sedimentary
Chert (Jasper/River Rock)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Yellow-brown, tan, or iron-stained; Luster: Dull to waxy when dry, vitreous (glassy) when wet; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture).
- Hardness
- 7 (Mohs scale)
- Color
- Yellow-brown, tan, or iron-stained
- Luster
- Dull to waxy when dry, vitreous (glassy) when wet
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Physical properties
Hardness: 7 (Mohs scale); Color: Yellow-brown, tan, or iron-stained; Luster: Dull to waxy when dry, vitreous (glassy) when wet; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed by the precipitation of silica from groundwater or as a biochemical sediment on the ocean floor, often replacing carbonate rocks. This specimen is a water-worn river cobble.
Uses & applications
Historically used for stone tools and fire-starting (flint). Currently used as landscaping stone, river rock for construction, and occasionally for lapidary work/tumbling.
Geological facts
Chert is so hard it can easily scratch glass and even common steel. It has been used by humans for hundreds of thousands of years to create sharp-edged tools like arrowheads.
Field identification & locations
Identified by its extreme hardness, smooth texture, and curved (conchoidal) breaks. Commonly found in riverbeds, glacial deposits, and gravel pits across North America.
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