sedimentary
Chert / Flint (with Iron Staining)
Silicon Dioxide (SiO2)
Hardness: 7 Mohs scale; Color: Grey, tan, brown with yellow/orange iron oxidation; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 7 Mohs scale
- Color
- Grey, tan, brown with yellow/orange iron oxidation
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Grey, tan, brown with yellow/orange iron oxidation; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed as nodules or beds in limestone or chalk through the precipitation of silica from groundwater. The silica often comes from biological sources like sponge spicules or radiolarians. Most findable specimens range from the Paleozoic to the Cretaceous periods.
Uses & applications
Historically used for stone tools/weapons due to sharp edges; currently used as construction aggregate, road ballast, or by hobbyists for flint knapping (arrowhead making) and fire-starting.
Geological facts
Chert was one of the first resources 'mined' by prehistoric humans. Because of its extreme hardness and way it breaks into razor-sharp edges, it was essential for survival. It is chemically identical to quartz but has a different internal structure.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture patterns and the fact that it will easily scratch glass. Commonly found in stream beds or weathering out of limestone cliffs. Collectors should look for 'glassy' textures inside rough exterior rinds.
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