sedimentary
Chert / Chalcedony
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs). Color: Dull green, brown, tan. Luster: Waxy to vitreous when wet or polished. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Dull green, brown, tan
- Luster
- Waxy to vitreous when wet or polished
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Physical properties
Hardness: 6.5-7 (Mohs). Color: Dull green, brown, tan. Luster: Waxy to vitreous when wet or polished. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich microfossils or petrification of organic matter in sedimentary environments. Common in marine limestone sequences or as river-worn nodules.
Uses & applications
Historically used for stone tools and arrowheads; today used as landscaping stone, river rock, or for lapidary tumbling if colorful.
Geological facts
Crystalline silica is so durable that it often survives long after the surrounding rock has eroded away, ending up as rounded pebbles in streambeds.
Field identification & locations
Identify by its hardness (can scratch glass) and smooth, shell-like fracture surfaces. Frequently found in riverbeds and glacial till throughout North America.
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