sedimentary
Chert (specifically Flint)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Gray to tan internal, white/brown exterior cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray to tan internal, white/brown exterior cortex
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray to tan internal, white/brown exterior cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from seawater or groundwater, often replacing organic matter or calcium carbonate in limestone/chalk beds. Commonly dates from the Cretaceous period.
Uses & applications
Historically used for stone tools (arrowheads, knives) and flintlock firearms. Currently used for road gravel, concrete aggregate, and occasional gemstone cabochons.
Geological facts
Due to its conchoidal fracture, it breaks into razor-sharp edges. It was arguably the most important technological material for early humans. It can create sparks when struck against steel.
Field identification & locations
Identifiable by its smooth, shell-like (conchoidal) fracture surfaces and waxy texture. Frequently found as nodules in chalk or limestone cliffs and along riverbeds. Look for a 'rind' or cortex on the outside.
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