sedimentary
Chert (Iron-rich)
Silicon Dioxide (SiO2) with Hematite (Fe2O3)
Hardness: 7 on Mohs scale; Color: Reddish-brown, tan, and grey; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Reddish-brown, tan, and grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Reddish-brown, tan, and grey; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (diatoms/radiolarians) or chemical precipitation from hydrothermal fluids. The reddish hues indicate iron oxidation during formation.
Uses & applications
Historically used for stone tools/arrowheads. Modern uses include abrasives, decorative landscaping, and use in the manufacturing of glass and ceramics.
Geological facts
Chert is so hard it can strike sparks against steel, making it a primary fire-starting tool for ancient civilizations. It is chemically identical to quartz but has a different microscopic structure.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its sharp, shell-like (conchoidal) edges when broken. Frequently found in limestone or dolomite beds as nodules or layers.
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