sedimentary
Gray Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Gray to light tan, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Gray to light tan, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed in marine environments through the accumulation of siliceous remains of organisms like diatoms or radiolarians, or by chemical precipitation from silica-rich fluids within limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, knives) due to its sharp edges; used today in road construction, as a source of silica in glassmaking, and by collectors.
Geological facts
Chert has been used by humans for over 2 million years. The gray variety is often referred to simply as flint, especially when found in chalk or limestone deposits.
Field identification & locations
Identified by its distinctive smooth, curved conchoidal fracture edges and extreme hardness (capable of scratching glass). Commonly found in nodular forms within limestone outcrops globally.
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