sedimentary
Chert (within Limestone matrix)
Silicon Dioxide (SiO2) within Calcium Carbonate (CaCO3)
Chert: Hardness 7 (Mohs), waxy to dull luster, conchoidal fracture, various tan/brown shades. Limestone: Hardness 3, dull luster, light gray/white, fizzes with acid.
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Physical properties
Chert: Hardness 7 (Mohs), waxy to dull luster, conchoidal fracture, various tan/brown shades. Limestone: Hardness 3, dull luster, light gray/white, fizzes with acid.
Formation & geological history
Formed by the accumulation of microcrystalline silica often in marine environments within carbonate sediments. The light gray host rock is limestone, while the orange-brown nodule is chert, formed via chemical replacement.
Uses & applications
Historically used for stone tools (arrowheads). Today used for landscaping, aggregate in construction, and as a geological study specimen.
Geological facts
Chert nodules like this one are often harder than the surrounding limestone, causing them to protrude as the limestone weathers away. Chert has been used by humans for millions of years to start fires and make sharp tools.
Field identification & locations
Identify in the field by checking for the conchoidal (curved, shell-like) fracture on the brown portion and its ability to scratch glass. Found globally in limestone layers.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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