sedimentary
Chert (specifically Cryptocrystalline Quartz)
Silicon dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, and gray mottled; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, and gray mottled
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, and gray mottled; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from hydrothermal fluids in sedimentary basins.
Uses & applications
Used historically for stone tools (flintknapping), as road aggregate in construction, and occasionally polished for lapidary use or aquarium stone.
Geological facts
Chert is extremely durable and chemically resistant. It was the primary material used by prehistoric humans to create arrowheads and scrapers due to its sharp, glass-like fracture edges.
Field identification & locations
Identify by its hardness (it will scratch glass) and its characteristic shell-like (conchoidal) fracture pattern. Commonly found in limestone and chalk deposits or as water-worn pebbles in riverbeds.
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Brown Sandstone (River Rock)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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