sedimentary
Chert (Red/Brown Variety)
Microcrystalline quartz (SiO2)
Hardness: 6.5-7 (Mohs scale). Color: Brownish-red to tan. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Brownish-red to tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale). Color: Brownish-red to tan. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by the precipitation of silica from groundwater within sedimentary layers like limestone or chalk. It can date from the Precambrian to recent eras.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges. Today used as decorative gravel, in road construction, and occasionally for lapidary work if highly colored.
Geological facts
Red chert is often colored by iron oxide (hematite). Because of its hardness and chemical stability, it is incredibly resistant to weathering and often found as rounded river pebbles long after the surrounding rock has eroded.
Field identification & locations
Identify by its waxy texture and conchoidal (shell-like) fracture pattern. It will easily scratch glass. Commonly found in riverbeds, beaches, and sedimentary outcrops globally.
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