sedimentary
Chert (Chert Nodule with Chalky Cortex)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale); Color: Gray to brown interior with white/cream outer cortex; Luster: Waxy to vitreous (glassy) on fresh breaks, dull on exterior; Crystal Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray to brown interior with white/cream outer cortex
- Luster
- Waxy to vitreous (glassy) on fresh breaks, dull on exterior
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray to brown interior with white/cream outer cortex; Luster: Waxy to vitreous (glassy) on fresh breaks, dull on exterior; Crystal Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica within porous sedimentary rocks like limestone or chalk. Often organic in origin, from the remains of sponges or radiolarians. Common in Paleozoic and Mesozoic sedimentary basins.
Uses & applications
Historically used for stone tools and weapons due to sharp conchoidal edges; currently used for road aggregate, laboratory surfaces, and occasionally as an abrasive or in decorative landscaping.
Geological facts
Chert is so hard it can strike sparks against steel. The dark center is the purest silica, while the white 'skin' (cortex) is where the silica transition into the surrounding limestone host rock was interrupted.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture pattern and hardness (it will scratch glass). Commonly found in stream beds, eroded limestone hills, or coastal chalk cliffs. Collectors look for pieces with high contrast between the core and cortex.
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