sedimentary
Chert Nodule (with Chalcedony)
Chert (SiO2) / Cryptocrystalline Quartz
Hardness: 7 (Mohs scale), Color: tan to brown exterior with white/blue-gray interior, Luster: waxy to vitreous on the fracture, Structure: microcrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan to brown exterior with white/blue-gray interior, Luster: waxy to vitreous on the fracture, Structure: microcrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within limestone or chalk beds, often replacing organic matter or filling voids. Most are from the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for making stone tools (arrowheads). Modernly used in lapidary work, jewelry (tumbled stones), and as a structural aggregate in construction.
Geological facts
Chert nodules are often called 'flint' when found in chalk. They are extremely durable and can survive millions of years of erosion, eventually accumulating in stream beds as 'river rocks'.
Field identification & locations
Identify in the field by its blocky or nodular shape and characteristic shell-like (conchoidal) fracture when broken. Commonly found in riverbeds, gravel pits, and eroding out of limestone cliffs.
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