sedimentary
Chert (variations include Flint/Jasper)
Cryptocrystalline Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan, brown, gray, or mottled. Luster: dull to waxy. Structure: cryptocrystalline (tiny crystals), Cleavage: none, breaks with a sharp shell-like (conchoidal) fracture. Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale), Color: tan, brown, gray, or mottled
- Color
- tan, brown, gray, or mottled
- Luster
- dull to waxy
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, brown, gray, or mottled. Luster: dull to waxy. Structure: cryptocrystalline (tiny crystals), Cleavage: none, breaks with a sharp shell-like (conchoidal) fracture. Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within limestone/chalk beds. Found in geological formations ranging from Precambrian to recent times.
Uses & applications
Historically used for stone tools/weapons (flint knapping) and fire-starting. Currently used in road construction as aggregate, as a component in glass manufacturing, and occasionally as decorative stone or jewelry (when colorful).
Geological facts
Chert is so hard it can scratch steel. Because it breaks into razor-sharp edges, it was the primary material for spearheads and arrowheads for thousands of years throughout human history.
Field identification & locations
Identify in the field by its smooth, shell-like fracture surfaces and extreme hardness. Often found as nodules within limestone or as river-worn cobbles. It is widely available across North America, especially in the Midwest and South.
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