sedimentary
Chert (White Variety)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: White to light grey, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to light grey, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed via the accumulation of siliceous oozes from microscopic organisms like radiolaria or diatoms in marine environments, or by replacement within limestone beds. Most deposits range from Paleozoic to Mesozoic eras.
Uses & applications
Historically used for making stone tools (flint knapping). Modernly used in construction aggregate, road base, and as decorative gravel. High-purity chert can be used in electronics.
Geological facts
Because it fractures into sharp edges, chert was the primary material for arrowheads and scrapers for thousands of years. It is chemically identical to quartz but contains impurities that give it an opaque appearance.
Field identification & locations
Identify in the field by its conchoidal fracture (curved, glass-like breaks) and that it cannot be scratched by a steel knife. Commonly found in limestone outcrops or as rounded pebbles in riverbeds.
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