sedimentary
Chert (Nodule fragment)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Dark brown to tan; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (systemic conchoidal fracture).
- Hardness
- 6
- Color
- Dark brown to tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Dark brown to tan; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (systemic conchoidal fracture).
Formation & geological history
Formed through the precipitation of silica from groundwater within limestone or chalk beds. Often associated with marine environments from the Paleozoic to Mesozoic eras.
Uses & applications
Historical use for stone tools (flint knapping); modern use in road construction aggregates and occasionally as an abrasive or in decorative hardscaping.
Geological facts
Chert is a cryptocrystalline variety of quartz. When it occurs in chalk, it is specifically called flint. Its conchoidal fracture allows it to produce razor-sharp edges.
Field identification & locations
Identify by its waxy luster, smoothness, and curved (conchoidal) breakage patterns. It will typically scratch glass and is resistant to acid.
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