sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Dark brown to black with grayish-green patches, Luster: Waxy to dull, Crystal structure: Microcrystalline (Cryptocrystalline), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark brown to black with grayish-green patches, Luster: Waxy to dull, Crystal structure: Microcrystalline (Cryptocrystalline), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the replacement of carbonate minerals in limestone or chalk beds. Often found in Phanerozoic sedimentary sequences.
Uses & applications
Historically used for making stone tools (flint knapping) due to its conchoidal fracture. Currently used in road construction, as an abrasive, and occasionally for lapidary work.
Geological facts
Chert is so hard that it can strike sparks from steel, which led to its famous use in flintlock firearms. It is chemically identical to quartz but has a different microscopic structure.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. Often found as nodules within limestone or as bedded layers.
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