sedimentary
Chert (Chalcedony / Gray Flint)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, light blue, white, and tan; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, light blue, white, and tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, light blue, white, and tan; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of silica-rich microfossils in limestone or chalk beds. Often found as nodules or layers in carbonate rocks ranging from the Precambrian to Cenozoic eras.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; modernly used as road surfacing material, in jewelry (agate varieties), and as a collector's specimen.
Geological facts
Chert was one of the most vital materials for early humans because of its ability to produce a razor-sharp edge when fractured. It is highly resistant to chemical weathering.
Field identification & locations
Identify by its waxy luster, extreme hardness (it will scratch glass), and its shell-like (conchoidal) fracture pattern. Commonly found in limestone outcrops, riverbeds, and ocean beaches.
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