mohs; Color: grey with yellow-ochre core; Luster: waxy to dull; Structure: microcrystalline quartz; Cleavage: none, exhibits conchoidal fracture
- Hardness
- 6
- Color
- grey with yellow-ochre core
- Luster
- waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: grey with yellow-ochre core; Luster: waxy to dull; Structure: microcrystalline quartz; Cleavage: none, exhibits conchoidal fracture
Formation & geological history
Formed as chemical precipitates or through the replacement of primary minerals by silica-rich fluids in sedimentary basins or marine environments. This specimen shows a weathered outer cortex/rind with a different colored core.
Uses & applications
Historically used for stone tools and fire-starting due to its hardness and sharp edges; currently used for lapidary work, decorative garden stones, and aggregate.
Geological facts
Chert is so durable that it often survives intense weathering that destroys surrounding limestone or chalk. Specimens with distinct internal rings or cores are often referred to as nodules.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, curved (conchoidal) break patterns. Common in gravel beds and limestone outcroppings.
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