sedimentary
Chert (specifically Flint or Common Chert node)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Typically gray, brown, greenish-gray, or black; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline or cryptocrystalline; Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Typically gray, brown, greenish-gray, or black
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Typically gray, brown, greenish-gray, or black; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline or cryptocrystalline; Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica (SiO2). It often occurs as nodules in sedimentary rocks like limestone or chalk when silica rich fluids replace carbonate minerals. They can vary in age from the Precambrian to more recent times.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges. In modern times, it is used as a road surfacing material, in some construction aggregates, and semi-translucent specimens are used for lapidary work and jewelry.
Geological facts
Chert is so hard it can strike sparks against steel, which was why it was famously used for flintlock firearms. Its predictable conchoidal fracture made it the most essential resource for stone-age tool production.
Field identification & locations
Identify in the field by its waxy appearance and 'scalloped' or shell-like conchoidal fracture when broken. It cannot be scratched by a steel knife. Common in limestone outcrops or as smooth pebbles in riverbeds.
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