sedimentary

Dark gray chert or flint nodule

Microcrystalline quartz (SiO₂), commonly chalcedony/quartz

AI-generated identification · may be incorrect

Typically very fine-grained, opaque gray to black, with dull to waxy luster and conchoidal fracture; hardness about 6.5–7 Mohs, no cleavage, and specific gravity near 2.6. The specimen appears weathered and knobby, with possible lighter silica or carbonate patches.

Identified August 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when microcrystalline silica replaces sediment or precipitates in limestone, chalk, or marine deposits; flint is a dark variety. Its age depends on the host formation and may range from Paleozoic to Cenozoic.

Uses & applications

Used historically for stone tools and fire-starting, and occasionally as aggregate, landscaping stone, or a lapidary material; ordinary pieces have little jewelry value.

Geological facts

Conchoidal edges can be extremely sharp, making chert important in prehistoric toolmaking. The image alone cannot confirm whether it is chert, flint, or another dark siliceous rock.

Field identification & locations

Check for a glassy waxy surface, curved fracture, absence of visible grains, and inability to scratch it with a steel knife; it should scratch glass. Common in limestone and chalk regions worldwide, including England, France, Texas, Kansas, and parts of the Middle East.