sedimentary
Black flint (chert)
Microcrystalline quartz, SiO₂; commonly called flint when dark and fine-grained
AI-generated identification · may be incorrectAppears dark gray to black with a dull-to-waxy surface and irregular, sharp-looking fracture. Hardness is about 7 Mohs; crystals are too fine to see, and it has no cleavage. The photo alone cannot confirm the identification.
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Physical properties
Appears dark gray to black with a dull-to-waxy surface and irregular, sharp-looking fracture. Hardness is about 7 Mohs; crystals are too fine to see, and it has no cleavage. The photo alone cannot confirm the identification.
Formation & geological history
Forms when silica accumulates and replaces material in sedimentary rocks, often as nodules in limestone or chalk. Its age varies with the host formation; no specific age or location can be determined from the image.
Uses & applications
Historically used for cutting tools, fire-starting, and gunflints; today it is collected and occasionally used in decorative work.
Geological facts
Flint commonly breaks with a curved, shell-like (conchoidal) fracture, producing sharp edges.
Field identification & locations
Look for a very hard, fine-grained surface and conchoidal breaks; a streak test and examination of fresh fracture surfaces can help distinguish it from slag or other dark stones. Flint occurs in sedimentary deposits worldwide.
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