sedimentary
Chert (Jasper/Agate variety)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: waxy translucent grey with reddish iron oxide inclusions; Luster: waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65
- Hardness
- 6
- Color
- waxy translucent grey with reddish iron oxide inclusions
- Luster
- waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: waxy translucent grey with reddish iron oxide inclusions; Luster: waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils or chemical precipitation in sedimentary basins. Red hues indicate secondary iron oxidation. Often found in nodules within limestone or as river-tumbled pebbles.
Uses & applications
Historically used for stone tools (knapping) due to sharp edges; currently used for lapidary work, jewelry, and as river rock for landscaping.
Geological facts
This specimen shows the transition between common chert and chalcedony. It was highly prized by prehistoric humans for creating arrowheads because of its predictable conchoidal fracturing.
Field identification & locations
Look for a smooth, waxy surface and shell-like (conchoidal) fractures. It will scratch glass easily. Commonly found in riverbeds, gravel pits, and coastal cliffs.
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