sedimentary
Chert (variously known as Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, gray, or tan with a waxy luster. Crystal structure: Hexagonal (microscopic). Breakage: Conchoidal (shell-like) fracture. High density and very sharp edges when broken.
- Color
- Brown, gray, or tan with a waxy luster
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, gray, or tan with a waxy luster. Crystal structure: Hexagonal (microscopic). Breakage: Conchoidal (shell-like) fracture. High density and very sharp edges when broken.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within limestone or chalk beds. Most common in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) and as striker stones for flintlock firearms. Currently used in road surfacing and as a component in some ceramics.
Geological facts
Chert was one of the most important minerals for early humans due to its ability to be knapped into extremely sharp tools. It is so hard that it can scratch steel and produce sparks when struck against iron.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it won't be scratched by a knife), and smooth, curved fracture surfaces. Often found as nodules within limestone or as river-worn pebbles.
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