sedimentary
Chert (Basanite)
Chert (Microcrystalline SiO2)
Hardness 6.5-7 (Mohs), dark grey to black color, waxy to dull luster, cryptocrystalline structure, conchoidal fracture, high specific gravity around 2.6
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Physical properties
Hardness 6.5-7 (Mohs), dark grey to black color, waxy to dull luster, cryptocrystalline structure, conchoidal fracture, high specific gravity around 2.6
Formation & geological history
Formed from the accumulation of silica-rich organic remains or chemical precipitation in marine environments, often associated with limestone or deep-sea deposits
Uses & applications
Used historically for stone tools and flintknapping; modern use as decorative gravel, in lapidary work, and for fire-starting (flint)
Geological facts
Black chert like this is often called Basanite by jewelers and 'flint' by geologists. It was used by many ancient cultures to create sharp arrowheads and knives due to its predictable breaking patterns.
Field identification & locations
Identify by its hardness (scratches glass), smooth waxy feel, and curved 'clamshell' fracture lines. Common in riverbeds and limestone outcrops.
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