sedimentary
Chert (Smooth River Siltstone/Basalt variant)
Silicon dioxide (SiO2) / Microcrystalline Quartz
Hardness: 6.5-7 Mohs. Color: Dull grey, greenish-grey, or black. Luster: Dull to waxy. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Dull grey, greenish-grey, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 Mohs. Color: Dull grey, greenish-grey, or black. Luster: Dull to waxy. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from seawater or the accumulation of silica-rich organic debris (such as radiolarians or diatoms) on the ocean floor. Most specimens are from the Paleozoic or Mesozoic era.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges when broken. Also used in construction aggregate, road base, and as a durable river stone in landscaping.
Geological facts
Chert is so hard it can scratch glass and steel. It was one of the most important commodities for prehistoric humans for crafting sharp cutting edges. When struck against steel, it can produce sparks, similar to flint.
Field identification & locations
Identify by its extreme hardness, waxy texture, and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds, glacial tills, and as nodules within limestone layers.
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Sandstone or Siltstone
Arenite (if sandstone)
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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