sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs, Color: tan, grey, yellow-brown, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 Mohs, Color: tan, grey, yellow-brown, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Forms primarily as nodules or layers in limestone or chalk through the replacement of calcium carbonate by silica-rich fluids. It can also form as primary chemical precipitates on the ocean floor.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Modernly used in construction aggregate, road surfacing, and as high-quality flint for fire-starting tools.
Geological facts
Because it breaks with a conchoidal fracture like glass, it creates edges that can be sharper than a surgical scalpel. It has been used by humans for over 2 million years.
Field identification & locations
Identify in the field by its sharp, curved fracture edges and its hardness (it will scratch glass). Look for it as nodules protruding from weathered limestone outcrops.
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