sedimentary
Chert (and Jasper/Flint variant)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Dark gray to black with lighter gray veins; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (has conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Dark gray to black with lighter gray veins
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark gray to black with lighter gray veins; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (has conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in deep-sea environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and arrowheads; currently used as a raw material for construction aggregate, road surfacing, and occasionally for lapidary work.
Geological facts
Chert was one of the first minerals used by early humans for tools because its conchoidal fracture allowed for the creation of sharp, durable edges. When struck against steel, it can produce sparks, though this is more commonly associated with its darker variety, Flint.
Field identification & locations
Identify in the field by its superior hardness (it will scratch glass and steel), its smooth waxy feel on breaks, and its characteristic curved (conchoidal) fracture pattern. Commonly found in riverbeds or eroding out of limestone cliffs.
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