sedimentary
Chert Nodules (Jasper)
Microcrystalline Quartz (SiO2)
Mohs Hardness: 7; Color: Reddish-brown, tan, ochre; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7
- Color
- Reddish-brown, tan, ochre
- Luster
- Waxy to vitreous
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Physical properties
Mohs Hardness: 7; Color: Reddish-brown, tan, ochre; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the precipitation of silica within sedimentary rock layers, often as nodules or concretions in limestone or shale. These often form from the accumulation of silica-rich organic remains like radiolaria or diatoms over millions of years.
Uses & applications
Historically used for making stone tools due to conchoidal fracturing. In modern times, used for lapidary work, jewelry (as jasper), and as decorative landscaping stones.
Geological facts
Iron oxide impurities give this specimen its characteristic reddish-brown color. Chert is so durable that it often survives as tumbled river or beach stones long after the surrounding softer limestone has eroded away.
Field identification & locations
Identify by its smooth, waxy texture and conchoidal (shell-like) fracture patterns. Look for the 'eyes' or circular growth patterns common in nodules. Commonly found in riverbeds and gravel pits in sedimentary basins.
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