sedimentary
Chert (varietal: Jasper or Ironstone nodule)
Microcrystalline Quartz (SiO2) with iron oxides
Hardness: 6.5-7 (Mohs scale); Color: Yellow-ochre, tan, and reddish-brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Yellow-ochre, tan, and reddish-brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Yellow-ochre, tan, and reddish-brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the precipitation of silica in sedimentary environments, often replacing organic or carbonate material. The coloration suggests the presence of iron oxides (limonite/hematite) during formation. These nodules are common in limestone or sandstone layers.
Uses & applications
Commonly used as lapidary material for tumbling/cabbing, historical use as strike-a-lights or flint tools, and for landscaping stone.
Geological facts
Chert and its variety, jasper, have been used by humans for hundreds of thousands of years to create sharp-edged tools because of their predictable conchoidal fracture pattern.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, waxy surface when broken. It often occurs as rounded nodules or pebbles in riverbeds or within sedimentary outcrops.
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