sedimentary
Chert (White/Gray variety)
Silicon Dioxide (SiO2 / Cryptocrystalline Quartz)
Hardness: 7 (Mohs scale), Color: White to light tan with iron staining, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to light tan with iron staining, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction aggregate, road surfacing, and as a decorative stone in landscaping.
Geological facts
Chert has a conchoidal fracture which allows it to produce razor-sharp edges. It is one of the most durable rocks on the surface of the Earth, frequently surviving long after other surrounding rocks have weathered away.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy luster on fresh breaks, and smooth, curved fracture surfaces. Found worldwide in sedimentary basins.
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