sedimentary
Chert (Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Grey, brown, or waxy olive-green, Luster: Vitreous to waxy, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Grey, brown, or waxy olive-green, Luster: Vitreous to waxy, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed as nodules or layers in sedimentary rocks (like limestone or chalk) through the precipitation of silica from groundwater or accumulation of siliceous organisms like diatoms and radiolarians.
Uses & applications
Historically used for stone tools and weapons (arrowheads/scrapers) and as a spark-generator (flint and steel). Modern uses include road surfacing and as a gemstone when colorful (Jasper).
Geological facts
The conchoidal fracture of chert produces sharp edges, which made it a critical material for human survival during the Stone Age. It is chemically identical to quartz but has a different internal structure.
Field identification & locations
Identify in the field by its smooth, shell-like fracture surfaces (conchoidal) and its ability to scratch glass. It is often found as pods or layers within chalk or limestone formations.
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