sedimentary
Chert (Nodule)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to black with tan/brown cortex, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.6
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey to black with tan/brown cortex, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.6
Formation & geological history
Formed by the precipitation of silica from seawater or groundwater within limestone or chalk beds. Often occurs as nodules or beds during the diagenesis of sedimentary layers.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its conchoidal fracture. Currently used in jewelry as cabochons, or for landscaping and construction aggregate.
Geological facts
Dark colored chert is often specifically called 'flint'. It is so hard that it can strike sparks from steel, which led to its extensive use in flintlock firearms and early fire-starting tools.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and smooth, shell-like fracture surfaces. Often found in riverbeds or protruding from limestone outcrops.
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Brecciated Chert (Microcrystalline Silica)
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