sedimentary
Chert (with Limestone matrix)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the precipitation of silica in groundwater within sedimentary host rocks like limestone during various geological eras.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, construction aggregate, and occasionally as an ornamental garden stone.
Geological facts
Chert was one of the most vital resources for prehistoric humans, often traded over long distances to produce arrowheads and scrapers. It can spark when struck against steel.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy feel, and curved 'eggshell' breaks. Found globally in limestone and chalk beds, often appearing as nodules or layers.
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