sedimentary
Chert / Flint (with quartz or calcite inclusion)
Silicon dioxide (SiO2)
Hardness of 7 on the Mohs scale; color varies from tan, gray, to white; dull to waxy luster; microcrystalline or cryptocrystalline structure; no cleavage; distinctive conchoidal (shell-like) fracture.
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Physical properties
Hardness of 7 on the Mohs scale; color varies from tan, gray, to white; dull to waxy luster; microcrystalline or cryptocrystalline structure; no cleavage; distinctive conchoidal (shell-like) fracture.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the precipitation of silica in groundwater within limestone or chalk beds. Most specimens date from the Phanerozoic eon.
Uses & applications
Historically used for stone tools and weapons (arrowheads/scrapers) due to sharp edges; currently used as road aggregate, in glass manufacturing, and as flint for fire-starting.
Geological facts
Chert is so hard it can scratch steel. The presence of a shiny or crystalline inclusion, like the one seen in the image, often indicates a small geode or 'vug' where secondary crystals like quartz or calcite formed in an empty cavity.
Field identification & locations
Identify in the field by its very smooth, curved break surfaces (conchoidal fracture) and its ability to scratch glass. Commonly found as nodules or layers within limestone formations globally.
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