sedimentary
Chert (Chert / Flint Variety)
Microcrystalline / Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark brown to black (chocolate chert), Luster: Dull to waxy, Crystal Structure: Hexagonal (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark brown to black (chocolate chert), Luster: Dull to waxy, Crystal Structure: Hexagonal (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by the chemical replacement of host rocks (like limestone). It typically forms as nodules in sedimentary strata.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used for road surfacing, flintknapping crafts, and occasionally lapidary work.
Geological facts
Chert breaks with a conchoidal fracture, creating razor-sharp edges. This property made it the most vital material for survival in the Stone Age for spearheads and knives.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth shell-like fracture surfaces, and lack of visible grains. Common in limestone outcrops and riverbeds.
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