sedimentary
Chert (Flint variety)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, grey, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Brown, grey, tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, grey, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed as nodules in sedimentary rocks like limestone or chalk when silica replaces carbonate material or precipitates from fluids. This process often occurs over millions of years.
Uses & applications
Historically used for stone tools and starting fires; currently used in jewelry (tumbled stones), road surfacing, and as a refractory material.
Geological facts
Because of its conchoidal fracture, it can be chipped to a razor-sharp edge, which led prehistoric humans to use it for arrowheads and knives.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its waxy texture, and its smooth, curved (conchoidal) broken edges. Commonly found in river beds and limestone outcrops.
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