sedimentary
Chert (often found within Limestone)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed as nodules or layers in carbonate rocks through the precipitation of silica from groundwater or the accumulation of siliceous skeletal remains (like diatoms or sponges) on the seafloor.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; today used in construction aggregate and as a collector's specimen.
Geological facts
Chert is extremely durable and weather-resistant; it often survives long after the surrounding limestone host rock has eroded away. It produces sparks when struck against steel.
Field identification & locations
Identify by its waxy texture, extreme hardness (it will scratch glass), and smooth, curved (conchoidal) breaks. Often found in limestone outcrops or as rounded pebbles in riverbeds.
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