sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Grey to Tan, Luster: Waxy to Dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs), Color: Grey to Tan, Luster: Waxy to Dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed primarily in deep-sea environments where silica-rich fluids or microscopic organisms like diatoms and radiolaria accumulate and recrystallize under pressure. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; modernly used as road aggregate and in some jewelry as 'Jasper' variants.
Geological facts
Because it breaks with a conchoidal fracture, it was one of the most critical materials for early humans to create survival tools. It can also produce a spark when struck against steel.
Field identification & locations
Identified by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. Commonly found in riverbeds or eroding out of limestone cliffs.
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