sedimentary
Chert (Black Flint)
Microcrystalline Quartz (SiO2)
Mohs hardness of 7; color is opaque black to dark grey; waxy to dull luster; cryptocrystalline structure; prominent conchoidal fracture (breaks with shell-like curved surfaces); specific gravity 2.65.
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Physical properties
Mohs hardness of 7; color is opaque black to dark grey; waxy to dull luster; cryptocrystalline structure; prominent conchoidal fracture (breaks with shell-like curved surfaces); specific gravity 2.65.
Formation & geological history
Formation occurs as nodules or layers in sedimentary rocks (like limestone) through the precipitation of silica from groundwater or the accumulation of siliceous biological remains (diatoms/radiolarians) on the ocean floor.
Uses & applications
Historically used for stone tools and fire-starting; currently used for decorative gravel, landscaping, and as a raw material for ceramics and glass when high purity.
Geological facts
Black flint gets its dark color from tiny inclusions of organic matter or carbon. It was the primary material for prehistoric survival tools because of its ability to hold a razor-sharp edge.
Field identification & locations
Identify by its smooth, waxy feel and the sharp edges formed by conchoidal fractures. It is commonly found in chalk or limestone outcrops and along coastal beaches where these rocks have eroded.
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Brecciated Chert (Microcrystalline Silica)
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