sedimentary
Chert (specifically Flint)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically dark gray, black, or brown with a lighter weathered cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically dark gray, black, or brown with a lighter weathered cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica in sedimentary rock layers, often replacing limestone or chalk. Many cherts form from the accumulation of silica-rich skeletal remains (like diatoms or radiolarians) on the deep ocean floor.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges. Used in flintlock firearms to create sparks. Modern uses include abrasives and aggregate in construction.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it produces sparks, which is why it was essential for starting fires and in early firearms for centuries.
Field identification & locations
Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It often feels 'slicker' or waxier than surrounding limestone. Common in riverbeds and limestone outcrops.
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