sedimentary
Chert (within Limestone)
Silicon Dioxide (SiO2) within Calcium Carbonate (CaCO3)
Hardness: 7 (chert) / 3 (limestone); Color: tan to honey-brown nodules in light grey matrix; Luster: waxy or dull; Structure: microcrystalline quartz; Cleavage: none (conchoidal fracture).
- Hardness
- 7 (chert) / 3 (limestone)
- Color
- tan to honey-brown nodules in light grey matrix
- Luster
- waxy or dull
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Physical properties
Hardness: 7 (chert) / 3 (limestone); Color: tan to honey-brown nodules in light grey matrix; Luster: waxy or dull; Structure: microcrystalline quartz; Cleavage: none (conchoidal fracture).
Formation & geological history
Formed by the precipitation of silica within limestone beds, often from the remains of prehistoric sponges or radiolarians. Common in Paleozoic and Mesozoic sedimentary formations.
Uses & applications
Historically used for making stone tools (flintknapping); current uses involve construction aggregate and occasionally as a decorative landscape stone.
Geological facts
Chert is a chemical sedimentary rock. It is so hard that it can strike sparks from steel, which led to its widespread use in early firearms and fire-starting tools.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its characteristic conchoidal fracture which looks like a curved shell-like chip. Common in regions with ancient seabed deposits.
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