sedimentary
Chert (Light Grey / Flint variant)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Light grey to white; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Trigonal; Fracture: Conchoidal; Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Light grey to white
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Light grey to white; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Trigonal; Fracture: Conchoidal; Specific Gravity: 2.65
Formation & geological history
Forms through the precipitation of silica in marine environments, often replacing organic matter or limestone. It can also form as primary chemical precipitates from hydrothermal fluids or silica-rich deep-sea ooze.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used as road aggregate, in glass manufacturing, and as a low-cost gemstone for lapidary work.
Geological facts
Chert is so hard it will scratch steel. It is nearly identical in composition to quartz but has a much finer grain, making it opaque rather than transparent. It was the most important material for Neolithic societies making arrowheads and spear points.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture and its ability to scratch glass. Found worldwide in sedimentary basins, often as nodules inside limestone or chalk beds.
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