sedimentary
Chert / Jasper and River Stones
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: Reddish-brown, dark brown, grey, black. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 6
- Color
- Reddish-brown, dark brown, grey, black
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Reddish-brown, dark brown, grey, black. 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 microfossils (like radiolaria or diatoms) or chemical precipitation from silica-rich fluids in ocean basins or groundwater. Ages range from Precambrian to recent.
Uses & applications
Commonly used in landscaping, as aggregate in construction, and historically used by indigenous peoples for stone tools (arrowheads). Colorful jasper varieties are used in lapidary work and jewelry.
Geological facts
Chert was one of the first materials used by early humans for tools because its conchoidal fracture produces extremely sharp edges. When struck against steel, it can produce sparks, though not as efficiently as true flint.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its shell-like (conchoidal) fracture pattern on broken edges, and its waxy feel. Often found in riverbeds or as nodules within limestone layers.
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