sedimentary
Chert (or Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan to light brown, Luster: waxy to dull, Crystal structure: cryptocrystalline, Fracture: conchoidal, Cleavage: none
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan to light brown, Luster: waxy to dull, Crystal structure: cryptocrystalline, Fracture: conchoidal, Cleavage: none
Formation & geological history
Forms primarily as nodules or layers in sedimentary rocks like limestone or chalk through the replacement of organic materials or precipitation from silica-rich fluids. Generally Mesozoic to Paleozoic in age.
Uses & applications
Historically used for stone tools and flintknapping. Modern uses include abrasive materials and sometimes used in landscaping or as a minor gemstone when colorful (Jasper/Agate).
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, leading to its historical use as 'flint' for fire-starting and in flintlock firearms.
Field identification & locations
Identify in the field by its smooth, waxy feel, conchoidal (shell-like) fracture edges, and extreme hardness. It won't be scratched by a knife blade. Found in riverbeds and limestone outcrops worldwide.
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